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Journal articles on the topic 'Wigner’s friend'

Author: Grafiati

Published: 4 June 2021

Last updated: 20 February 2023

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1

Sokolovski, Dmitri, and Alexandre Matzkin. "Wigner’s Friend Scenarios and the Internal Consistency of Standard Quantum Mechanics." Entropy 23, no. 9 (September 9, 2021): 1186. http://dx.doi.org/10.3390/e23091186.

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Wigner’s friend scenarios involve an Observer, or Observers, measuring a Friend, or Friends, who themselves make quantum measurements. In recent discussions, it has been suggested that quantum mechanics may not always be able to provide a consistent account of a situation involving two Observers and two Friends. We investigate this problem by invoking the basic rules of quantum mechanics as outlined by Feynman in the well-known “Feynman Lectures on Physics”. We show here that these “Feynman rules” constrain the a priori assumptions which can be made in generalised Wigner’s friend scenarios, because the existence of the probabilities of interest ultimately depends on the availability of physical evidence (material records) of the system’s past. With these constraints obeyed, a non-ambiguous and consistent account of all measurement outcomes is obtained for all agents, taking part in various Wigner’s Friend scenarios.

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2

Castellani, Leonardo. "No Relation for Wigner’s Friend." International Journal of Theoretical Physics 60, no. 6 (May 24, 2021): 2084–89. http://dx.doi.org/10.1007/s10773-021-04826-9.

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3

d’Espagnat, Bernard. "Consciousness and the Wigner’s Friend Problem." Foundations of Physics 35, no. 12 (December 2005): 1943–66. http://dx.doi.org/10.1007/s10701-005-8656-1.

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4

Proietti, Massimiliano, Alexander Pickston, Francesco Graffitti, Peter Barrow, Dmytro Kundys, Cyril Branciard, Martin Ringbauer, and Alessandro Fedrizzi. "Experimental test of local observer independence." Science Advances 5, no. 9 (September 2019): eaaw9832. http://dx.doi.org/10.1126/sciadv.aaw9832.

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The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics the objectivity of observations is not so clear, most markedly exposed in Wigner’s eponymous thought experiment where two observers can experience seemingly different realities. The question whether the observers’ narratives can be reconciled has only recently been made accessible to empirical investigation, through recent no-go theorems that construct an extended Wigner’s friend scenario with four observers. In a state-of-the-art six-photon experiment, we realize this extended Wigner’s friend scenario, experimentally violating the associated Bell-type inequality by five standard deviations. If one holds fast to the assumptions of locality and free choice, this result implies that quantum theory should be interpreted in an observer-dependent way.

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5

Yang, Jianhao M. "Consistent Descriptions of Quantum Measurement." Foundations of Physics 49, no. 11 (October 24, 2019): 1306–24. http://dx.doi.org/10.1007/s10701-019-00305-8.

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Abstract The Wigner’s friend type of thought experiments manifest the conceptual challenge on how different observers can have consistent descriptions of a quantum measurement event. In this paper, we analyze the extended version of Wigner’s friend thought experiment (Frauchiger and Renner in Nat Commun 3711:9, 2018) in detail and show that the reasoning process from each agent that leads to the no-go theorem is inconsistent. The inconsistency is with respect to the requirement that an agent should make use of updated information instead of outdated information. We then apply the relational formulation of quantum measurement to resolve the inconsistent descriptions from different agents. In relational formulation of quantum mechanics, a measurement is described relative to an observer. Synchronization of measurement result is a necessary requirement to achieve consistent descriptions of a quantum system from different observers. Thought experiments, including EPR, Wigner’s Friend and it extended version, confirm the necessity of relational formulation of quantum measurement when applying quantum mechanics to composite system with entangled but space-like separated subsystems.

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DeBrota, John B., Christopher A. Fuchs, and Rüdiger Schack. "Respecting One’s Fellow: QBism’s Analysis of Wigner’s Friend." Foundations of Physics 50, no. 12 (August 18, 2020): 1859–74. http://dx.doi.org/10.1007/s10701-020-00369-x.

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7

Sudbery, Anthony. "Single-World Theory of the Extended Wigner’s Friend Experiment." Foundations of Physics 47, no. 5 (April 7, 2017): 658–69. http://dx.doi.org/10.1007/s10701-017-0082-7.

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8

Bong, Kok-Wei, Aníbal Utreras-Alarcón, Farzad Ghafari, Yeong-Cherng Liang, Nora Tischler, Eric G. Cavalcanti, Geoff J. Pryde, and Howard M. Wiseman. "A strong no-go theorem on the Wigner’s friend paradox." Nature Physics 16, no. 12 (August 17, 2020): 1199–205. http://dx.doi.org/10.1038/s41567-020-0990-x.

