Sources

Every source on this list was checked against its Crossref / arXiv / Stanford Encyclopedia of Philosophy record (title, year, author). The summary under each source is in our own words; it is not the source's own text.

• 't Hooft 1993 — 't Hooft, Dimensional reduction in quantum gravity. arXiv:gr-qc/9310026
  In brief, what does it say?

  The question: how can gravitational collapse, that is, black-hole-type events, be reconciled with the principles of quantum mechanics? The author argues that this reconciliation requires that at the Planck scale our world is not 3+1 dimensional. According to him, the observable degrees of freedom can be described as Boolean variables defined on a two-dimensional lattice evolving in time. He says this conclusion is reached resting on little other than 'unitarity', entropy and counting arguments. The author uses cellular automata as examples and argues that this dimensional reduction imposes more constraints than our freedom in model building. This is a theory paper; not an experiment, and the author's own claim. According to him, this is why a fully consistent mathematical model for quantum black holes has not yet been found. What gives one pause is the idea that the information of a world that looks three-dimensional can be held on a two-dimensional surface.

• Aad 2012 — Aad, Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC. Phys. Lett. B 716, 1 (ATLAS Collaboration)
  In brief, what does it say?

  The ATLAS collaboration reported a new boson around 126 GeV at the LHC.

  The last untested piece of the Standard Model was the Higgs boson. ATLAS’s question is whether there is an excess in proton–proton collisions at the LHC that fits this particle.

• Adam 2021 — Adam, Measurement of e+e- momentum and angular distributions from linearly polarized photon collisions. Phys. Rev. Lett. 127, 052302
  In brief, what does it say?

  The question: when particles of light (photons) collide, can matter and its antimatter be born? This process is known as the Breit-Wheeler process. In the experiment, gold nuclei were collided at very high speed; because the nuclei carry a large charge, there are extraordinarily strong electromagnetic fields around them, and these fields can make photons collide with one another. The team observed 6085 electron-positron pairs coming out of these collisions. A marked modulation and orientation were found in the angular distribution of the pairs; the mass distribution came out smooth, and no trace of known intermediary particles was seen. These features agree with quantum electrodynamics calculations that assume linearly polarized photons colliding. So this is an experimental finding; its agreement with theory is the paper's own determination. What gives one pause here is that a pair of matter being born from light in a vacuum softens the boundary between matter and light. The paper also says the finding has consequences for vacuum birefringence and for mapping magnetic fields.

• Adlam 2021 — Adlam, Foundations of Quantum Mechanics. Cambridge University Press (Elements in the Philosophy of Physics)
  In brief, what does it say?

  The question was: although quantum mechanics has been a very successful theory for more than a hundred years, why is it still not understood what it says? This short book by Emily Adlam (from Cambridge's Elements series) introduces the most surprising questions at the foundations of quantum mechanics and presents the main answers given to them, together with the costs each one requires. Topics such as the measurement problem, entanglement and Bell's theorem are taken up one by one. The book does not declare one interpretation correct; it shows how the interpretations answer which questions and what they leave behind.

• Aharonov 1959 — Aharonov, Significance of Electromagnetic Potentials in the Quantum Theory. Physical Review 115, 485-491
  In brief, what does it say?

  Aharonov and Bohm showed that even in a region where the field is zero, the vector potential can have a measurable effect on the interference pattern of a charged particle.

  The paper considers the following setup: a beam of electrons is split in two, passes on either side of a long, thin coil (a solenoid), and recombines on a screen. There is a magnetic field inside the coil but none outside it; that is, the electrons feel no magnetic force. Even so, the vector potential outside the coil is not zero.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Aharonov 1988 — Aharonov, How the result of a measurement of a component of the spin of a spin-1/2 particle can turn out to be 100. Phys. Rev. Lett. 60, 1351
  In brief, what does it say?

  The question: when we measure the spin of a particle, the result is 'plus one half' or 'minus one half.' But what comes out if we make the measurement very gentle and select only particles that have certain states at the beginning and at the end? The authors showed that in such a setup the average result can give an unusual value; they called it a 'weak value.' The example in the paper's title says that a value that looks like 100 can come out for a spin component of a spin-one-half particle. For this reason a weak value is not read like an ordinary measurement result; it is a kind of average that a selected ensemble gives under a gentle measurement. We limited the detail in this summary to what is in the paper's raw abstract and title. It is a theoretical definition and calculation; its interpretation (whether this value is 'real') is disputed. What gives one pause is that a measurement result can fall far outside the possible range.

• Allen 1938 — Allen, Flow of Liquid Helium II. Nature 141, 75
  In brief, what does it say?

  Allen and Misener measured the flow of helium II through narrow capillary tubes and showed that it differs from the usual viscous flow.

  This short paper, which appeared in the same issue as Kapitza’s note, approaches the same phenomenon with an independent experiment. The question is again this: how does liquid helium below the λ-point behave while passing through a narrow tube?

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Almheiri 2013 — Almheiri, Black holes: complementarity or firewalls?. J. High Energy Phys. 2013(2), 62
  In brief, what does it say?

  Almheiri, Marolf, Polchinski and Sully argued that three widely held assumptions cannot all be true together, and said that the most 'conservative' way out is a firewall at the horizon.

  The paper sets out three statements and says they cannot all be true together. The first is that Hawking radiation is in a pure quantum state, that is, that information is preserved. The second is that the information carried by the radiation comes from the region near the horizon, and that beyond a microscopic distance from the horizon the low-energy effective field theory holds. The third is that an observer falling in sees nothing out of the ordinary at the horizon; this is the equivalence principle that comes from general relativity.

• Altaie 2009 — Altaie, Re-creation: a possible interpretation of quantum indeterminism. arXiv:0907.3419
  In brief, what does it say?

  The paper reviews the main interpretations that try to explain why quantum measurements give uncertain, unpredictable results, and argues that a corner is left open in all of them. The author puts forward his own proposal: physical properties may be continuously re-created. The rate of re-creation is taken as the measure of how determinate measurements are. According to the author, from this standpoint the existence of uncertainties, the meaning of quantities that cannot be measured together, and the origin of the Heisenberg uncertainty principle are better understood. This is not an experiment; it is an interpretation the author proposes himself, that is, contested ground. Relating quantum uncertainty to an idea of 'continuous re-creation' is what makes it interesting in this source list.

• Anderson 1933 — Anderson, The positive electron. Phys. Rev. 43, 491
  In brief, what does it say?

  The question was: among the tracks opened by cosmic rays, is there a light particle other than the electron? Anderson placed a cloud chamber (Wilson chamber), which makes visible the paths of charged particles passing through it, inside a strong magnetic field and took about 1300 photographs. In 15 of them the track belonged to a positively charged particle whose mass could not be as large as the proton's; from energy loss and ionization the charge was found equal to the proton's. The curvatures and the ionization required the mass to be less than twenty times the electron mass. Anderson named these particles positrons and wrote that the tracks appeared in groups together with other tracks, and that they therefore had to be secondary particles ejected from atomic nuclei. The finding gives one pause because it showed that the electron really has an oppositely charged twin.

• Anderson 1995 — Anderson, Observation of Bose-Einstein Condensation in a Dilute Atomic Vapor. Science 269, 198-201
  In brief, what does it say?

  Anderson and colleagues observed Bose-Einstein condensation in a dilute vapor of rubidium atoms.

  The condensation Einstein predicted around 1925 could not be observed directly for decades. In a dilute atomic gas this requires reaching a very low temperature: the temperature must fall so low that the average distance between atoms and the atom's quantum wavelength are about the same.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Arndt 1999 — Arndt, Wave-particle duality of C60 molecules. Nature 401, 680
  In brief, what does it say?

  The question was this: is behaving like a wave peculiar only to small things such as electrons and photons, or can a fairly large molecule do it too? The team passed C60 molecules, shaped like a soccer ball and made of sixty carbon atoms, through a thin grating and looked at where they landed on the screen behind. The molecules struck one at a time, at a point, like particles; but when many strikes accumulated, an interference pattern peculiar to waves, that is, light and dark bands, appeared on the screen. This is an experimental finding and admits no dispute. What gives one pause is this: the pattern comes out only when it is calculated as if each molecule had passed 'through more than one path', not as if it had passed through a single hole of the grating. What this means, that is, what the molecule really does, is a matter of interpretation, and theorists are still divided. The paper shows only the existence of the pattern and its agreement with the expected wave calculation.

• Arute 2019 — Arute, Quantum supremacy using a programmable superconducting processor. Nature 574, 505–510
  In brief, what does it say?

  The Google paper claiming quantum supremacy by random circuit sampling on the 53-qubit superconducting Sycamore processor.

  The paper is presented as an experimental test of the promise that quantum computers can perform certain tasks exponentially faster than classical computers. The difficulty is building a processor that works with high fidelity and can run quantum algorithms in a very large computational space.

• Aspect 1982 — Aspect, Experimental test of Bell's inequalities using time-varying analyzers. Phys. Rev. Lett. 49, 1804
  In brief, what does it say?

  Einstein, Podolsky and Rosen expected a 'local' nature: two things in separate places should not act on each other instantly. In 1964 John Bell turned this into a testable inequality. Alain Aspect and colleagues measured the polarizations of two photon pairs; by changing the measurement direction very quickly while the photons were in flight, they tried to close off the possibility of communication at the speed of light between the two measurements. The result agreed with the prediction of quantum theory and exceeded Bell's inequality by about five standard deviations. This is an experiment. What gives one pause is this: the relations of two things measured separately cannot be explained by a local 'hidden plan'. What this points to (a non-local influence, or another description of reality) is a matter of interpretation and is disputed.

• Bailey 1977 — Bailey, Measurements of relativistic time dilation for positive and negative muons in a circular orbit. Nature 268, 301
  In brief, what does it say?

  The question was: does a fast-moving clock really run slow? Bailey and colleagues circulated muons in a circular orbit at a speed very close to the speed of light in a muon storage ring at CERN and measured how long they lived until they decayed. Since the muon is a very short-lived particle, it works as a clock on its own. Moving muons lived markedly longer than stationary muons, and the lengthening agreed with the prediction of time dilation in special relativity. The finding gives one pause because it showed that time dilation is not a method of calculation but a measurable fact. We do not have the paper's own summary; the generally known framework is given here.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Bardeen 1957 — Bardeen, Theory of Superconductivity. Phys. Rev. 108, 1175
  In brief, what does it say?

  Bardeen, Cooper and Schrieffer gave the first microscopic theory of superconductivity, starting from electrons.

  Some metals, below a critical temperature, lose electrical resistance entirely and push magnetic fields out of their interior. Up to 1957 these phenomena had been observed, but how electrons established such an order was not known. The question was: how do electrons that repel one another establish a collective state that can flow without friction?

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Bekenstein 1973 — Bekenstein, Black Holes and Entropy. Phys. Rev. D 7, 2333
  In brief, what does it say?

  Bekenstein argued that the entropy of a black hole is proportional to the area of the event horizon, and that a generalized second law must include this entropy as well.

  Physicists knew that black holes appear to an outside observer through three quantities (mass, rotation, charge). This means that the scattered, detailed information that enters a black hole cannot be read from outside. Bekenstein asked this question: if a hot body is thrown into a black hole and its entropy disappears, is the second law not violated?

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Bell 1964 — Bell, On the Einstein Podolsky Rosen paradox. Physics 1, 195
  In brief, what does it say?

  Einstein, Podolsky and Rosen had asked in 1935: might quantum theory be incomplete, that is, is there a depth in which things are definite even before measurement, which the theory does not state? Bell made this question testable. Imagine we measure two particles that are tied to each other in distant places. If there is a 'local' hidden order that determines in advance the result of each particle and is not affected from afar, the relation between the two measurement results cannot exceed a certain limit. Bell wrote this limit as an inequality; the predictions of quantum theory exceed this limit at some measurement settings. That is, a theory that explains quantum results with local and predetermined values cannot be constructed. This is a theoretical result; testing experiments were carried out later and matched the quantum prediction. The interpretive side is open: what the result leaves behind (locality, or predetermination) is still debated.

• Bennett 1982 — Bennett, The thermodynamics of computation—a review. Int. J. Theor. Phys. 21, 905
  In brief, what does it say?

  Bennett surveyed the thermodynamics of computation and set out clearly the view that resolves the Maxwell's demon puzzle through the cost of erasing information.

  This paper presents neither a new experiment nor a quantum model; it is a review. Its question is: what is the least energy a computation must expend?

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Bennett 1984 — Bennett, Quantum cryptography: Public key distribution and coin tossing. IEEE Int. Conf. on Computers, Systems and Signal Processing, Bangalore, 1984; yeniden basım: Theoretical Computer Science 560 (2014) 7–11
  In brief, what does it say?

  The paper proposing secret key distribution with polarized photons (the BB84 protocol) and quantum coin tossing.

  The paper's starting point is this: when digital information is carried with basic quantum systems such as polarized photons, the uncertainty principle gives rise to cryptographic possibilities that do not exist in conventional media. In particular, because measurement disturbs the system, someone listening in on the transmission channel is noticed with high probability.

• Bennett 1993 — Bennett, Teleporting an unknown quantum state via dual classical and Einstein-Podolsky-Rosen channels. Phys. Rev. Lett. 70, 1895
  In brief, what does it say?

  Teleportation is the sending not of an object itself but of its information. The proposal of Bennett and colleagues works like this: Alice and Bob share in advance an entangled pair of particles. Alice makes a joint measurement on the particle in the unknown state she wants to teleport and on her own particle; this measurement destroys the original state but gives an ordinary result of two bits. Alice tells Bob this result by an ordinary route such as telephone or letter. When Bob applies a correction to the entangled particle he holds, according to the result, he obtains an exact copy of the original state. This is a theoretical proposal; it was later realized in experiments. Nothing travels faster than light, because ordinary news cannot exceed the speed of light; and no copying occurs, because the original is destroyed. What gives one pause is that the information is carried half by entanglement and half by the ordinary channel.

• Bennett 1996 — Bennett, Concentrating partial entanglement by local operations. Physical Review A 53, 2046–2052
  In brief, what does it say?

  A paper showing that shared pure, partially entangled pairs can be converted by local operations into maximally entangled pairs, and that the rate is determined by entropy.

  The question is this: if two distant observers hold many copies of the same partially entangled particle pair, can they, by applying only local operations to their own particles and talking to each other classically, concentrate this entanglement into a smaller number of maximally entangled (EPR "singlet") pairs?

• Bennett 1996 — Bennett, Mixed-state entanglement and quantum error correction. Physical Review A 54, 3824–3851
  In brief, what does it say?

  A paper showing that sending a quantum state through a noisy channel is equivalent to distilling maximal entanglement from mixed entangled states.

  This paper examines what can be done when entanglement is mixed with noise. Two ways of protection are compared: entanglement purification (extracting a small number of maximally entangled pairs from the mixed state held by the two parties) and quantum error-correcting codes (sending an arbitrary quantum state through a noisy channel without corruption).

• Bernstein 1993 — Bernstein, Quantum complexity theory. Proc. 25th Annual ACM Symposium on Theory of Computing (STOC '93), 11–20; tam sürüm: SIAM J. Comput. 26 (1997) 1411–1473
  In brief, what does it say?

  A paper that founded the complexity theory of quantum Turing machines and related the class BQP to classical classes.

  The paper examines quantum computation from the standpoint of complexity theory. The first result is that, in Deutsch's quantum Turing machine model, an efficient universal quantum Turing machine exists. The authors note that this construction is much more complicated than its classical counterpart; they show that even simple principles such as looping, branching and composition are laborious in the quantum setting.

• Berry 1977 — Berry, Level clustering in the regular spectrum. Proceedings of the Royal Society A 356, 375-394
  In brief, what does it say?

  The paper that suggests that level spacings in regular (nonchaotic) systems are randomly (Poisson) distributed; it shows the opposite case.

  Berry and Tabor examine systems in which the classical motion is regular. In such an f-dimensional system, each energy level is labeled by f quantum numbers, and these numbers come from the f constants of the classical motion. Levels with very different quantum numbers can fall at similar energies.

• Berry 1984 — Berry, Quantal phase factors accompanying adiabatic changes. Proceedings of the Royal Society A 392, 45-57
  In brief, what does it say?

  Berry showed that a slowly rotated quantum system acquires a geometric phase in addition to the usual temporal phase.

  The paper considers a system in which the parameters of the Hamiltonian are changed slowly and carried around a cyclic path back to the start. A system that begins in an eigenstate keeps tracking the same eigenstate as long as the slowness condition holds. It is shown that, in addition to the familiar "dynamic" phase that accumulates with time, the system acquires another phase.

• Binnig 1982 — Binnig, Tunneling through a controllable vacuum gap. Appl. Phys. Lett. 40, 178
  In brief, what does it say?

  In quantum physics a particle can pass to the other side of a barrier that seems impossible to cross; this is called tunneling. Binnig and colleagues left a gap between two metal surfaces, measured the current between them, and were able to vary the gap in a controlled way. In earlier attempts an insulating layer was placed in between; here for the first time tunneling was seen across an adjustable 'gap.' The finding: the resistance to the current increases exponentially as the gap widens; that is, when the gap opens a little, the current falls quickly. This is an experiment; it is the observation, in a gap, of a behavior expected beforehand on theoretical grounds. This setup later became the basic foundation of the scanning tunneling microscope, which made it possible to see individual atoms on surfaces. Why it gives one pause: one can pass, with a measurable probability, beyond a barrier that cannot be crossed by classical thinking.

• Bohigas 1984 — Bohigas, Characterization of Chaotic Quantum Spectra and Universality of Level Fluctuation Laws. Physical Review Letters 52, 1-4
  In brief, what does it say?

  The paper that suggests that the level fluctuations of chaotic quantum spectra are universal.

  Bohigas, Giannoni and Schmit examine the statistics of the quantum energy levels of a classically chaotic system. The levels of a quantum system are like points on a ruler; the real question is how the spacings between neighboring levels are distributed.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Bohm 1952 — Bohm, A suggested interpretation of the quantum theory in terms of hidden variables. I. Phys. Rev. 85, 166
  In brief, what does it say?

  In the usual quantum account the state of a particle is specified only by a wave function and only probability is given for measurement results. Bohm asked: is this account 'complete', or are there 'hidden variables' that we cannot see for now? In the picture he proposed every particle always has a definite position; this position follows an influence in the form of a 'guiding wave' directed by the wave function. The paper shows that this gives exactly all the predictions made by the usual theory; that is, this is not an experiment but a proposal for an interpretation, and it cannot be told apart by the same experiments. This picture allows a continuous and definite description, but it has a price: a structure that binds distant particles instantly, that is, a nonlocal one, is needed. In conclusion the paper gives one pause in that it shows that quantum theory does not have a single reading. Which interpretation is right is an open debate even today.

• Bohr 1913 — Bohr, On the constitution of atoms and molecules. Phil. Mag. 26, 1
  In brief, what does it say?

  Bohr treats the hydrogen atom as a system with definite energy levels and proposes that the emitted light is determined by the differences between these levels.

  According to Rutherford's model of the atom, an atom consists of a small nucleus and electrons orbiting around it. Classical electrodynamics requires that an orbiting charge radiate continuously, lose energy and collapse into the nucleus. Yet atoms are stable, and the light emitted by gases comes in sharp lines at definite frequencies.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Bohr 1928 — Bohr, The Quantum Postulate and the Recent Development of Atomic Theory. Nature 121, 580-590
  In brief, what does it say?

  Bohr first explained the quantum postulate and the idea of complementarity in his 1927 lecture at Como, Italy (Nature 121, 580).

  This piece is the printed version of the lecture Bohr gave at a congress in Como in September 1927. Bohr's question is how we are to use the concepts of classical physics (position, momentum, energy, wave, particle) when describing events at the atomic level.

• Bohr 1935 — Bohr, Can Quantum-Mechanical Description of Physical Reality be Considered Complete?. Phys. Rev. 48, 696-702
  In brief, what does it say?

  Bohr replied, under the same title, to the claim of Einstein, Podolsky and Rosen that the quantum description is incomplete (Phys. Rev. 48, 696).

  Bohr's reply was published in the same journal and under the same title shortly after the EPR paper appeared. EPR's question was this: if, without disturbing a system in any way, we can predict with certainty the value of a physical quantity by measurement, then there is a reality corresponding to that quantity. If this criterion is accepted, a link set up between two particles that have moved apart shows the quantum description to be incomplete.

• Bohr 2007 — Bohr, Niels Bohr. "Anyone who is not shocked by quantum theory has not understood it." Barad 2007, s. 254 aktarıyor; kesin kaynağı belirsiz, varyantları var
  In brief, what does it say?

  This is not a paper but a saying attributed to Niels Bohr: whoever is not shocked by quantum theory has not understood it. The saying is reported by Barad (2007); its source is a secondhand report coming via Wikiquote, and its exact source is not fixed; there are different ways of putting it. For this reason it is not verified that the attribution belongs to Bohr; we mention it not as Bohr's definite word but as a saying attributed to him. The saying is neither an experiment nor a theory; it summarizes a stance often voiced by those who work on quantum physics.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Born 1926 — Born, Zur Quantenmechanik der Stoßvorgänge. Z. Phys. 37, 863
  In brief, what does it say?

  Born treated the problem of a particle scattering off an atom with wave mechanics and proposed that the square of the wave function gives a probability (Z. Phys. 37, 863).

  Born's paper is about scattering, that is, a beam of particles striking an atom and being dispersed in various directions. Until then Schrödinger's wave mechanics had been used for stationary states inside the atom; Born asked whether the same equation could also describe particles flying freely and striking an obstacle.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Bose 1924 — Bose, Plancks Gesetz und Lichtquantenhypothese. Zeitschrift für Physik 26, 178-181
  In brief, what does it say?

  Bose derived Planck's radiation law by counting photon states, without appealing to classical electrodynamics.

  In 1900 Planck had proposed a law for the spectrum of thermal radiation that fitted experiment, but the derivations up to that time also appealed in part to classical electrodynamics. Bose's question was: can we obtain the law with an argument that only counts light quanta?

• Bousso 2002 — Bousso, The holographic principle. Rev. Mod. Phys. 74, 825
  In brief, what does it say?

  The question: how much information fits into a given region of space? Physicists studying black holes arrived at something surprising: the information a region can carry is limited not by its volume but by the area of the surface that encloses it. According to the paper, this limit is about 1.4 times 10 to the power of 69 bits per square meter. The author describes how this limit was found, the role of the quantum properties of black holes here, and to which regions it can be applied by means of a construction called 'light sheets'; he says the limit has been tested in many examples. This is a theoretical review, not a direct measurement. The side the author states plainly: the reason for this relation between geometry and information has not yet been explained. The idea called the 'holographic principle' suggests that its root lies in the degrees of freedom of a unified theory of spacetime and matter, but this is a proposal. What gives one pause is that it is the surface, not the volume, that decides.

• Bousso 2011 — Bousso, The multiverse interpretation of quantum mechanics. Phys. Rev. D 85, 045007
  In brief, what does it say?

  Bousso and his colleague argue that two separate 'many' ideas, the many worlds of quantum mechanics and the multiverse of cosmology, may in fact be the same thing. Their starting point is that decoherence, the modern counterpart of the 'collapse' of the wave function after measurement, depends on which degrees of freedom we count as the 'environment'. The authors propose that, taking as the basis the region a measurement event can causally reach (the causal diamond), the boundary of this region determines the environment in a natural way. They argue that to fully verify the probabilities of quantum predictions, it must be possible to perform an infinite number of identical experiments and every outcome must be definite. In diamonds with a finite surface, holographic entropy bounds make this impossible; so such a full verification could be possible only in certain regions of infinite extent ('hats'). This is a proposal, not a measured finding. The notable side is that it forces an everyday question such as the definiteness of a measurement to be thought about in the same frame as the geometry of the universe.

• Bouwmeester 1997 — Bouwmeester, Experimental quantum teleportation. Nature 390, 575
  In brief, what does it say?

  Is it possible to 'send' a quantum state somewhere else? Bouwmeester and colleagues re-created the polarization state of a photon in another photon without physically carrying that photon (quantum teleportation). For this they put two photons into an entangled state; the sender side made a special joint measurement and communicated the result through a classical channel; at the receiver the state was re-formed according to this information. The original state is destroyed in this process. This is an experiment. What gives one pause is that the whole information of the state reappears elsewhere through communication no faster than the speed of light. It is important why this does not mean 'matter is being teleported', that is, that only the state is transferred.

• Brunner 2014 — Brunner, Bell nonlocality. Rev. Mod. Phys. 86, 419
  In brief, what does it say?

  What Bell showed in 1964 was that the results of quantum theory cannot be explained by any local theory: the relation between the measurement results of two distant particles can be stronger than the relation that could be given by an order that determines each one's result in advance and is not affected from afar. This paper does not report a new experiment; it is a review. It tells how, over the last twenty years, especially thanks to quantum information science, this subject has become a field in its own right: how the relation is defined, how it is measured, with what tools it is studied. It notes that this 'non-locality' also has concrete uses in information processing. What is left to the reader: the part settled by experiment (that the inequalities are exceeded) and the interpretive part (what this means) must be kept apart. The review lists the mathematics of the theory and the various interpretive options.

• Budroni 2022 — Budroni, Kochen-Specker contextuality. Rev. Mod. Phys. 94, 045007
  In brief, what does it say?

  The question was: is quantum theory compatible with classical models that say the outcome of a measurement is independent of the other measurements that can be made together with it? The Kochen–Specker theorem gives the answer: it is not. If you try to assign, for every measurement of a system, a definite and context-independent value before the measurement, you fall into contradiction for some sets of measurements. This review article (a survey published in a peer-reviewed journal) gathers the theorem, its various forms, its experimental tests and its quantum information applications (for example, using contextuality as a resource in quantum computing). What gives one pause here is that the result does not say the measured property is not definite on its own before measurement; it says only that assigning a context-independent value is not possible; which of the two to give up depends on interpretation.

• Busch 2014 — Busch, Colloquium: Quantum root-mean-square error and measurement uncertainty relations. Rev. Mod. Phys. 86, 1261
  In brief, what does it say?

  The Heisenberg uncertainty principle is often explained as 'when we measure something, we touch it and disturb it.' This paper shows that this account confuses two separate things: the error of a measurement and the disturbance a measurement causes to the system. The authors examine two separate ways of measuring 'error.' In one, two quantities are measured together and the difference between them is looked at; in the other, the distributions that come out of two separate rounds of measurement are compared. The result calms a debate that has gone on for years: Heisenberg's first statement does not hold in the way we assumed in some cases, but the idea that 'there is a price between information and disturbance' survives in another, more careful form. So this is a work of theory and definition; it is not an experimental result. What gives one pause is that even the most repeated sentence of quantum physics turns into a matter of definition when read carefully.

• Casimir 1948 — Casimir, On the attraction between two perfectly conducting plates. Proc. K. Ned. Akad. Wet. B51, 793
  In brief, what does it say?

  Casimir calculated the attractive force between two perfectly conducting parallel plates from the zero-point energy of the vacuum.

  Casimir asks how two uncharged, perfectly conducting plates, parallel to each other, behave in empty space. In classical electrodynamics no force is expected between them. Casimir, however, counts the allowed modes of the electromagnetic field between the plates: the field’s boundary conditions at the conducting surfaces allow only certain wavelengths to fit between the plates. Outside there is no such restriction.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Chatrchyan 2012 — Chatrchyan, Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC. Phys. Lett. B 716, 30 (CMS Collaboration)
  In brief, what does it say?

  The CMS collaboration, independently and in the same period, reported a new boson around 125 GeV.

  CMS, the second experiment asking the same question as ATLAS, works with the Compact Muon Solenoid detector at the LHC. Its data are about 5.1 fb⁻¹ at 7 TeV and about 5.3 fb⁻¹ at 8 TeV.

• Chou 2010 — Chou, Optical Clocks and Relativity. Science 329, 1630
  In brief, what does it say?

  The theory of relativity says that the rate of clocks changes with motion and with gravity. For a long time one had to look for this at speeds near the speed of light or on satellites. Chou and colleagues, using extremely precise optical atomic clocks, showed that these effects can be measured on the ground, at everyday scales as well: a clock moving at a few meters per second runs slower, and a clock raised by about one meter also runs at a different rate. This is an experiment and it has turned out consistent with the predictions of relativity. The paper itself rests on measurement rather than interpretation. What gives one pause is that our intuition that 'time flows the same for everyone' is disturbed in a measurable way even by a height difference of one meter. The details we use here are only those in the publication abstract.

• Ciampini 2018 — Ciampini, Experimental signature of quantum Darwinism in photonic cluster states. Physical Review A 98, 020101
  In brief, what does it say?

  The experiment that first measured the traces of Quantum Darwinism in a small 'environment' model built with photons.

  The previous two works were theory; this paper is an experiment. The team sets up in the laboratory a system in which the 'environment' consists of a few photon pieces. The source they use is a hyperentangled source that entangles the polarization and the path information of photons at the same time; the entangled photons form a 'graph state' (cluster state), that is, a multipartite state connected like a network.

• Clauser 1969 — Clauser, Proposed experiment to test local hidden-variable theories. Phys. Rev. Lett. 23, 880
  In brief, what does it say?

  Einstein and his colleagues thought that quantum theory might be incomplete, that particles carry definite properties even before measurement, and that distant measurements do not influence each other instantly (the idea of local hidden variables). In 1964 Bell had shown that this view has a measurable limit. In this paper Clauser, Horne, Shimony and Holt put the same idea into a form that can be carried out in a real laboratory: two photons, in separate places, are passed through filters set to different directions, and the agreement of the results is counted. Any theory with local hidden variables has an upper bound for this agreement count; quantum mechanics, on the other hand, predicts a number that exceeds this bound. The paper is an experimental proposal and reports no result; the idea is that whichever one nature chooses can be seen by experiment. What gives one pause is that this question stops being philosophy and becomes measurable. The experiments later mostly supported the quantum side; what this means is still open to interpretation.

• Crooks 1999 — Crooks, Entropy production fluctuation theorem and the nonequilibrium work relation for free energy differences. Phys. Rev. E 60, 2721
  In brief, what does it say?

  Crooks derived a more general fluctuation theorem and briefly proved the Jarzynski equality as a consequence of it.

  Few exact relations are known for nonequilibrium processes. One is the fluctuation theorem, which places a condition on the distribution of entropy production; the other is Jarzynski's work equality. This paper brings the two together under the same roof.

• Davisson 1927 — Davisson, Diffraction of electrons by a crystal of nickel. Phys. Rev. 30, 705
  In brief, what does it say?

  Davisson and Germer scatter an electron beam from a nickel crystal and observe that the electrons are concentrated at certain angles.

  In the early 1920s de Broglie had suggested that a wavelength might exist for the particles of matter as well. Davisson and Germer's work is one of the first examples of an experimental test of this proposal. The aim of the work is to measure whether, when an electron beam is scattered from a crystal surface, the electrons come back in all directions or only in certain directions.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• de Broglie 1927 — de Broglie, La mécanique ondulatoire et la structure atomique de la matière et du rayonnement. J. Phys. Radium 8, 225-241
  In brief, what does it say?

  De Broglie proposed a picture in which the particle is guided by a wave; he also presented the same idea at the 1927 Solvay Congress (J. Phys. Radium 8, 225).

  De Broglie's idea of matter waves, proposed in 1924, had led to Schrödinger's wave equation. The 1927 paper takes up the relation of 'wave mechanics to atomic structure and radiation' and proposes a picture that holds wave and particle together.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Degen 2017 — Degen, Quantum sensing. Reviews of Modern Physics 89, 035002
  In brief, what does it say?