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9

Łukaszyk, Szymon. "Making Mistakes Saves the Single Observer’s World of the Extended Wigner’s Friend Experiment." Journal of Quantum Information Science 12, no. 01 (2022): 1–12. http://dx.doi.org/10.4236/jqis.2022.121001.

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10

Cavalcanti, Eric G., and Howard M. Wiseman. "Implications of Local Friendliness Violation for Quantum Causality." Entropy 23, no. 8 (July 21, 2021): 925. http://dx.doi.org/10.3390/e23080925.

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We provide a new formulation of the Local Friendliness no-go theorem of Bong et al. [Nat. Phys. 16, 1199 (2020)] from fundamental causal principles, providing another perspective on how it puts strictly stronger bounds on quantum reality than Bell’s theorem. In particular, quantum causal models have been proposed as a way to maintain a peaceful coexistence between quantum mechanics and relativistic causality while respecting Leibniz’s methodological principle. This works for Bell’s theorem but does not work for the Local Friendliness no-go theorem, which considers an extended Wigner’s Friend scenario. More radical conceptual renewal is required; we suggest that cleaving to Leibniz’s principle requires extending relativity to events themselves.

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11

Baumann, Veronika, Flavio Del Santo, Alexander R. H. Smith, Flaminia Giacomini, Esteban Castro-Ruiz, and Caslav Brukner. "Generalized probability rules from a timeless formulation of Wigner's friend scenarios." Quantum 5 (August 16, 2021): 524. http://dx.doi.org/10.22331/q-2021-08-16-524.

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The quantum measurement problem can be regarded as the tension between the two alternative dynamics prescribed by quantum mechanics: the unitary evolution of the wave function and the state-update rule (or "collapse") at the instant a measurement takes place. The notorious Wigner's friend gedankenexperiment constitutes the paradoxical scenario in which different observers (one of whom is observed by the other) describe one and the same interaction differently, one –the Friend– via state-update and the other –Wigner– unitarily. This can lead to Wigner and his friend assigning different probabilities to the outcome of the same subsequent measurement. In this paper, we apply the Page-Wootters mechanism (PWM) as a timeless description of Wigner's friend-like scenarios. We show that the standard rules to assign two-time conditional probabilities within the PWM need to be modified to deal with the Wigner's friend gedankenexperiment. We identify three main definitions of such modified rules to assign two-time conditional probabilities, all of which reduce to standard quantum theory for non-Wigner's friend scenarios. However, when applied to the Wigner's friend setup each rule assigns different conditional probabilities, potentially resolving the probability-assignment paradox in a different manner. Moreover, one rule imposes strict limits on when a joint probability distribution for the measurement outcomes of Wigner and his Friend is well-defined, which single out those cases where Wigner's measurement does not disturb the Friend's memory and such a probability has an operational meaning in terms of collectible statistics. Interestingly, the same limits guarantee that said measurement outcomes fulfill the consistency condition of the consistent histories framework.

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12

Elouard, Cyril, Philippe Lewalle, Sreenath K. Manikandan, Spencer Rogers, Adam Frank, and Andrew N. Jordan. "Quantum erasing the memory of Wigner's friend." Quantum 5 (July 8, 2021): 498. http://dx.doi.org/10.22331/q-2021-07-08-498.

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The Wigner's friend paradox concerns one of the most puzzling problems of quantum mechanics: the consistent description of multiple nested observers. Recently, a variation of Wigner's gedankenexperiment, introduced by Frauchiger and Renner, has lead to new debates about the self-consistency of quantum mechanics. At the core of the paradox lies the description of an observer and the object it measures as a closed system obeying the Schrödinger equation. We revisit this assumption to derive a necessary condition on a quantum system to behave as an observer. We then propose a simple single-photon interferometric setup implementing Frauchiger and Renner's scenario, and use the derived condition to shed a new light on the assumptions leading to their paradox. From our description, we argue that the three apparently incompatible properties used to question the consistency of quantum mechanics correspond to two logically distinct contexts: either one assumes that Wigner has full control over his friends' lab, or conversely that some parts of the labs remain unaffected by Wigner's subsequent measurements. The first context may be seen as the quantum erasure of the memory of Wigner's friend. We further show these properties are associated with observables which do not commute, and therefore cannot take well-defined values simultaneously. Consequently, the three contradictory properties never hold simultaneously.

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13

Muciño, R., and E. Okon. "Wigner's convoluted friends." Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 72 (November 2020): 87–90. http://dx.doi.org/10.1016/j.shpsb.2020.07.001.

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14

Schumann, Thomas G. "Schrödinger's cat and Wigner's friend." Physics Essays 29, no. 4 (December 26, 2016): 482–84. http://dx.doi.org/10.4006/0836-1398-29.4.482.

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15

Albert, David Z., and Hilary Putnam. "Further adventures of Wigner's friend." Topoi 14, no. 1 (March 1995): 17–22. http://dx.doi.org/10.1007/bf00763474.