  A Reviews of Modern Physics review that introduces the basic principles, methods and concepts of quantum sensing from an experimentalist's point of view.

  The review defines quantum sensing as the use of a quantum system, of quantum properties or of quantum phenomena to measure a physical quantity. Historical examples mentioned include superconducting quantum interference devices (SQUIDs), magnetometers based on atomic vapors, and atomic clocks.

• Deutsch 1985 — Deutsch, Quantum theory, the Church–Turing principle and the universal quantum computer. Proceedings of the Royal Society A 400, 97–117
  In brief, what does it say?

  A paper that formulates the Church-Turing hypothesis as a physical principle and introduces the notions of the quantum Turing machine and the universal quantum computer.

  The paper's starting point is this: the Church-Turing hypothesis says that everything that is "computable" can be computed by a Turing machine. Deutsch argues that there is a tacit physical claim underlying this hypothesis, and writes it out explicitly as a principle: "every finitely realizable physical system can be perfectly simulated by a universal model computing machine operating by finite means."

• Dieks 1982 — Dieks, Communication by EPR devices. Phys. Lett. A 92, 271
  In brief, what does it say?

  Dieks's short paper starts from this question: can entangled particle pairs serve faster-than-light communication? A proposal in this direction hoped to read out by duplicating the state at one end of the pair. Dieks showed that a perfect copy of an unknown state is incompatible with the rules of quantum mechanics; so the proposal does not work. The result coincides with the result Wootters and Zurek published in the same year. This is one of the reasons entanglement cannot be turned into faster-than-light communication. We do not have the paper's own summary; the generally known framework is given here.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Dirac 1926 — Dirac, On the theory of quantum mechanics. Proceedings of the Royal Society A 112, 661-677
  In brief, what does it say?

  Dirac derived the same result in the matrix language of quantum mechanics and with antisymmetric wave functions.

  In 1926 Dirac takes Heisenberg's matrix mechanics as a general framework and asks how a system made of identical particles is to be described. When we exchange two identical particles the physical state must not change; this requires that the wave function either remain the same or change sign under the exchange.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Dirac 1928 — Dirac, The quantum theory of the electron. Proceedings of the Royal Society A 117, 610-624
  In brief, what does it say?

  Dirac set up a wave equation for the point electron that conforms both to relativity and to the general transformation theory of quantum mechanics.

  The paper begins with a shortcoming. The quantum mechanics of the day, with the electron thought of as a point charge, could not fully describe atomic spectra: the number of observed stable states was twice what the theory gave. Goudsmit and Uhlenbeck had met this doubling by giving the electron a model spinning about its own axis and having a magnetic moment; Pauli had placed this in quantum mechanics. Dirac asks why this model is the one nature chose, and thinks the shortcoming may lie elsewhere: the earlier theories did not conform to relativity.

• Donadi 2021 — Donadi, Underground test of gravity-related wave function collapse. Nature Phys. 17, 74-78 (çevrimiçi 2020)
• Dressel 2014 — Dressel, Colloquium: Understanding quantum weak values: Basics and applications. Reviews of Modern Physics 86, 307-316
  In brief, what does it say?

  A review that explains what weak values are and what they are good for, through measurable quantities.

  The paper is a review written about 25 years after the first definition of the weak value, and it aims to clear up historical confusion. The authors' approach is to introduce the weak value through measurable quantities and through how it is determined in the laboratory.

• Einstein 1905 — Einstein, Zur Elektrodynamik bewegter Körper. Ann. Phys. 322, 891
  In brief, what does it say?

  If the speed of light is found to be the same for everyone, how do the things that two moving observers call 'at the same time' fit together? In 1905 Einstein set out from two principles: the laws of physics look the same to all observers moving uniformly; the speed of light in a vacuum does not depend on the motion of the source. From this he concluded that simultaneity is not absolute and that length and duration change according to the observer. This is a theory, and it has been tested by many experiments in the following century. What gives one pause is that the 'now' is not single for everyone. What this means is a question of interpretation about the nature of time. (For the details of the electrodynamics part, the article should be consulted.)

• Einstein 1905 — Einstein, Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt. Ann. Phys. 322, 132
  In brief, what does it say?

  Einstein sets out to explain the photoelectric effect and related phenomena by supposing that light in vacuum behaves as if made up of shares of energy of a definite size.

  Maxwell's theory describes light as a continuous wave. Matter, on the other hand, consists of separate atoms and electrons. At the start of the paper Einstein recalls the gap between these two descriptions and says that the wave description alone may fall short in phenomena concerning the production and transformation of light. The word 'heuristic' in the title indicates that what is put forward is not a definitive theory but a viewpoint that works.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Einstein 1925 — Einstein, Quantentheorie des einatomigen idealen Gases. Zweite Abhandlung. Sitzungsberichte der Preussischen Akademie der Wissenschaften (1925); yeniden basım: Albert Einstein: Akademie-Vorträge, Wiley-VCH 2005, s. 245-257
  In brief, what does it say?

  Einstein applied Bose's counting to a massive ideal gas and predicted that at low temperature the particles could gather in the lowest level.

  Einstein realized that the counting Bose used for light was not peculiar to light and could also be applied to a massive gas. In the works he presented at the Prussian Academy of Sciences in 1924-25, he rederived the properties of an ideal gas by counting the particles as indistinguishable. These works were published in German; the Crossref record we have is of the reprint of this paper by Wiley-VCH in 2005.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Einstein 1935 — Einstein, Can quantum-mechanical description of physical reality be considered complete?. Phys. Rev. 47, 777
  In brief, what does it say?

  Einstein, Podolsky and Rosen put this question to quantum theory: is the description the theory gives for a system (the wave function) a complete description of reality? They set a criterion: if we can predict something with certainty without touching it at all, then there is an element of reality that exists in that thing. Then they considered two systems that had interacted with each other. A measurement made on one lets us state with certainty the value of the other, far away. In quantum theory, however, some quantities cannot be definite together. From this the authors concluded: either the theory is incomplete or these quantities do not have reality at the same time; they chose the first, that is, they said the wave function is not a complete description. This is a thought experiment, not a measurement; and years later Bell's theorem and the experiments made the 'local reality' assumption on which this argument rests testable. What gives one pause is that the question of what a system 'is' before it is measured was left open from the very beginning.

• Englert 1964 — Englert, Broken Symmetry and the Mass of Gauge Vector Mesons. Phys. Rev. Lett. 13, 321
  In brief, what does it say?

  Englert and Brout solved the same problem independently, with a model in which gauge fields acquire mass through spontaneous symmetry breaking.

  This paper was published a few weeks before Higgs’s and asks the same question: when a gauge symmetry is spontaneously broken, can vector mesons acquire mass?

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Englert 1996 — Englert, Fringe visibility and which-way information: an inequality. Phys. Rev. Lett. 77, 2154
  In brief, what does it say?

  In the double-slit experiment, light or a particle can pass through two paths; an interference pattern (light and dark stripes) appears on the screen. The question is this: what happens to the pattern if we try to find out which path the particle took? Englert described this with an inequality, that is, an upper bound: how distinct the stripes are (the clarity of the pattern) and how much of the 'which path' information can be obtained can together reach at most a certain limit. The clearer the pattern, the less path information; as path information increases, the pattern is erased to that degree. This ties the 'wave-particle duality,' formerly stated in words, to a measure. The side that draws attention: the derivation does not rest on the Heisenberg uncertainty principle; so this limit is not a restatement of that principle. This is a theoretical result; experiments have later confirmed this kind of trade-off, but the question 'what is this an expression of' remains open as a matter of interpretation.

• Everett 1957 — Everett, Relative state formulation of quantum mechanics. Rev. Mod. Phys. 29, 454
  In brief, what does it say?

  Quantum theory says that a system develops over time according to a smooth and definite equation; but at the moment of measurement it adds a separate rule called 'collapse'. Everett does not like this duality. His proposal: let us drop the collapse rule and describe everything, the observer included, with the same equation. Then after the measurement the observer carries not a single result but a separate state associated with each result; no result is 'more real' than the others, each gains meaning relative to the other ('relative state'). This is not an experiment but a proposal for a theory; 'many worlds' is not Everett's own language, this name was given later by others. What gives one pause is that it is an interpretation that cannot be told apart by ordinary experiment, and that what the word 'probability' corresponds to here is still debated. So neither certain nor refuted: one of the interpretations.

• Facchi 2008 — Facchi, Quantum Zeno dynamics: mathematical and physical aspects. J. Phys. A 41, 493001
  In brief, what does it say?

  If something is observed continuously, does it not change? Quantum theory gives a strange answer: if you measure very often whether a system is still in its initial state, its passage to another state is prevented. This is called the Zeno effect (with reference to Zeno's paradox 'the flying arrow is always at rest somewhere'). This review examines a broader case: if the measurement asks not about a single state of the system but about a set of states, the system does not freeze; it keeps moving within that set and cannot get out of it. For example, if it is asked continuously whether a particle is in a certain region, the particle behaves as if it were in a box with walls in that region. The article treats the mathematics of this and its applications that give the same result by different routes. The effect itself has been shown in theory and in experiments; 'why measurement does this' is a matter of interpretation.

• Fein 2019 — Fein, Quantum superposition of molecules beyond 25 kDa. Nat. Phys. 15, 1242
  In brief, what does it say?

  The question was: is wave-like behavior, that is, superposition and interference, peculiar only to small objects such as electrons and atoms, or does it extend to large molecules as well? The Vienna group's work reports that, with interference experiments on large molecules of many atoms, it demonstrated quantum superposition in a setup where the molecule's mass goes beyond 25,000 atomic mass units (25 kDa). The title says this; we do not give the details and numbers here since we do not have our own summary. The importance of the finding is the demonstration that quantum behavior does not get stuck by itself at a size limit. We do not have the paper's own summary; the generally known framework is given here.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Fermi 1926 — Fermi, On the Quantization of the Monoatomic Ideal Gas. Fermi, Rendiconti della Accademia dei Lincei (1926), 'Sulla quantizzazione del gas perfetto monoatomico'; İngilizce çeviri: A. Zannoni, arXiv:cond-mat/9912229
  In brief, what does it say?

  Fermi treated an ideal gas as a quantum gas by using the Pauli principle.

  Fermi's 1926 paper was published in Italian; it was translated into English by Zannoni in 1999 and placed on arXiv, and the account below rests on the abstract of that translation. According to the translation's abstract, in this paper Fermi for the first time sets out the statistical law obeyed by most of the particles in nature.

• Feynman 1949 — Feynman, Space-Time Approach to Quantum Electrodynamics. Physical Review 76, 769-789
  In brief, what does it say?

  Feynman rebuilt QED over spacetime, in the language of the paths particles follow, and gave the rules that simplify the calculations.

  Feynman's paper is another way of presenting QED. In quantum mechanics the probability of a particle going from one point to another is found by adding up the contributions of all possible paths; Feynman applies this to the electromagnetic interaction. Electrons and photons are treated as objects that send and receive one another, like light, in spacetime, and each interaction corresponds by a rule to a definite contribution.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Feynman 1965 — Feynman, Richard Feynman. The Character of Physical Law, bölüm 6, s. 129: "I think I can safely say that nobody understands quantum mechanics." (Wikiquote kaydı; kitaba ikinci el)
  In brief, what does it say?

  This is not an experimental or theoretical paper but a sentence from Feynman's book 'The Character of Physical Law': 'I think I can safely say that nobody understands quantum mechanics.' The book is based on the lectures he gave at Cornell in 1964; we know the sentence not from the book but from the Wikiquote entry, a collection of quotations, so our source is second-hand. Feynman is not saying here that the calculations do not work; quantum theory gives some of the most precise predictions known. What he says is that we have no story that fits our everyday intuition about why these calculations work as they do. This is not a physical finding but an honest statement by a physicist about his own field. Why it matters: even one of the masters of the theory has not said 'I understand'. In this work this sentence stands not as a ground for comparing physics with another door, but only as a statement cited in his own words.

• Feynman 1982 — Feynman, Simulating physics with computers. Int. J. Theor. Phys. 21, 467
  In brief, what does it say?

  Feynman argued that classical computers could not efficiently simulate quantum systems and proposed a computer made of quantum elements.

  Feynman’s question is what it means to simulate physics with a computer and what limits it is subject to. He argues that this is possible for classical and probabilistic systems; the difficulty begins with quantum systems.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Fuchs 2014 — Fuchs, An introduction to QBism with an application to the locality of quantum mechanics. Am. J. Phys. 82, 749
  In brief, what does it say?

  In quantum mechanics, is a 'wave function', a quantum state, directly a state of the world, or is it an expression of the knowledge and expectation of the person who uses it? The interpretation called QBism says the second: a quantum state is an agent's (an observer's) personal belief about probabilities, and a measurement is that agent's gaining of experience. In this paper Fuchs introduces the interpretation and shows an application: he explains why the idea of 'quantum nonlocality', the mysterious connection between two distant measurements, is not a problem in this interpretation. This is a defense of an interpretation, not an experimental result; other interpretations give the same predictions with the same mathematics, and QBism's claims (that it removes paradoxes and pseudo-problems) are disputed. What gives one pause is that it takes seriously the question of where the line between 'reality' and 'knowledge' runs in quantum theory.

• Gamow 1928 — Gamow, Zur Quantentheorie des Atomkernes. Z. Phys. 51, 204-212
  In brief, what does it say?

  The paper that calculates the emission of an alpha particle from an atomic nucleus as the tunneling of the particle through the nucleus’s potential barrier.

  In radioactive alpha decay, half-lives vary over a very wide range: from fractions of a second to billions of years. There was a regular relation (the Geiger–Nuttall relation) between the energy of the emitted alpha particle and the half-life, and it could not be explained. Moreover, the alpha particle’s energy was far below the top of the repulsive electric barrier outside the nucleus; classically the particle would not be expected to get out.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Gerlach 1922 — Gerlach, Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld. Z. Phys. 9, 349
  In brief, what does it say?

  In which direction can an atom be turned when it passes through a magnetic field? According to the old physics every direction was possible; that is, the atoms should have spread on the screen like a smear. Some theorists, however, claimed that directions could only have certain values. Gerlach and Stern passed silver atoms through a non-uniform magnetic field and looked at what happened on the screen. Instead of the expected smear, two separate traces appeared: the atoms were as if splitting into only two directions. This is the 'directional quantization' shown by experiment. The real cause of the results (the electron's spin) was not known at that time and was understood a few years later. What gives one pause is that when a measurement is made, discrete rather than continuous results come out. The experimental result is firm; its first interpretation differed from today's explanation. The author's name appears in the source list as Gerlach, but the experiment was done together with Stern.

• Ghirardi 1986 — Ghirardi, Unified dynamics for microscopic and macroscopic systems. Phys. Rev. D 34, 470
  In brief, what does it say?

  Quantum theory describes a particle as if it could be in several places at once, but in daily life we do not see a table in two places. What happens at the moment of measurement is the theory's most debated question. Ghirardi, Rimini and Weber proposed changing the equation of the theory: let every particle have a chance, at very rare and random times, of spontaneously 'localizing' in one place. For a single particle this is hardly noticed; but for a large body with many particles these localizations multiply one another quickly, so that the body practically stays in a single place and obeys classical physics. The paper sets this up as a model: the microscopic world and large bodies are described with a single dynamics, and no separate rule is needed for measurement. This is a theoretical proposal; it has not been confirmed by experiment, and works that try to narrow its limits through experiments continue. I am not giving detail on the choice of parameters; it is in the paper.

• Giovannetti 2004 — Giovannetti, Quantum-Enhanced Measurements: Beating the Standard Quantum Limit. Science 306, 1330-1336
  In brief, what does it say?

  A Science review that explains how quantum tricks (squeezing, entanglement) get past the standard quantum limit.

  The paper begins by noting that quantum mechanics places a limit on measurement precision through the Heisenberg uncertainty principle. Conventional measurement techniques often cannot reach these limits.

• Giovannetti 2006 — Giovannetti, Quantum Metrology. Physical Review Letters 96, 010401
  In brief, what does it say?

  The paper that sets up quantum metrology in a general framework and proves that the square-root gain is the best possible.

  The authors present a general framework that covers most cases in which quantum effects increase precision in the estimation of a parameter. Quantum metrology is the subject of this framework.

• Giovannetti 2011 — Giovannetti, Advances in quantum metrology. Nature Photonics 5, 222-229
  In brief, what does it say?

  A review of progress in quantum measurement science (metrology): it describes how entangled and squeezed states can carry measurement precision beyond the classical limit.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Giovannetti 2015 — Giovannetti, Quantum time. Phys. Rev. D 92, 045033
  In brief, what does it say?

  The question: how can time be described in quantum theory? In the theory there is no clock that supplies a 'now' from outside for the things being measured. The paper starts from the conditional probability idea put forward earlier by Page and Wootters: time is described by looking at the reading of a clock that is correlated with the rest of the system. The authors give a version of this proposal that withstands the criticisms once leveled at it. What they show is that the model can reproduce the correct statistics of successive measurements made at different moments. This is a theoretical work; not an experiment. What gives one pause is that time can be set up as something arising from relation, instead of being an external parameter.

• Giustina 2015 — Giustina, Significant-loophole-free test of Bell's theorem with entangled photons. Phys. Rev. Lett. 115, 250401
  In brief, what does it say?

  The question: can two particles be bound so specially that when one is measured, the result of the other cannot be explained by something 'predetermined', even though no signal passes between them? The Bell inequality makes this question testable: every account based on local and measurement-independent properties cannot exceed a certain bound, while quantum theory does exceed it. The team produced entangled photon pairs, chose measurement settings quickly and at random at two distant stations, and counted with very sensitive detectors. In old experiments 'loopholes' remained, for example that some photons were never caught; here all the most important holes were closed at the same time. Result: the inequality was exceeded by a margin that is extremely hard to explain by chance. This is an experimental finding. What the finding shows is the collapse of the picture of local and predetermined properties; but what is to be put in its place, that is, the interpretation, is disputed.

• Gorini 1976 — Gorini, Completely positive dynamical semigroups of N-level systems. Journal of Mathematical Physics 17, 821-825
  In brief, what does it say?

  The paper in which a separate team reached the same result for N-level systems.

  Gorini, Kossakowski and Sudarshan examine the completely positive dynamical semigroups of N-level (finite-dimensional) quantum systems. The work was published in the same year as Lindblad's paper, independently of it.

• Greenberger 1989 — Greenberger, Going Beyond Bell's Theorem. M. Kafatos (ed.), Bell's Theorem, Quantum Theory and Conceptions of the Universe, Kluwer, s. 69-72
  In brief, what does it say?

  Greenberger, Horne and Zeilinger showed that in a four-particle quantum state the assumptions of local reality contradict definite results ('Going Beyond Bell's Theorem').

  The paper's starting point is a difference between Bell's theorem and EPR's reality criterion. EPR counted cases in which we can predict the value of a quantity with certainty as 'elements of reality'. Bell's theorem, on the other hand, does not contain this case of certain prediction; it is a statistical inequality. The authors show that if a more complex model is used, locally determinate models will not work even in cases where certain prediction is possible.

• Greiner 2002 — Greiner, Quantum phase transition from a superfluid to a Mott insulator in a gas of ultracold atoms. Nature 415, 39
  In brief, what does it say?

  At absolute zero temperature thermal fluctuations freeze; only quantum fluctuations remain. The question: can these microscopic fluctuations carry the whole of a many-particle system into another phase? In the experiment, a Bose-Einstein condensate made of repelling atoms was placed in a three-dimensional lattice built from light. As the depth of the lattice was increased, the system passed from a superfluid to a phase called a 'Mott insulator'. In the superfluid phase each atom is spread over the whole lattice and there is long-range phase coherence. In the insulator phase, by contrast, a whole number of atoms sits settled in each lattice site, there is no phase coherence across the lattice, and there is a gap in the excitation spectrum. The team was able to make the transition between these two phases in both directions. This is an experimental finding; what gives one pause is that a single atom being spread over the whole and its being settled in one site can be seen as two phases of the same system.

• Grover 1996 — Grover, A fast quantum mechanical algorithm for database search. Proc. 28th Annual ACM Symposium on Theory of Computing (STOC '96), 212–219
  In brief, what does it say?

  A paper giving a quantum algorithm with O(√N) steps for search in an unsorted database.

  The author explains the problem with a telephone directory example: N names are arranged at random and one wants to find the number of a given person. For a fifty percent probability of success, a classical algorithm (deterministic or probabilistic) must check at least N/2 names.

• Gurney 1928 — Gurney, Wave Mechanics and Radioactive Disintegration. Nature 122, 439
• Hafele 1972 — Hafele, Around-the-World Atomic Clocks: Predicted Relativistic Time Gains. Science 177, 166
  In brief, what does it say?

  General relativity says that time runs slowly where gravity is strong, and special relativity says that a fast-moving clock falls behind. The question: can this knowledge be tested with real clocks on a laboratory scale? In October 1971 Hafele and Keating flew four cesium atomic clocks around the world on commercial airplanes, once eastward and once westward. This paper calculates from the flight paths what the theory predicts: the clocks going east should fall slightly behind the reference clocks on the ground, and those going west should go ahead. The predictions are on the nanosecond scale, that is, one billionth of a second. This paper is only the prediction; the observed results are given in a separate accompanying paper. What gives one pause is that time itself runs differently according to where and at what speed the clock is. This is not an interpretation but a measured fact.

• Hafele 1972 — Hafele, Around-the-World Atomic Clocks: Observed Relativistic Time Gains. Science 177, 168
  In brief, what does it say?

  The theory of relativity says that a clock that moves and a clock that sits at height will run at a different rate from a clock on the ground. The question: can this be measured at airliner speeds and with real clocks? Hafele and Keating carried four cesium atomic clocks around the world on commercial flights in October 1971, once eastward and once westward, and compared them with the Naval Observatory's time scale. On the eastward trip the flown clocks lost about 59 nanoseconds (plus or minus 10), and on the westward trip they gained about 273 nanoseconds (plus or minus 7). This two-directional difference agreed well with the prediction of the theory of relativity. Here the measurement is the experiment; which part of which effect comes from speed and which from altitude is a theoretical calculation. What gives one pause is that it has been shown with tangible clocks that time is not a single flow that is the same for everyone.

• Harrigan 2010 — Harrigan, Einstein, incompleteness, and the epistemic view of quantum states. Found. Phys. 40, 125
  In brief, what does it say?

  The question: does the quantum state represent reality itself, or our knowledge about reality? The authors make this distinction precise and place hidden-variable models of quantum theory into a new classification. They show that in models where the quantum state is counted as 'a real thing', nonlocality can be demonstrated with an easier argument than Bell's theorem. This is a work of theory and history, not an experiment. They point out that the same argument is also found in Einstein's incompleteness claim going back to 1935. From this they conclude that Einstein was seeking a completion in which quantum states represent only information; this is their own claim, a historical interpretation. What makes the source valuable is that it describes the distinction 'is the state information or reality' explicitly.

• Hawking 1974 — Hawking, Black hole explosions?. Nature 248, 30
  In brief, what does it say?

  Hawking reported that, by looking at the behavior of quantum fields around a black hole, he found that the black hole emits thermal radiation.

  The title of the paper ends with a question: 'Black hole explosions?'. It is a short Nature article and the first announcement of the result; the detailed calculation was published a year later in a separate paper.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Hawking 1976 — Hawking, Breakdown of predictability in gravitational collapse. Phys. Rev. D 14, 2460
  In brief, what does it say?

  Hawking argued that in the process of black hole evaporation a pure quantum state would turn into a mixed state and predictability would break down.

  The paper questions the consequence of the 1974 radiation calculation. If a black hole forms and evaporates completely, the detailed quantum state used at the start becomes unreadable from the final state.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Heisenberg 1927 — Heisenberg, Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik. Z. Phys. 43, 172
  In brief, what does it say?

  In this 1927 paper Heisenberg derived for the first time the uncertainty relation, which states that paired quantities such as position and momentum cannot be determined at the same time with arbitrary precision. He explained it with a thought experiment: if we try to measure the position of a single electron with a gamma-ray microscope, the light strikes the electron and changes its momentum in an uncontrollable way. The more sharply we measure the position, the more the momentum is disturbed. Whether this limit is merely a defect of measuring instruments or the structure of nature itself has been put differently in later years depending on the interpretation; the relation itself is a firm part of the theory. We do not have the paper's own summary; the generally known framework is given here.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Hensen 2015 — Hensen, Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres. Nature 526, 682
  In brief, what does it say?

  Einstein and his colleagues had asked how the measurement results of two distant particles can agree so well; Bell, in 1964, had calculated which limit an ordinary explanation called 'local hidden variables' cannot exceed. In this experiment a team in Delft entangles the electron spins of two defects in diamonds 1.3 kilometres apart and measures them. In all earlier experiments there were gaps ('loopholes') that allowed the result to be rescued by an ordinary explanation; here two of them are closed together. The result is a violation of the Bell bound: correlations that cannot be given by an ordinary, local calculation. This is an experimental result and it is firm. How it is to be interpreted (action at a distance, or giving up something about reality) is an open debate; the experiment says only 'there can be no local hidden variables', it does not say that there is communication.

• Higgs 1964 — Higgs, Broken Symmetries and the Masses of Gauge Bosons. Phys. Rev. Lett. 13, 508
  In brief, what does it say?

  Higgs showed how gauge bosons can acquire mass through symmetry breaking.

  Gauge theories derive forces from a symmetry principle; but in these theories the gauge bosons come out massless. The weak force has a short range, so its carriers must be heavy. Another obstacle is the Goldstone theorem: when a symmetry is spontaneously broken, massless scalar particles were expected, and these were not observed. Higgs’s question is whether this obstacle can be overcome.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Hong 1987 — Hong, Measurement of subpicosecond time intervals between two photons by interference. Phys. Rev. Lett. 59, 2044
  In brief, what does it say?

  What happens if two photons arrive at a half-silvered mirror from the two sides at the same time? By an ordinary calculation four possible outcomes are considered. According to quantum theory, however, if the photons are completely identical, the cases in which the two come out from separate exits cancel each other; the two always come out together from the same exit. Hong, Ou and Mandel measured this: when the small difference between the arrival times of the two photons is changed, the number of simultaneous signals at two separate detectors collapses when the photons coincide exactly. The width of this collapse gave how 'long' the photons are in time, about a hundred femtoseconds (on the order of one ten-quadrillionth of a second). This is an experimental result and it is very firm. What gives one pause is that two separate particles, when they cannot be told apart from each other, are obliged to 'behave together'.

• Horodecki 1996 — Horodecki, Separability of mixed states: necessary and sufficient conditions. Physics Letters A 223, 1-8
  In brief, what does it say?

  The paper that gives the necessary and sufficient conditions for telling whether mixed states are entangled (separability).

  The paper asks whether mixed quantum states are separable. A separable state is one that carries only classical correlation between the two parties. Non-entangled states are exactly these.

• Horodecki 1998 — Horodecki, Mixed-State Entanglement and Distillation: Is there a "Bound" Entanglement in Nature?. Physical Review Letters 80, 5239-5242
  In brief, what does it say?

  The paper that shows there are two separate kinds of entanglement: distillable (usable) and nondistillable ('bound') entanglement.

  The paper asks this question: can every entangled mixed state be converted into a pure pair (a singlet) that can be used in tasks such as teleportation? This conversion is called distillation; it is done with local operations and classical communication alone.

• Horodecki 2009 — Horodecki, Quantum entanglement. Reviews of Modern Physics 81, 865-942
  In brief, what does it say?

  The Reviews of Modern Physics review that brings together the characterization, detection, distillation and quantification of entanglement.

  The review gives a compact account of the basic aspects of entanglement: characterization, detection, distillation and quantification. The authors discuss the various faces of entanglement through Bell inequalities, entropic inequalities, entanglement witnesses and quantum cryptography, and show the connections among them.

• Itano 1990 — Itano, Quantum Zeno effect. Phys. Rev. A 41, 2295
  In brief, what does it say?

  If we 'look' at something very often, we fail to let it change: the quantum Zeno effect. Theory says that the transition of a system from one state to another can be prevented if it is measured frequently. Itano and colleagues held beryllium ions in a trap, cooled them with a laser, and while driving the transition between two energy levels with a radio wave, 'measured' it frequently with short light pulses. The more frequent the measurement, the more the transition was suppressed; the result agreed well with the calculations. This is an experiment. What gives one pause is that measurement is not only taking information but affects the course of the system; whether this is 'the collapse of the wave function' or something else is a matter of interpretation.

• Jacques 2007 — Jacques, Experimental realization of Wheeler's delayed-choice gedanken experiment. Science 315, 966
  In brief, what does it say?

  In a setup that a photon enters through two paths, it can appear both as a wave (through both paths together, interference) and as a particle (which path it took is known); which one we will see depends on the choice of setup. John Wheeler had asked 'what if the choice is made after the photon has entered the setup?' Jacques and colleagues carried this out with single photons: the choice of open or closed configuration was made by a random number generator and took place at a moment so separated from the photon's entry that no communication at the speed of light was possible. The result agreed with the prediction of quantum theory: even a late choice determined the outcome. That is the experiment. What gives one pause is that the photon seems not to have decided 'what it is' before the entry; its meaning varies from interpretation to interpretation, and no one has to say 'we changed the past'.

• Jaffe 2005 — Jaffe, Casimir effect and the quantum vacuum. Phys. Rev. D 72, 021301
  In brief, what does it say?

  It is said that even in what we call empty space, quantum fields have a lowest energy called 'zero-point energy.' In the Casimir effect, two metal plates very close together attract each other in a vacuum, and this attraction has been measured. It is often said: this attraction is proof that zero-point energy is real. Jaffe objects to this view. He shows that the Casimir force can also be calculated, without any appeal to zero-point energy, as a relativistic and quantum interaction of the charges and currents in the plates. Moreover, the known result appears not to depend on the fine-structure constant, the measure of the strength of the interaction, but in fact if that constant goes to zero the force goes to zero as well. This is a theoretical dispute; the measured force is not disputed, what is disputed is what it 'proves.' Why it gives one pause: a famous experiment is not condemned to a single interpretation, as it is often told.

• Jarzynski 1997 — Jarzynski, Nonequilibrium Equality for Free Energy Differences. Phys. Rev. Lett. 78, 2690
  In brief, what does it say?

  Jarzynski derived an exact equality that gives the equilibrium free-energy difference from the distribution of work done in a rapidly driven system.

  The free-energy difference gives the least work needed to take a system slowly (in equilibrium) between two configurations. Real experiments, however, are usually done quickly, and each repetition gives a different work result: work is a random variable.

• Johansson 2012 — Johansson, QuTiP: An open-source Python framework for the dynamics of open quantum systems. Comput. Phys. Commun. 183, 1760
  In brief, what does it say?

  The question was: is it possible to calculate, with a tool anyone can use, how quantum systems interacting with their environment (open systems) change over time? The authors presented a framework called QuTiP, written in Python, object-oriented and open source. The user defines states and operators with a quantum object class; they can set up any system they want, including time-dependent Hamiltonians, and give it to different equation-of-motion solvers. The paper describes the structure of the tool and examples of use. The four small calculations on the site's "Compute and See" page were made with this framework. Since it is a software paper, it does not report a physical finding; it only documents how the calculation is done.

• Joos 1985 — Joos, The emergence of classical properties through interaction with the environment. Z. Phys. B 59, 223
  In brief, what does it say?

  The question was this: if according to quantum theory the smallest things can carry two states at once, why does a speck of dust on a table, or a cat, never look like that? Joos and Zeh calculated that a body inevitably 'talks' with its surroundings, that is, with light, air and temperature. Every photon or molecule that comes from the surroundings and strikes the body carries away a trace of where the body is. Because of this trace, the interference between the body's states of being 'here and there together' becomes invisible in a very short time. This is called decoherence: the state in which two possibilities enter the calculation together is dispersed into the environment and lost. This is a theoretical calculation and was later supported by experiments. But why a single result is realized, that is, why the measurement gives 'only one', this calculation does not explain by itself; this is still a debate among interpretations.

• Jönsson 1961 — Jönsson, Elektroneninterferenzen an mehreren künstlich hergestellten Feinspalten. Z. Phys. 161, 454
  In brief, what does it say?

  Electrons are thought of as particles; but do they also behave like waves, that is, when they pass through two narrow slits do they give an interference pattern like water waves? In 1961 Claus Jönsson sent a beam of electrons through very thin, artificially made slits and saw light and dark stripes characteristic of waves on the screen opposite. Electron diffraction had been shown earlier with crystals; here, the very slit arrangement known from light was built with electrons. This is an experiment; the interpretation comes later. What gives one pause is this: each electron arrives at the screen as a single point, but when many electrons accumulate, the wave pattern emerges. What it means for this pattern to count as 'passing through both slits together' is still a disputed matter of interpretation. (For detailed numbers, the article should be consulted.)

• Kapitza 1938 — Kapitza, Viscosity of Liquid Helium below the λ-Point. Nature 141, 74
  In brief, what does it say?

  Kapitza measured the viscosity of liquid helium below the λ-point and reported that he found it very small; the name ‘superfluidity’ for the phenomenon is associated with this note.

  Helium-4 passes, below about 2.17 K (the λ-point), into a separate state called liquid helium II. Below this temperature it was known to behave like a liquid that conducts heat unusually well and seeps from the walls of vessels through thin cracks. Kapitza’s question was simple: what, in fact, is the viscosity of this liquid, that is, its internal resistance to flow?

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Kaufman 2016 — Kaufman, Quantum thermalization through entanglement in an isolated many-body system. Science 353, 794
  In brief, what does it say?

  That a hot cup of tea comes to the temperature of its surroundings over time is something we know, but how can a perfectly isolated quantum system, if it starts in a completely definite (pure) state, behave as if 'heated'? Kaufman and colleagues examined, with a quantum microscope, six rubidium atoms placed in the sites of a lattice set up with light. When they suddenly switched on tunneling between the atoms, the whole of the six atoms stayed in a pure state; but when small parts of two or three atoms were looked at, those behaved in accordance with a thermal distribution. The source of the effect is entanglement: the parts are tied to each other in such a way that, even if the whole is definite, a part by itself looks uncertain. This is an experimental result. Why it gives one pause: the parts of a fully known whole look random. How we shall tie this to the foundation of statistical physics is separately debated.

• Kennard 1927 — Kennard, Zur Quantenmechanik einfacher Bewegungstypen. Z. Phys. 44, 326
  In brief, what does it say?

  The more precisely we know a particle's position, the more uncertain we find its velocity (momentum), and vice versa: the uncertainty principle. Heisenberg had put this forward in 1927 by means of a thought experiment; in the same year Earle Kennard derived the principle by mathematics from within the theory. The result is that the product of the position uncertainty and the momentum uncertainty cannot fall below a certain lower bound (half the reduced Planck constant). This is a result of the theory: it arises not from the crudeness of our measuring instrument but from the structure of the state itself. What gives one pause is this: the bound can be read as a lack of information, or as a property belonging to the state itself; this distinction is a matter of interpretation.

• Kim 2000 — Kim, Delayed choice quantum eraser. Phys. Rev. Lett. 84, 1
  In brief, what does it say?

  Light passing through two slits forms an interference pattern; if we try to find out which slit it passed through, the pattern is lost. The question was this: what happens if this 'which-path' information is erased afterwards, even after the photon has struck the screen? The team used entangled photon pairs: one (the 'signal') is recorded at the screen, while its twin later reaches a splitter and there either reveals the path information or erases it. When the records were brought together afterwards, the signal photons corresponding to twins whose path information was erased showed an interference pattern among themselves, those whose information was revealed did not. The important point: looking at all the signal photons there is no pattern; the pattern emerges only when they are sorted according to the twin's result. For this reason this experiment does not change the past; saying so is a common misunderstanding. The finding is an experimental result; 'influence on the past' is an interpretation and is not certain as to the real state of affairs.

• Kimble 2008 — Kimble, The quantum internet. Nature 453, 1023-1030
  In brief, what does it say?

  A short vision paper in Nature that describes the opportunities and challenges of quantum networks.

  Kimble says that quantum networks present a common set of opportunities and challenges for quantum computation, communication and metrology. For a network made up of many nodes and channels to be realized, new scientific capabilities are needed in producing and characterizing quantum coherence and entanglement.

• Kirchmair 2009 — Kirchmair, State-independent experimental test of quantum contextuality. Nature 460, 494
  In brief, what does it say?

  The question: if we assume that the measured properties have definite values before measurement, do we contradict quantum predictions? Quantum contextuality is the fact that the result of measuring a property can change according to which other measurement is made alongside it; that is, the result is not the property's possession by itself. In this work the researchers measured, on trapped ions, an inequality that tests this idea, and it is important that it is 'state-independent': the violation of the inequality appears not for a special state in which the system is prepared but whatever state it is prepared in. The result showed that the picture that assumes predetermined and context-independent values does not agree with experiment. This is an experimental finding. What is to be put in its place is a matter of interpretation. The experiment uses not distant particles as in the Bell test but multiple measurements of a single system.

• Kitaev 2003 — Kitaev, Fault-tolerant quantum computation by anyons. Annals of Physics 303, 2-30
  In brief, what does it say?

  Kitaev proposed computing by moving excitations called anyons around one another in a two-dimensional system, and that this is inherently fault-tolerant.

  The abstract of the paper is short and clear: a two-dimensional quantum system can be thought of as a quantum computer if it carries anyon excitations. In three dimensions there are only two kinds of fundamental particles, fermions and bosons; in two dimensions "anyons," which occupy a place between them, are possible. When two anyons are exchanged, the state of the system undergoes a definite transformation.

• Kochen 1967 — Kochen, The Problem of Hidden Variables in Quantum Mechanics. Indiana University Mathematics Journal (J. Math. Mech.) 17, 59-87
  In brief, what does it say?

  The original mathematical paper showing that quantum mechanics contradicts the assumption of noncontextuality.

  Kochen and Specker examine mathematically whether 'hidden variables' can be added to quantum mechanics. A hidden variable theory is the idea that the uncertainty of quantum results comes only from our lack of knowledge and that there is a predetermined value for every measurement.

• Kocsis 2011 — Kocsis, Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer. Science 332, 1170-1173
  In brief, what does it say?

  The experiment that reconstructs, by weak measurement, the average paths of photons inside a double-slit interference.

  In the double-slit experiment, single photons leave dots on a screen; when enough photons are collected, an interference pattern appears. The question is what path a single photon follows after the slits. In quantum mechanics, measuring the path destroys the pattern. This work, instead of 'measuring' the path, reconstructs the average path by weak measurement.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Lamoreaux 1997 — Lamoreaux, Demonstration of the Casimir force in the 0.6 to 6 μm range. Phys. Rev. Lett. 78, 5
  In brief, what does it say?

  The question was: is 'empty space' really nothing? According to quantum theory, even in empty space a stirring remains in the lowest state of the electromagnetic field (zero-point fluctuation). The theory said that two conducting surfaces brought very close together would attract each other because they change this stirring; this is called the Casimir force. Lamoreaux measured this very small force on a torsion pendulum, at separations between the surfaces of about 0.6 to 6 micrometers. The result agreed with the theory to about 5 percent. So 'empty space' can give rise to a measurable force. Here the experiment is the existence and size of the force; the interpretation, whether this means 'the vacuum is full' or is another way of describing the surfaces' interaction with each other, is still discussed among physicists. What gives one pause is that a tangible effect was measured 'where there is nothing.'

• Leifer 2014 — Leifer, Is the quantum state real? An extended review of psi-ontology theorems. Quanta 3, 67
  In brief, what does it say?

  The question of whether the quantum state is something that really exists in the world (ontic) or our knowledge (epistemic) is an old one. In late 2011 Pusey, Barrett and Rudolph put forward a theorem, and it drew much interest and debate. Leifer's long review first tells the background of the subject, then presents the theorem in plain language, then goes over the related work done afterward. It defines what it means for a state to be 'ontic' or 'epistemic' and lists the arguments before the theorem, for and against. An important point: to prove the reality of the state is to place a very hard constraint on realist theories. This is a theoretical review; not an experiment. What is firm: the mathematics of the theorems. What is disputed: whether the assumptions are in place and what conclusion will come out.

• Lindblad 1976 — Lindblad, On the generators of quantum dynamical semigroups. Communications in Mathematical Physics 48, 119-130
  In brief, what does it say?

  The paper that gives the most general form of the generators of quantum dynamical semigroups.

  Lindblad examines the semigroups that describe the time evolution of a quantum system interacting with its environment. A semigroup is a time evolution in which the state after a certain time step can be obtained by applying two half steps in succession; this corresponds to the assumption that the environment is memoryless (Markov).

• Lloyd 1996 — Lloyd, Universal Quantum Simulators. Science 273, 1073
  In brief, what does it say?

  Lloyd showed that quantum computers can efficiently simulate local quantum systems.

  In 1982 Feynman had intuitively proposed that a quantum computer could imitate quantum systems. Lloyd’s question is whether this is mathematically correct.

• Ma 2016 — Ma, Delayed-choice gedanken experiments and their realizations. Rev. Mod. Phys. 88, 015005
  In brief, what does it say?

  When light or a particle enters a device with two possible paths, whether it will behave like a wave or like a particle depends on what is measured at the end of the device. Wheeler had asked: 'what if we decide what to measure at the end only after the particle has entered the device?' This review article tells the history of this 'delayed choice' idea and collects its versions carried out in the laboratory, such as having the choice made by a fast random number generator. The finding is this: the results come out as the theory predicts, that is, whether interference appears or not agrees with the final choice. What gives one pause is that this can be read as 'changing the past'. The article objects to this reading: the experimental result is firm, but 'influencing the past' is an interpretation and the usual theory does not need it. An additional topic of the article is the duality between separability and entanglement.

• Maldacena 1997 — Maldacena, The large N limit of superconformal field theories and supergravity. Adv. Theor. Math. Phys. 2, 231
  In brief, what does it say?

  There are two great frameworks in physics: quantum field theories (the quantum physics of particles) and gravity (Einstein's theory of spacetime). Uniting the two is an unsolved question. Maldacena proposed that a certain kind of quantum field theory (with many components, in the 'large N' limit, of very high symmetry) can carry the same information as a string theory with gravity in a curved spacetime of another dimension. That is, a description without gravity on a surface is like the counterpart of a description with gravity in a depth. The paper puts this forward as a 'conjecture' and supports it with reasons derived from string theory. This is a theoretical proposition; it has no direct experiment. It has passed many tests and mathematical checks and is used very heavily, but whether it is true for our universe is a separate question. Why it gives one pause: that a world with an extra dimension can be fully matched by a description on a boundary with fewer dimensions.

• Maldacena 2013 — Maldacena, Cool horizons for entangled black holes. Fortschr. Phys. 61, 781
  In brief, what does it say?

  Maldacena and Susskind put forward a proposal that identifies entanglement with a spacetime bridge, and used it as a way out of the firewall paradox.

  General relativity contains solutions in which two distant black holes are connected through the interior region by a bridge, that is, an Einstein–Rosen bridge. The authors interpret these solutions as a maximally entangled state of two black holes; that is, a complex EPR pair.

• Micadei 2019 — Micadei, Reversing the direction of heat flow using quantum correlations. Nat. Commun. 10, 2456
  In brief, what does it say?

  Heat flows spontaneously from hot to cold; this is the established rule of thermodynamics. However, this rule assumes that the two systems are initially unrelated to each other. Micadei and colleagues prepared two spins by nuclear magnetic resonance in a correlated (entangled) state and observed heat flowing from cold to hot. The direction of the flow was determined by a trade-off between the correlations and entropy. This is an experiment. What gives one pause is that the correlations can change the direction without violating the second law of thermodynamics; the law is being re-read with a broader view of information. As an application, heat control at the micro scale has been proposed.

• Minev 2019 — Minev, To catch and reverse a quantum jump mid-flight. Nature 570, 200
  In brief, what does it say?

  In quantum physics a measurement can give discrete and random outcomes; the 'quantum jump' Bohr proposed in 1913, an atom's sudden transition between two energy levels, is the symbol of this. When jumps would occur was held to be fundamentally unpredictable. The question: is it possible to know when a jump is about to happen? The team, in an artificial atom with three levels (a superconducting circuit), tracked the jump from the ground level to the excited level by monitoring the occupation of an auxiliary level. The finding: every completed jump proceeds with a continuous, coherent and deterministic course. With real-time monitoring and feedback, the team caught a jump midway and reversed it, that is, prevented its completion. The results agree with the predictions of quantum trajectory theory for jumps. This is an experimental finding; the paper says it refutes Bohr's view. What gives one pause is that there turns out to be a course within an event thought to be 'sudden'.

• Misra 1977 — Misra, The Zeno's paradox in quantum theory. J. Math. Phys. 18, 756
  In brief, what does it say?

  An unstable particle, for example an atomic nucleus about to decay, turns into other particles over time. Misra and Sudarshan asked: what happens if we observe this particle continuously, that is, if we keep looking and asking 'has it decayed?' In their calculation with the mathematics of quantum theory, if the looks are frequent enough, the result is that the particle never decays, something like 'a watched pot never boils'. This result is called the quantum Zeno paradox. The paper is a theory and calculation paper; it rests on the assumption that measurement restarts the state each time. Later, other groups showed in the laboratory that frequent measurement really does slow decay; this is the result not of this paper but of the later experiments. What gives one pause is that looking, that is, measuring, is not merely taking in information but affects the course of the system.

• Mohideen 1998 — Mohideen, Precision Measurement of the Casimir Force from 0.1 to 0.9 μm. Phys. Rev. Lett. 81, 4549
  In brief, what does it say?

  Mohideen and Roy measured the Casimir force much more precisely, with an atomic force microscope, over a range of 0.1–0.9 μm.

  This work, carried out a few years after Lamoreaux’s measurement, aims to measure the same force at much smaller separations and with smaller uncertainty. The question: does the Casimir force agree with theory at the one percent level once the effects of real materials and of temperature are taken into account?

• Monroe 1996 — Monroe, A "Schrödinger cat" superposition state of an atom. Science 272, 1131
  In brief, what does it say?

  Monroe and colleagues prepare a single trapped beryllium ion in a superposition of two spatially separated states and verify it by interference.

  According to the paper’s abstract, the authors have produced a “Schrödinger cat”-like state at the level of a single atom. A 9Be+ ion held in a trap is first laser-cooled to its zero-point energy. The ion is then prepared in a superposition of two spatially separate coherent harmonic oscillator states.

• Moreva 2014 — Moreva, Time from quantum entanglement: An experimental illustration. Phys. Rev. A 89, 052122
  In brief, what does it say?

  For someone looking at the universe from outside, does time flow? According to a proposal by Page and Wootters, the total state of the universe may be unchanging, 'frozen'; time arises from the entanglement of one part of the system (a clock) with the other parts. This paper shows a small scene of this idea in the laboratory. Two photons are prepared in an entangled state; one is used like a clock, the other is measured with it. An observer from inside, attached to the clock photon, sees the other photon change over time. An observer from outside who looks only at the joint state of the two sees no change at all: the state stands still. This is a demonstration experiment: it brings the idea to life in a small setup, it does not show that the universe really works this way. Whether time really arises in this way is a theoretical and open question.

• Ollivier 2004 — Ollivier, Objective properties from subjective quantum states: Environment as a witness. Physical Review Letters 93, 220401
  In brief, what does it say?

  One of the first formulations that treats the environment not as a passive destroyer but as an active witness that multiplies information.

  The paper asks how objective properties emerge in open quantum systems. An open system is one that interacts with its environment; photons, air molecules or other particles pass by the system and carry a trace of it. In the classical account of decoherence, the environment was seen as a 'reservoir' that selectively destroys quantum coherence. The authors give the environment a second role: a communication channel that carries and broadcasts information about the system.

• Ozawa 2003 — Ozawa, Universally valid reformulation of the Heisenberg uncertainty principle on noise and disturbance in measurement. Phys. Rev. A 67, 042105
  In brief, what does it say?

  The Heisenberg uncertainty principle is often told like this: the more sharply you measure the position of a particle, the more you disturb its velocity (more precisely its momentum); the product of the two cannot fall below a certain lower bound. Ozawa showed mathematically that this lower bound on the product of 'measurement error and disturbance' does not hold in every case. In the correct relation there are extra terms; if there is a certain link between the measuring instrument and the thing measured, the product can be smaller than Heisenberg's bound. The paper also built a measurement model showing this and proposed an experiment that could be done with light. These are results at the level of theory; the paper itself presents not an experiment but an experimental design. What gives one pause is not the collapse of the principle itself but the correction of its often-told measurement-disturbance form. Other forms, that is, the relations concerning the spreads of the states themselves, remain valid as they are.

• Page 1983 — Page, Evolution without evolution: Dynamics described by stationary observables. Phys. Rev. D 27, 2885
  In brief, what does it say?

  The equations of physics always take time as a 'parameter'; but if the whole universe is a closed system, who has a clock outside the universe? Don Page and William Wootters offered a proposal: the universe can be static, and change arises from comparing one part of the system with another part (an internal clock). That is, the impression that 'time is passing' is read from the relation between what the clock shows and the rest. This is a theoretical proposal, not an experiment. What gives one pause is that change is not seen in the universe as a whole and that change emerges only from the relation between parts. Whether it is really so is a matter of interpretation and dispute.

• Page 1993 — Page, Information in black hole radiation. Phys. Rev. Lett. 71, 3743
  In brief, what does it say?

  Page estimated how fast and in what way information would appear if it comes back in the black hole radiation.

  Page's paper begins with a question: if black hole formation and evaporation can be described by an S-matrix, that is, a unitary mapping from the initial state to the final state, then information must come out together with the radiation. When and to what extent does this exit take place?

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Pauli 1925 — Pauli, Über den Zusammenhang des Abschlusses der Elektronengruppen im Atom mit der Komplexstruktur der Spektren. Z. Phys. 31, 765-783
  In brief, what does it say?

  The paper that proposes an exclusion rule by relating the closing of electron shells in atoms to the complex splittings of spectral lines in a magnetic field.

  The paper’s starting point was two separate puzzles. The first was that the shells in the periodic table fill up to 2, 8, 18, 32 electrons and why these numbers close at these values. The second was that spectral lines split in a magnetic field in a more complicated way than expected, that is, the anomalous Zeeman effect. Moreover, it was also unclear why all the electrons in an atom do not gather in the lowest-energy state.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Pednault 2019 — Pednault, Leveraging Secondary Storage to Simulate Deep 54-qubit Sycamore Circuits. arXiv:1910.09534 (önbaskı)
  In brief, what does it say?

  A response arguing that Sycamore circuits could be simulated within days on a classical supercomputer by using secondary storage.

  An earlier work by the authors had shown that secondary storage (disks) can extend the range of quantum circuits that can be practically run in classical simulation. This paper improves those techniques and applies them to Sycamore circuits of 53 and 54 qubits. The paper appeared as a preprint shortly before the publication of Google's Nature paper.

• Penrose 1996 — Penrose, On Gravity's role in Quantum State Reduction. Gen. Relativ. Gravit. 28, 581-600
  In brief, what does it say?

  The proposal that describes quantum state reduction (collapse) by tying it to the difference in the geometry given to space-time by two mass distributions.

  Penrose’s starting point is the measurement problem: quantum theory can describe a system as a superposition of several states, but a measurement gives a single outcome. According to Penrose, this requires quantum theory and general relativity to be considered together. His reason is this: the superposition of a mass located in two separate positions means a superposition of two different space-time geometries.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Petrosky 1990 — Petrosky, Quantum Zeno effect. Phys. Lett. A 151, 109
  In brief, what does it say?

  The paper is on the quantum Zeno effect; we do not have this source's own summary. The effect says that measuring an unstable system frequently can slow its decay.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Planck 1901 — Planck, Ueber das Gesetz der Energieverteilung im Normalspectrum. Ann. Phys. 309, 553
  In brief, what does it say?

  Planck derives the energy distribution law of black-body radiation by treating energy as divided into shares of a definite size.

  The distribution by wavelength of the light emitted by a heated body was being measured at the time. The distribution that came out of classical theory contradicted experiment at short wavelengths, while the laws that fit the measurements lacked a theoretical basis. In this paper Planck tries to ground in a theoretical calculation a law that fits the measured distribution.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Popescu 1994 — Popescu, Quantum nonlocality as an axiom. Found. Phys. 24, 379-385
  In brief, what does it say?

  Popescu and Rohrlich showed a hypothetical model that is compatible with relativity but builds stronger correlations than quantum mechanics ('Quantum nonlocality as an axiom', Found. Phys. 24, 379).

  The starting point of this paper is the question from which principles the nonlocal correlations of quantum mechanics can be derived. Two principles come to mind: the correlations cannot be explained by a local theory (Bell), but these correlations cannot be used to send a message (relativity).

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Procopio 2015 — Procopio, Experimental superposition of orders of quantum gates. Nat. Commun. 6, 7913
  In brief, what does it say?

  In an operation, doing A then B and doing B then A often give different results. Does quantum theory allow these two orders to exist together? In this experiment, which gate a photon passes through first is tied to the state of a second photon, so that 'the two orders together' is realized. In this setup, whether two operations give the same result when their order is swapped can be learned with fewer trials than by applying each operation one by one many times; this situation, known as a theoretical advantage, is demonstrated in the laboratory. This is an experimental result. The authors say that this can be read as a 'superposition of causal orders'; but this reading is an interpretation. What gives one pause is that the order of events can also behave like a quantum possibility.

• Proietti 2019 — Proietti, Experimental test of local observer independence. Sci. Adv. 5, eaaw9832
  In brief, what does it say?

  The scientific method rests on facts that are independent of who observes and are accepted by everyone. In quantum mechanics, however, the objectivity of observations is not so clear; in Wigner's famous thought experiment two observers seem to live different realities from one another. Can these realities be reconciled independently of the observer? Recently an extended version with four observers was set up, which made it possible to test this question. The team realized this version in a six-photon experiment and violated the relevant Bell-type inequality by five standard deviations. This is an experimental finding. The interpretive part is conditional: if the assumptions of locality and free choice are kept, quantum theory must be interpreted as observer-dependent. So the result, without giving up one of the assumptions, strains the idea of a 'fact that is the same for everyone'.

• Pusey 2012 — Pusey, On the reality of the quantum state. Nat. Phys. 8, 475
  In brief, what does it say?

  The quantum state is the basic mathematical object of the theory. Yet physicists have not agreed on what it actually represents. One view says that a pure quantum state corresponds directly to reality. Another view has long held that the state represents only information about some aspect of the system. The paper gives a theoretical result: any model that counts the quantum state as mere information about an underlying physical state, and that assumes independently prepared systems have independent physical states, must make predictions that contradict quantum theory. This is not an experiment but a mathematical result; its validity depends on these two assumptions. What gives one pause is that approaches saying 'the state is only information' are closed off under these conditions.

• Ringbauer 2015 — Ringbauer, Measurements on the reality of the wavefunction. Nat. Phys. 11, 249
  In brief, what does it say?

  Quantum mechanics describes nature with extraordinary success, but what the wave function is remains unclear: does it represent our limited knowledge about a system, or does it correspond directly to reality? Earlier theoretical results said that if an objective reality exists the wave function must be real, but they rested on contested assumptions. This work takes another path without those assumptions and, with single photons, places an experimental bound on how much information interpretations can explain of quantum phenomena. The finding: in three- and four-dimensional states, no information interpretation can fully explain the limited distinguishability of non-orthogonal quantum states. This is an experimental result. The interpretive part is conditional: if the idea of an objective reality is accepted, the result strengthens the view that the wave function corresponds directly to reality. The debate is still going on.

• Ritchie 1991 — Ritchie, Realization of a measurement of a "weak value". Physical Review Letters 66, 1107-1110
  In brief, what does it say?

  The first laboratory measurement of a weak value (with the polarization of a light beam).

  This paper is the first experimental demonstration of the weak value concept proposed theoretically in 1988. The experiment is done in an optical setup: the polarization of the light is preselected, a crystal shifts the light beam very slightly depending on polarization, and the beam is then postselected on another polarization. The measured quantity is the shift of the beam.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Robertson 1929 — Robertson, The uncertainty principle. Phys. Rev. 34, 163
  In brief, what does it say?

  In 1927 Heisenberg had said that the position and the velocity (momentum) of a particle cannot be known at the same time with arbitrary precision. Robertson takes this idea from a single pair and ties it to a general rule: for any two measurable quantities that do not 'commute' with each other, the product of their uncertainties has a lower bound. That is, the more sharply one is known, the blurrier the other remains. Here 'uncertainty' does not mean the crudeness of the measuring instrument but the spread of measurement results for the same state. This is a result of the theory, an inequality proved by mathematics; it has been tested many times by experiment. What is disputed is whether this concerns nature itself or our knowledge; Robertson does not enter this question. What gives one pause is not 'something we do not know' but two certainties that do not exist at the same time.

• Rovelli 1996 — Rovelli, Relational quantum mechanics. Int. J. Theor. Phys. 35, 1637
  In brief, what does it say?

  Rovelli asks: does quantum theory describe the state of a system 'on its own,' or the relation of one system to another? His proposal is this: the properties of a thing exist not absolutely but relative to another system that observes it or interacts with it. For the same event, two different observers, each in accordance with the rules, may say different things; this is the strange-looking part. The author argues that the unusual features of quantum theory follow naturally from this idea of 'being relative to a viewpoint.' This is not an experiment; it is a proposal about the interpretation of the theory, and it is as disputed as the other interpretations; no experiment has decided between one and another. The attractive and unsettling side of the approach is that it puts the sentence 'it is so relative to this' in place of the sentence 'the world is thus.'

• Rovelli 2009 — Rovelli, Forget time. arXiv:0903.3832 (Found. Phys. 41, 1475, 2011)
  In brief, what does it say?

  The paper argues that the best way to understand quantum gravity is to build a picture of the world in which the notion of time plays no role. The author summarizes this standpoint: in a fundamental description 'we have to forget time'. He explains how this can be done in classical theory and in quantum theory. The idea is to develop a formalism that treats dependent and independent variables on the same level. In short, the author's proposal is to interpret mechanics not as a theory of the evolution of variables in time but as a theory of the relations between variables. This is a proposal of theory and standpoint; not an experiment, and the author's own advocacy. What gives one pause is that time may be, fundamentally, not a precondition but something derived from relations.

• Rozema 2012 — Rozema, Violation of Heisenberg's measurement-disturbance relationship by weak measurements. Phys. Rev. Lett. 109, 100404
  In brief, what does it say?

  The idea Heisenberg first stated was this: the more precisely we measure something, the more we disturb it as we measure. This 'measurement-disturbance' relation is in many places taught together with the uncertainty principle; yet what has been rigorously proved is the relation in which the uncertainties within a system itself (for example the spreads of position and velocity) are tied to each other. Rozema and colleagues followed the polarization of a photon, a particle of light, before and after with a method called 'weak measurement,' which hardly disturbs the system, and extracted the error and the disturbance separately. In the end they saw that the relation in Heisenberg's original form was exceeded, while the corrected relation proposed later by Ozawa remained valid. This is an experimental result. Why it gives one pause: the widely told principle does not turn out to be true as it stands; what is firm and what is disputed must still be kept apart. What weak measurement means is also a separately debated subject.

• Rubino 2017 — Rubino, Experimental verification of an indefinite causal order. Sci. Adv. 3, e1602589
  In brief, what does it say?

  The raw summary we have is very short: the researchers report that they observed and characterized a quantum process that has no predefined causal order. The question is whether the order of which event comes first and which comes after has to be definite. From the title, as far as we can tell, this is an experimental work, aimed at verifying a situation called 'indefinite causal order'. Beyond this summary we do not know the details of the experiment, the setup used, or the numerical results from the text provided; we do not write them. What gives one pause is the idea that even an order that seems as basic as sequencing may not be predefined in quantum situations.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Rugh 2002 — Rugh, The quantum vacuum and the cosmological constant problem. Stud. Hist. Phil. Mod. Phys. 33, 663
  In brief, what does it say?

  The cosmological constant problem arises where general relativity and quantum field theory meet, and is counted among the fundamental problems of modern physics. The paper recounts the historical and conceptual origin of this problem; the problem is closely tied to the concept of the vacuum in quantum field theory. The authors critically discuss how the problem rests on the idea of a 'physically real vacuum energy'. They also lay out which relations between general relativity and quantum field theory are assumed for the problem to be well defined. This is not an experiment; it is a conceptual and historical study, that is, at the level of interpretation. What gives one pause is that whether the vacuum is counted as a real thing carrying energy determines the problem itself.

• Schlosshauer 2005 — Schlosshauer, Decoherence, the measurement problem, and interpretations of quantum mechanics. Rev. Mod. Phys. 76, 1267
  In brief, what does it say?

  According to quantum theory, before it is measured a system can be in a state in which several possibilities enter the calculation together (superposition). Yet when we measure, we always see a single result; this mismatch is called the measurement problem. Schlosshauer's paper is a review of the phenomenon called decoherence and of how much it helps with this problem. Decoherence is this: a small system inevitably interacts with its environment (air, light, heat), and because of this the traces by which the joint presence of possibilities could be captured, that is, interference, are erased very quickly. This is a matter of experiment and calculation and is well understood. The paper also asks whether this explains why a single result appears, and examines the main interpretive approaches (Copenhagen, many worlds, hidden variables and others) one by one within this frame. The point the article reaches is that decoherence explains why results look classical, but does not by itself close the question 'why only one'; that part depends on interpretation.

• Schlosshauer 2013 — Schlosshauer, A snapshot of foundational attitudes toward quantum mechanics. Stud. Hist. Phil. Mod. Phys. 44, 222
  In brief, what does it say?

  The foundations of quantum mechanics, through experimental and theoretical work, gave birth to quantum information science, but fundamental questions are still fiercely debated in the physics community and there is no agreement on the main issues. This work is neither a theory nor an experiment but a survey: 16 multiple-choice questions were put to 33 people attending a conference on the foundations of quantum mechanics. The participants included physicists, philosophers and mathematicians. The authors extract the common views and the strong, moderate and weak correlations between answers. What emerges is a snapshot of views in the field at that moment; that is, a picture drawn from a sample of 33 people. It should not be read as a general verdict as it stands. What gives one pause is that even experts in this field still do not meet at the same place.

• Schlosshauer 2019 — Schlosshauer, Quantum decoherence. Phys. Rep. 831, 1
  In brief, what does it say?

  According to quantum theory, before it is measured a system can be in a state in which several possibilities come together (superposition). In the everyday world, however, we always see the table in a single place. This review article describes a process called 'decoherence': the system inevitably interacts with its environment (air, light, heat), and the environment carries a kind of trace about the system; because of this, the interference between the possibilities disappears and the system begins to look like something classical. Decoherence can be calculated and has been measured in experiments; this part is solid. What is disputed is whether it explains 'why a measurement gives a single result'. According to most authors it does not explain this by itself; it only explains why the interference is not seen. The article is a review that also speaks openly about this limit.

• Schrödinger 1926 — Schrödinger, An Undulatory Theory of the Mechanics of Atoms and Molecules. Phys. Rev. 28, 1049
  In brief, what does it say?

  Schrödinger summarized, in English, his wave equation and how the energy levels of the hydrogen atom are found with it (a summary of the four-part series 'Quantisierung als Eigenwertproblem' in Annalen der Physik).

  This paper is an English summary of the work Schrödinger published in German in 1926 in four parts. The starting point was this: why the energy of atoms can take only certain values was being explained at the time by rules called 'quantum conditions' that were added from outside. Instead of adding these rules from outside, Schrödinger proposed to show that the solutions of a wave equation are, by themselves, well-behaved (finite and single-valued) only for certain values. Much as a taut string can vibrate only at certain frequencies.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Schrödinger 1935 — Schrödinger, Die gegenwärtige Situation in der Quantenmechanik. Naturwissenschaften 23, 807
  In brief, what does it say?

  Schrödinger reviews the then-current state of quantum mechanics and, in this paper, introduces the concept of entanglement and the cat thought experiment.

  The paper was published in the journal Naturwissenschaften in three parts and reviews the then-current presentation of quantum mechanics with a critical eye. The text is read within a chain of replies to the EPR paper; for there it takes up a situation in which the states belonging to two systems cannot be separated from each other, and Schrödinger gives this situation the name “Verschränkung”, that is, entanglement.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Schwinger 1948 — Schwinger, On Quantum-Electrodynamics and the Magnetic Moment of the Electron. Physical Review 73, 416-417
  In brief, what does it say?

  Schwinger calculated the first quantum correction to the electron's magnetic moment and compared it with the measured value.

  This is a short two-page letter. The Dirac equation said that the electron's magnetic moment would come out at an exactly definite value from its spin and charge. Measurements made in 1947, however, showed that this value was larger by a few parts in a thousand. The question was: could this small deviation be a correction arising from the interaction of the light field with the electron?

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Scovil 1959 — Scovil, Three-Level Masers as Heat Engines. Phys. Rev. Lett. 2, 262
  In brief, what does it say?

  Scovil and Schulz-DuBois read a three-level maser (a microwave amplifier) as a heat engine and compared its efficiency with the thermodynamic limit.

  A maser is a device that exploits transitions between the energy levels of atoms in a material to amplify microwave radiation. In a three-level maser, atoms are lifted from the lowest level to the highest level by a "pump" radiation; then, as they drop to the intermediate level, they give off the amplifier radiation.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Scully 1982 — Scully, Quantum eraser: A proposed photon correlation experiment concerning observation and "delayed choice" in quantum mechanics. Phys. Rev. A 25, 2208-2213
  In brief, what does it say?

  The theoretical paper that discusses the effect on the interference pattern of the choice of reading or erasing path information afterwards in a double-slit-like arrangement, and proposes the “quantum eraser”.

  In a double-slit experiment, if there is information that determines which slit the particle went through, the interference pattern disappears. The question is: what happens if this information is stored in another system (for example a photon or the internal state of an atom) and, instead of being read afterwards, is erased?

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Shalm 2015 — Shalm, Strong loophole-free test of local realism. Phys. Rev. Lett. 115, 250402
  In brief, what does it say?

  A Bell test asks this: could nature be working by a rule in which everything is 'local' and 'predetermined'? Local reality means that the effect of a thing does not spread faster than light and that it already has a definite value before being measured. In older Bell experiments there were small 'loopholes' that allowed the result to be fitted into this rule. This work used entangled photon pairs. The parties were placed so far apart that not even light could travel between the measurements; the measurement choices were made with fast random number generators; the detectors were efficient enough. Thus the main loopholes that could save local reality were closed at the same time. By the authors' calculation, the result makes local reality untenable even at a very small probability (the paper gives p-values on the order of 10 to the power of minus 9 and 10 to the power of minus 7). This is an experiment and it is firm; what is disputed is which interpretation the conclusion drawn from it should be told in. What gives one pause is that the intuition we express as 'it was definite even before being measured' was really put to the test and did not hold.

• Shor 1994 — Shor, Algorithms for quantum computation: discrete logarithms and factoring. Proc. 35th Annual Symposium on Foundations of Computer Science (FOCS 1994), 124–134; tam sürüm: SIAM J. Comput. 26 (1997) 1484–1509, arXiv:quant-ph/9508027
  In brief, what does it say?

  A paper giving polynomial-time algorithms on a quantum computer for factoring integers and computing discrete logarithms.

  The paper's starting question is whether the computer is a universal computing device: can a classical computer simulate any physical computing device with at most a polynomial loss of time? Once quantum mechanics is taken into account, this is uncertain. The paper takes up two problems: factoring integers and finding the discrete logarithm. Both are considered hard on a classical computer and are the basis of many cryptographic proposals.

• Shor 1995 — Shor, Scheme for reducing decoherence in quantum computer memory. Phys. Rev. A 52, R2493
  In brief, what does it say?

  Quantum computers hold information in superposition, and this state is disrupted by the smallest interaction with the environment (decoherence). This seemed to make long computations impossible. Shor proposed a quantum counterpart of classical error-correcting codes: by spreading a single quantum bit over several physical bits, one detects and corrects a disturbance arising in one bit without reading the information. The idea is to measure only the question 'is there an error?' without looking at the information itself. The paper shows that this route works under the assumption that each bit is disturbed separately. This is a proposal of theory and method; it was tried experimentally in the following years. What gives one pause is that, although we cannot copy quantum information, a way of protecting it has been found.

• Slusher 1985 — Slusher, Observation of squeezed states generated by four-wave mixing in an optical cavity. Phys. Rev. Lett. 55, 2409
  In brief, what does it say?

  Even in empty space the field of light is not perfectly calm; according to quantum theory there is always a small fluctuation, 'vacuum noise'. The question was this: can a state of light be made that goes below this noise? Slusher and his colleagues used, inside an optical cavity with sodium atoms, a process called 'four-wave mixing'. In this process, while one direction of the light field is amplified, the perpendicular direction is suppressed along with it: like squeezing one side of a balloon and the other side swelling (an aid to description). The measurement was made with a setup reading the two directions separately, and the noise in the suppressed direction fell below the usual noise of the vacuum. This state is called a 'squeezed state'. This is an experimental finding; it requires no interpretation but shows the limit of the principle: the product of the noises of the two directions cannot fall below a lower bound, but one of them can.

• Spekkens 2007 — Spekkens, Evidence for the epistemic view of quantum states: A toy theory. Phys. Rev. A 75, 032110
  In brief, what does it say?

  Spekkens built a toy theory: a model based on a simple principle, which is not quantum mechanics but shows behaviors similar to it. The principle is this: even if we have the most knowledge possible about a system, half of the questions that could be asked remain unanswered. With this single rule, many peculiarities of the quantum reappear in the model: the dependence of measurements on order, interference, the impossibility of copying an unknown state, teleportation, and others. According to the author, so many similarities can be taken as evidence for the view that quantum states are states not of the world but of incomplete knowledge. But there are also things the model cannot reproduce: the violation of Bell's inequality and the Kochen-Specker theorem. So this is a theoretical study; it shows similarity, it does not give a definitive result. What gives one pause is how much quantum appearance 'not knowing' can produce once it is structured by a certain rule.

• Steane 1996 — Steane, Multiple-particle interference and quantum error correction. Proceedings of the Royal Society A 452, 2551–2577
  In brief, what does it say?

  A paper presenting quantum error-correcting codes and their limits by treating multi-particle interference in the light of classical coding theory.

  The paper brings two things together: the mathematics of classical error-correcting codes and many-particle quantum interference. Using this connection, the author explains how error correction can be done in quantum communication channels and quantum computers, and evaluates its limits.

• Susskind 1995 — Susskind, The world as a hologram. J. Math. Phys. 36, 6377
  In brief, what does it say?

  The question: if we bring quantum theory and gravity together, how much information can be stored in a region? According to 't Hooft's proposal the three-dimensional world can be described completely by information written on a boundary surface, that is, on a two-dimensional projection; each very small piece of the surface carries only one discrete degree of freedom. In the paper Susskind summarizes this idea and draws a first, informal picture of how it could be applied. One consequence of this picture is that particles must grow in length as their energies become very high. The author also discusses string theory as a possible candidate for the realization of this idea. This is a theory paper; there is no experiment and the idea was later developed in different forms. What gives one pause is the thought that the information contained in what we call 'volume' cannot exceed that of the surface. How this holds in the real universe is still open.

• Tegmark 2003 — Tegmark, Parallel universes. Sci. Am. 288(5), 40 (arXiv:astro-ph/0302131)
  In brief, what does it say?

  Tegmark lines up the idea of 'other universes' not as a single speculation but as the natural consequence of four separate theories in physics. At the first level, if space is infinitely large, there are infinitely many regions of the same size as the one we can see, and among them there may be copies very similar to ours, even identical. At the second level, in some forms of inflation theory, other regions of space may have formed with different constants of nature and different sets of particles. The third level is the 'many worlds' reading of quantum mechanics; the paper says this reading adds nothing new and merely accepts the consequences of the same equations. At the fourth level it is thought that different mathematical structures give different physical equations. The author discusses how testable these are, and openly admits that as long as we do not know how to count probabilities (the measure problem), no testable prediction comes from the higher levels. What gives one pause is that the phrase 'other universe' is taken seriously here not as science fiction but as a possibility arising from equations; yet the distance between possibility and evidence is not hidden in the paper either.

• Thomas 1926 — Thomas, The Motion of the Spinning Electron. Nature 117, 514
• Tonomura 1989 — Tonomura, Demonstration of single-electron buildup of an interference pattern. Am. J. Phys. 57, 117
  In brief, what does it say?

  Does an electron behave like a wave or like a particle? This paper describes the famous double-slit experiment as it was actually carried out with electrons. The researchers placed in an electron microscope a thin wire that joins the two paths (a biprism) and a detector that counts the arriving electrons one by one. The electrons were sent so sparsely that at most one electron was in the apparatus at any moment. On the screen, at first, single dots appear scattered at random; each dot lands in one place, like a particle. But as thousands of dots accumulate, light and dark stripes emerge among them: the interference pattern peculiar to waves. This experiment is not an interpretation of the theory but a directly observed result; what gives one pause is that even a lone electron leaves a trace as if it had 'passed through both paths at once'. What this means is a matter of interpretation; the experiment shows only the pattern.

• Tsui 1982 — Tsui, Two-Dimensional Magnetotransport in the Extreme Quantum Limit. Physical Review Letters 48, 1559-1562
  In brief, what does it say?

  Tsui, Störmer and Gossard observed that the Hall resistance forms fractional steps at a very clean semiconductor interface.

  The experiment was carried out on a two-dimensional electron gas accumulated at the interface between the layers of a structure made of gallium arsenide and aluminium gallium arsenide. The sample was cooled to a very low temperature (below 1 kelvin) and placed in a very strong magnetic field; the "extreme quantum limit" mentioned in the title describes this regime: all the electrons fit into the lowest-energy magnetic level.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Uhlenbeck 1925 — Uhlenbeck, Ersetzung der Hypothese vom unmechanischen Zwang durch eine Forderung bezüglich des inneren Verhaltens jedes einzelnen Elektrons. Naturwissenschaften 13, 953-954 (Crossref kaydında yazar P. Ehrenfest olarak görünür; not Uhlenbeck ve Goudsmit'e atfedilir)
  In brief, what does it say?

  The short note that proposes, as a physical content for Pauli’s fourth quantum number, that the electron has an intrinsic angular momentum and a magnetic moment.

  The note’s title proposes that, in place of Pauli’s “non-mechanical constraint”, a condition on the internal behavior of each electron be put. That is, the question is what Pauli’s fourth quantum number corresponds to.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• von Klitzing 1980 — von Klitzing, New Method for High-Accuracy Determination of the Fine-Structure Constant Based on Quantized Hall Resistance. Physical Review Letters 45, 494-497
  In brief, what does it say?

  von Klitzing and colleagues showed that the Hall resistance in a silicon transistor forms steps, and that these steps are exact enough to measure the fine-structure constant.

  The title of the paper states what was done: the quantity measured is the quantized Hall resistance, and the use made of it is a high-precision determination of the fine-structure constant. The experiment was carried out in a silicon field-effect transistor that forms a two-dimensional electron layer, at low temperature and under a strong magnetic field.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• von Neumann 1932 — von Neumann, Mathematische Grundlagen der Quantenmechanik. Springer, Berlin (1932; yeniden basım 1996)
  In brief, what does it say?

  Von Neumann placed quantum mechanics on a sound mathematical foundation over Hilbert space and for the first time analyzed the measurement process in detail within the theory (the book Mathematische Grundlagen der Quantenmechanik).

  This work is not a paper but a book. Its aim is to gather quantum mechanics, independently of the various calculation methods used until then (matrix, wave), into a single mathematical framework. The framework is a vector space called Hilbert space: states are vectors in this space, and measurable quantities are certain operators acting on the space.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Wehner 2018 — Wehner, Quantum internet: A vision for the road ahead. Science 362, eaam9288
  In brief, what does it say?

  A Science vision paper that describes the gradual stages of development of the quantum internet and its applications.

  This paper is written as a 'roadmap'. The idea of the quantum internet is to support long-distance communication in the classical internet with quantum communication; that is, for any two points on Earth to be able to exchange quantum information.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Weinberg 1989 — Weinberg, The cosmological constant problem. Rev. Mod. Phys. 61, 1
  In brief, what does it say?

  Even empty space may have an energy; this is called the cosmological constant, and it affects the expansion rate of the universe. Astronomical observations show that this value is very small. Yet when the theories of particle physics calculate the energy expected from vibrations in the vacuum, they obtain a number far larger than what is observed; the difference is so great that it takes many digits to write. In this paper Weinberg briefly recounts the history of the problem and reviews five approaches put forward to solve it; none was considered a full solution at the time. Observation and calculation are firm; an explanation is either absent or disputed. What gives one pause is that one of the greatest numerical mismatches in physics arises at the place where two well-established theories (quantum fields and general relativity) meet.

• Wigner 1961 — Wigner, Remarks on the mind-body question. I. J. Good (ed.), The Scientist Speculates, Heinemann, Londra, 1961; yeniden basım: Philosophical Reflections and Syntheses, Springer 1995, s. 247-260
  In brief, what does it say?

  Wigner proposed a thought experiment relating measurement to the observer's consciousness ('Wigner's friend') and an interpretation tied to it.

  Wigner's essay appeared in a collection titled 'The Scientist Speculates', and its title tells the nature of the piece: this is not an experiment but a piece of thinking. Wigner's question is at what point the measurement process within quantum mechanics must end in a single result.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Wilson 1971 — Wilson, Renormalization Group and Critical Phenomena. I. Renormalization Group and the Kadanoff Scaling Picture. Physical Review B 4, 3174-3183
  In brief, what does it say?

  Wilson combined the renormalization group, which describes scale change in critical phenomena, with Kadanoff's scaling picture.

  At first sight the paper is concerned not with field theory but with critical phenomena. When a fluid approaches its critical point or a magnet reaches the Curie temperature, the system's behavior begins to look similar at every scale and the measured quantities follow power laws carrying definite exponents. Kadanoff had proposed an idea to explain this: grouping neighboring spins into blocks and thinking of the block as a new "single spin".

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Wootters 1982 — Wootters, A single quantum cannot be cloned. Nature 299, 802
  In brief, what does it say?

  The question was: can a machine be built that duplicates an unknown quantum state exactly? Wootters and Zurek showed, starting from the linearity of the equations of quantum theory, that it cannot. That is, a perfect second copy of a state we do not know contradicts the theory. This no-cloning theorem is a theoretical result; it is a consequence not of a particular experiment but of the structure of the theory. Re-preparing a known state, however, is possible; the prohibition concerns the copy of an unknown state. We do not have the paper's own summary; the generally known framework is given here.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• Zurek 2003 — Zurek, Decoherence, einselection, and the quantum origins of the classical. Rev. Mod. Phys. 75, 715
  In brief, what does it say?

  Quantum theory describes the world of atoms, yet the tables and cats around us appear in 'either this or that' states, not both at once. How does this transition happen? Wojciech Zurek compiles studies arguing that it comes largely from interaction with the environment. The environment constantly 'watches' the system; because of this, the delicate coherence of states in which possibilities enter the calculation together (superposition) is disturbed, which is called decoherence. A few stable states that the environment does not disturb remain, and these appear like the states we see in the classical world; Zurek calls this 'einselection' (selection made by the environment). The theory and calculation here are undisputed; the real dispute is whether this explains why a single result emerges in measurement. That is, the article shows how the classical appearance arises, but does not close the 'single result' question for everyone.

• Zurek 2009 — Zurek, Quantum Darwinism. Nat. Phys. 5, 181
  In brief, what does it say?

  Zurek's review in Nature Physics presenting the idea of Quantum Darwinism as a whole.

  Zurek approaches the quantum measurement problem from a new angle: the robustness of the classical world may arise from a fragile quantum state leaving many copies of traces in the environment. According to the paper's abstract, Quantum Darwinism describes the multiplying records in the environment of the selected states of a system. Thanks to these records, the fragility of a single quantum system turns into a classical durability in the many records that carry correlations about that system.

• SEP: The Nobel Prize in Physics 2022 (press release) — Nobel Committee, The Nobel Prize in Physics 2022 (press release). nobelprize.org
  In brief, what does it say?

  The 2022 Nobel Prize in Physics was awarded to Alain Aspect, John Clauser and Anton Zeilinger. All three experimented with entangled photons: entanglement is when the outcomes of two particles, even when they are in separate places, turn out connected in a way that cannot be explained when taken one by one. The experiments showed that the Bell inequality is violated; that is, the results did not fit a local description of the kind in which particles carry definite, hidden properties before measurement. The prize citation also notes that these works opened the way for quantum information science. We do not have the prize announcement's own summary; only the generally known framework is given here.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• SEP: Uncertainty Principle — SEP, Uncertainty Principle. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  This is the entry on the uncertainty principle in the Stanford Encyclopedia of Philosophy; it is not an experiment but a well-organized introduction to the subject. Classical physics assumes that a definite value can be given to every property of a body at the same time. Quantum mechanics rejects this: the more precisely the position of a particle is stated, the more uncertain its velocity (momentum) remains, and the reverse also holds. The entry tells of the first and simple statement of this principle, the more rigorous forms that came later and the role it plays in philosophical debates. The following distinction stands out in particular: is the principle the disturbing effect of a measurement, or a property of the state itself? The entry shows that these two readings rest on different relations and that their consequences are separate. This is a review source; we report its verdict not in our own words but as far as it is told in the entry.

• SEP: Bell's Theorem — SEP, Bell's Theorem. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  Bell's theorem is not a single result but a family of results. All proceed like this: from the idea of 'local causality' (a thing is affected only by effects near it) and a few auxiliary assumptions, a limit is derived for the relations between the results of measurements made far apart. These limits are called Bell inequalities. Quantum theory predicts relations that exceed this limit in some cases, and experiments show that it is exceeded. Therefore no theory that rests on the idea of local causality and also accepts the auxiliary assumptions can produce the quantum predictions. This encyclopedia entry is a summary of theory and philosophy, not an experiment; which of the assumptions is to be given up (locality, freedom of measurement choice, or something else) is disputed. What gives one pause is that the result raises not the question 'is quantum theory right' but 'which of our intuitions shall we give up.'

• SEP: Decoherence — SEP, Decoherence. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  One of the best-known traces of quantum physics is interference: as in the double slit, probabilities strengthen or cancel one another. But in everyday life we see nothing like this. Decoherence theory examines how, when a system interacts with its environment, for example air, light or heat, the traces of interference are suppressed very quickly. The Stanford Encyclopedia of Philosophy entry tells this from two sides: first the mathematics of this suppression, second its relation to the measurement problem, the direction of time and the question of 'how the classical world emerges.' What is firm: that interaction with the environment practically destroys interference has also been shown by experiments. What is disputed: whether this is a solution to the measurement problem. Most authors say it is not enough by itself, because the result is still an 'all at once' state. The entry works through these distinctions together with the interpretive options.

• SEP: Copenhagen Interpretation — SEP, Copenhagen Interpretation. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  This is the Stanford Encyclopedia of Philosophy entry on the Copenhagen interpretation; the text provided consists only of the introduction. The introduction counts quantum mechanics as one of the most successful theories in history: it serves to calculate experimental results and to build advanced technology. But it is also a theory that strains our imagination; it seems to violate some principles of classical physics that have been part of common sense since the Renaissance. The Copenhagen interpretation is the first general attempt to understand the atomic world; its founder is mainly Niels Bohr, but Heisenberg and Born also contributed. The entry notes that Bohr and Heisenberg never fully agreed on how to understand the mathematical formalism, and that neither used the term 'Copenhagen interpretation' as a common name. The term is a label put on it later by those who opposed Bohr's idea of complementarity. The rest of the introduction is cut off in the text we have; we do not write the remainder.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• SEP: Many-Worlds Interpretation — SEP, Many-Worlds Interpretation. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  One of the answers to the question of what happens in measurement in quantum theory is the 'many-worlds interpretation'. According to this interpretation, as a result of measurement one of the possibilities is not selected while the others are erased; every possibility is realized in its own world and these worlds continue independently of one another. The interpretation does not put a 'collapse' rule added to the equations of the theory, and promises to remove randomness in measurement and action at a distance. This is not an experiment but an interpretation; it is disputed because it cannot be distinguished experimentally from the other interpretations. The entry describes the objections to the interpretation and its defenses, and hard questions such as what probability means in this framework. (This is an encyclopedia entry; for detail the source should be consulted.)

• SEP: Bohmian Mechanics — SEP, Bohmian Mechanics. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  Quantum theory tells only with probability where a particle will be. Bohmian mechanics (the de Broglie-Bohm theory, the 'pilot wave') proposes another way: the particle has a real position at every moment, and the wave function guides these particles. The rule is this: the wave evolves according to the usual quantum equation, and the velocity of the particles is determined by the wave. In this case, in the double-slit experiment the particle passes through only one of the slits, and where it will land is determined by its initial position and the wave; probability comes from our not knowing the initial positions. This is an interpretation (a variety of theory) and gives the same results as the usual quantum predictions; that is, it cannot be told apart by experiment. Its price: instantaneous effects between positions, that is, a nonlocal structure. This entry is an introduction from the Stanford Encyclopedia of Philosophy.

• SEP: Relational Quantum Mechanics — SEP, Relational Quantum Mechanics. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  This is the entry on relational quantum mechanics in the Stanford Encyclopedia of Philosophy; it is not an experiment but an introduction to an interpretation. The interpretation was put forward in 1996 and has drawn interest in recent years. Its starting point is this: the confusion in quantum theory is not resolved by adding a new equation, a new fact or an inaccessible reality; it is said that the theory proposes to us a radical renewal of our framework of thought. According to the entry, the role of the 'observer' in the traditional account can be played not only by large, classical bodies but by every physical system. Thus the value of a property is spoken of not by itself but together with which system it is relative to. This is a proposal for an interpretation; it cannot be told apart by experiment and is not accepted among physicists. The entry tells not that this is true but what it says.

• SEP: Quantum-Bayesian and Pragmatist Views of Quantum Theory — SEP, Quantum-Bayesian and Pragmatist Views of Quantum Theory. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  This entry describes the views that read quantum theory not as a description that tells the world directly, but as a description of our observation, our knowledge, or our interaction with the world. This idea has been expressed in various forms since the 1920s. The entry's focus is a newer branch, Quantum Bayesianism or QBism, and its influences from pragmatist thought. In QBism a quantum state is not the world itself but a belief that shows what a person making a measurement expects from the measurement (we write this as an interpretation of the entry's presentation). The entry also takes up the common objections to this view. This is not an experiment or a theory but a summary of the interpretation literature, and the debate over it continues. What gives one pause is that in this view the 'measurement result' and the 'person making the prediction' become inseparable.

• SEP: Collapse Theories — SEP, Collapse Theories. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  Quantum mechanics says that before measurement a system can be a combination of several states; but in measurement we see a single result. 'Collapse theories' try to solve this by changing the equation: let measurement not be a separate rule, let collapse be inside the law of physics, spontaneous and objective. The Stanford Encyclopedia of Philosophy entry tells this approach within the framework of the general problems raised by quantum mechanics: is there a world independent of the observer, can we obtain reliable knowledge of it, can some properties really belong to bodies? A section of the entry discusses particular models such as Ghirardi-Rimini-Weber. What is firm: these theories differ from quantum mechanics for now in a few predictions and attempts are being made to test them with experiments. What is disputed: whether they are true, and their superiority over other interpretations. This is a map of interpretations, not an experimental result.

• SEP: Philosophical Issues in Quantum Theory — SEP, Philosophical Issues in Quantum Theory. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  This is the entry in the Stanford Encyclopedia of Philosophy that offers a general overview of the philosophical problems raised by quantum theory. The text provided is only a one-sentence introduction: the piece is designed as a map of the philosophical issues the theory raises and aims to point to the deeper entries in the encyclopedia. We do not know from the text we have the detailed content of the entry, which problems it lists, or which view it takes; so we do not write them. This is not an experiment or a theory but a pointer source; it counts as a secondhand compilation. Its value for us is that it shows that interpretive questions in quantum theory have a place in the philosophical literature.

  This summary was written, because we could not reach the source's own abstract, only from the general knowledge we are sure of; it should be checked against the source.

• SEP: Quantum Entanglement and Information — SEP, Quantum Entanglement and Information. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  Two quantum systems can be so related that they cannot be described separately but only as a whole: entanglement. This entry describes how entanglement gives rise to relations with no counterpart in the classical world, that it can be measured, transformed and purified, and that it can be used like a kind of resource. Entangled pairs are used in computation and encryption tasks that cannot be done with classical systems. It also covers its relation to information theory and the debates about what entanglement points to. This is an encyclopedia entry: it presents experimental results, theory and interpretations separately. What gives one pause is that there is a shared wholeness between two things that are far apart and that there is no firm consensus on how this is to be read.

• SEP: Quantum Field Theory — SEP, Quantum Field Theory. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  This is the entry on quantum field theory in the Stanford Encyclopedia of Philosophy; it is not an experiment but an introduction to the theory and its philosophical questions. Quantum field theory is the conceptual and mathematical framework of today's high-energy physics, that is, particle physics; it is also used in other fields such as condensed matter physics. Roughly, it is the form of quantum mechanics that has passed from particles to fields: systems with an infinite number of degrees of freedom. The entry stresses that the picture of the world the theory gives, if taken seriously, does not agree with the classical understanding of particles and fields, nor even with some aspects of quantum mechanics. Among this is the fact that the very concept of 'particle' is problematic. The entry also tells of the theory's present standing and its place among other theories. This is a review source, that is, the interpretation should not be read as a firm verdict.

• SEP: Identity and Individuality in Quantum Theory — SEP, Identity and Individuality in Quantum Theory. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  The question: does quantum physics allow us to understand particles as individuals of which we can say 'this one' or 'that one' separately? According to the entry, the 'received view' that became widespread in the period when quantum theory was built says that elementary particles cannot be counted as individuals in this sense. On that basis special logical systems have been developed that cannot count individuals and that set up the concept of 'identity' differently. On the other hand, it has also been argued that quantum physics may still be compatible with a metaphysics that speaks of individuals; only in this case these individuals are so indistinguishable from one another that Leibniz's principle of the 'identity of indiscernibles' is violated. This claim has also met objections. The entry is not an experiment but a philosophical inquiry; the conclusions are not firm but disputed. What gives one pause is that the question 'which of the two electrons is which' may lose its meaning.

• SEP: Being and Becoming in Modern Physics — SEP, Being and Becoming in Modern Physics. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  Is time real, does it flow, do the past and the future exist as much as the present? These questions have been debated for more than two thousand years and there is no resolution. The Stanford Encyclopedia of Philosophy entry says that modern physics has given tools that sharpen these old questions, and puts its focus on that. A sample question: does special relativity show that there is no flow of time, or that the future too is as real as the present? Because in relativity the 'same moment' can change with the person looking, it seems everyone has no common 'now'; what follows from this is disputed. What is firm: the equations of relativity and the measurements of time dilation. What is disputed: what these say about metaphysics, that is, about whether time flows. The entry proceeds by deliberately separating these two areas and lists the arguments of different philosophers.

• SEP: The Kochen-Specker Theorem — SEP, The Kochen-Specker Theorem. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  The Kochen-Specker theorem shows that a certain kind of 'hidden variable' interpretation of quantum mechanics is impossible. The idea of hidden variables is this: even before a measurement is made, every measurable quantity has a definite value, and the measurement only brings it out. The theorem shows that in systems of three or more dimensions, if you try to assign values to all quantities at once and independently of the way of measuring, you fall into contradiction with the rules of quantum theory. This is a mathematical result, not an experiment, but some aspects of it have later been tested by experiments as well. An important detail: the theorem brings forward the notion of 'context', that is, that the value of a quantity may depend on which other measurements are made together with it. This entry presents the theorem at different depths; detailed proofs are in supplementary documents. What gives one pause is that the intuition 'measurement only reads what is there' cannot be kept in this form.

• SEP: Quantum Gravity — SEP, Quantum Gravity. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  General relativity describes gravity as the curvature of spacetime; quantum theory describes all the other forces. Quantum gravity is the search for a theory that will cover both, and it has been 'under construction' for more than a hundred years. What is expected is that it will describe how spacetime looks at very small scales (the Planck scale); but that scale is very far from our experiments, and for this reason it is hard to test the proposals by experiment, though some new approaches may change this. In most proposals the gravitational field is also quantized. The entry describes the different approaches and their philosophical questions, for example whether time and space are fundamental. This is a search for a theory; there is no settled result.

• SEP: Ibn 'Arabi — SEP, Ibn 'Arabi. Stanford Encyclopedia of Philosophy
  In brief, what does it say?

  According to the SEP entry, Ibn Arabi (1165-1240) was born in Murcia, and after going to Mecca settled in Damascus and died there; the entry says his writings cover not only Sufism but all the Islamic sciences, from Qur'anic exegesis to jurisprudence, from theology to philosophy. In describing his method it places the concept of tahqiq at the center: the entry reports tahqiq, as the opposite of taqlid, as giving everything its due and realizing the truth in itself (relying on Chittick). In the ontology section, according to what the entry reports, existence is al-Haqq, absolute (unconditioned) existence manifests itself as limited existence, and al-Haqq is known at once through both tanzih (incomparability) and tashbih (similarity). In the imagination section, according to the entry (citing Corbin), reason distinguishes and limits, while imagination sees the self-disclosure of al-Haqq; these are the two eyes of the heart. For the barzakh, the entry gives a description as that which both separates and joins two things and has no two faces of its own, and identifies the greatest barzakh with absolute imagination. The immutable entities, ta'ayyun (a term made current by Qunawi) and the perfect human are also presented in the entry under separate headings, mostly with translated quotations from Ibn Arabi's own texts. The entry is a peer-reviewed entry of the Stanford Encyclopedia of Philosophy, first published in 2008, with a substantial revision in December 2025; for this reason it can be read as a summary of the current academic literature. According to the entry's own cautions, Ibn Arabi himself never used the expression wahdat al-wujud (the unity of being) attributed to him, and this formula was attached to him later, first through Ibn Taymiyya's criticism; the entry also mentions that some studies read him as a forerunner of trends in physics or in Western philosophy, and notes that there are studies questioning the validity of such modern lenses and of perennialist readings.