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16

Matzkin, A., and D. Sokolovski. "Wigner's friend, Feynman's paths and material records." EPL (Europhysics Letters) 131, no. 4 (September 3, 2020): 40001. http://dx.doi.org/10.1209/0295-5075/131/40001.

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17

Belinsky, Aleksandr V. "Wigner's friend paradox: does objective reality not exist?" Uspekhi Fizicheskih Nauk 190, no. 12 (May 2020): 1335–42. http://dx.doi.org/10.3367/ufnr.2020.05.038767.

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18

Thaheld, Fred H. "A Feasible Experiment Concerning the Schrödinger's Cat and Wigner's Friend Paradoxes." Physics Essays 14, no. 2 (June 2001): 164–70. http://dx.doi.org/10.4006/1.3025477.

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19

Moreno, George, Ranieri Nery, Cristhiano Duarte, and Rafael Chaves. "Events in quantum mechanics are maximally non-absolute." Quantum 6 (August 24, 2022): 785. http://dx.doi.org/10.22331/q-2022-08-24-785.

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The notorious quantum measurement problem brings out the difficulty to reconcile two quantum postulates: the unitary evolution of closed quantum systems and the wave-function collapse after a measurement. This problematics is particularly highlighted in the Wigner's friend thought experiment, where the mismatch between unitary evolution and measurement collapse leads to conflicting quantum descriptions for different observers. A recent no-go theorem has established that the (quantum) statistics arising from an extended Wigner's friend scenario is incompatible when one try to hold together three innocuous assumptions, namely no-superdeterminism, parameter independence and absoluteness of observed events. Building on this extended scenario, we introduce two novel measures of non-absoluteness of events. The first is based on the EPR2 decomposition, and the second involves the relaxation of the absoluteness hypothesis assumed in the aforementioned no-go theorem. To prove that quantum correlations can be maximally non-absolute according to both quantifiers, we show that chained Bell inequalities (and relaxations thereof) are also valid constraints for Wigner's experiment.

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20

Baumann, Veronika, and Stefan Wolf. "On Formalisms and Interpretations." Quantum 2 (October 15, 2018): 99. http://dx.doi.org/10.22331/q-2018-10-15-99.

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One of the reasons for the heated debates around the interpretations of quantum theory is a simple confusion between the notions of formalism versus interpretation. In this note, we make a clear distinction between them and show that there are actually two inequivalent quantum formalisms, namely the relative-state formalism and the standard formalism with the Born and measurement-update rules. We further propose a different probability rule for the relative-state formalism and discuss how Wigner's-friend-type experiments could show the inequivalence with the standard formalism. The feasibility in principle of such experiments, however, remains an open question.

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21

Sokolovski, D., and E. Akhmatskaya. "Wigner's friends, tunnelling times and Feynman's “only mystery of quantum mechanics”." Europhysics Letters 136, no. 2 (October 1, 2021): 20001. http://dx.doi.org/10.1209/0295-5075/ac3cd0.

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Abstract Recent developments in elementary quantum mechanics have seen a number of extraordinary claims regarding quantum behaviour, and even questioning internal consistency of the theory. These are, we argue, different disguises of what Feynman described as quantum theory's “only mystery”.

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22

Belinsky, A. V. "Objective Reality and the Paradox of Wigner Friends." Optics and Spectroscopy 128, no. 9 (September 2020): 1421–24. http://dx.doi.org/10.1134/s0030400x20090039.

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23

Seyfarth, U., A. B. Klimov, H. de Guise, G. Leuchs, and L. L. Sanchez-Soto. "Wigner function for SU(1,1)." Quantum 4 (September 7, 2020): 317. http://dx.doi.org/10.22331/q-2020-09-07-317.

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In spite of their potential usefulness, Wigner functions for systems with SU(1,1) symmetry have not been explored thus far. We address this problem from a physically-motivated perspective, with an eye towards applications in modern metrology. Starting from two independent modes, and after getting rid of the irrelevant degrees of freedom, we derive in a consistent way a Wigner distribution for SU(1,1). This distribution appears as the expectation value of the displaced parity operator, which suggests a direct way to experimentally sample it. We show how this formalism works in some relevant examples.Dedication: While this manuscript was under review, we learnt with great sadness of the untimely passing of our colleague and friend Jonathan Dowling. Through his outstanding scientific work, his kind attitude, and his inimitable humor, he leaves behind a rich legacy for all of us. Our work on SU(1,1) came as a result of long conversations during his frequent visits to Erlangen. We dedicate this paper to his memory.

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24

Green, HS, and T. Triffet. "Quantum Mechanics, Real and Artificial Intelligence." Australian Journal of Physics 44, no. 3 (1991): 323. http://dx.doi.org/10.1071/ph910323.

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Some incompletely resolved problems in the quantal theories of measurement and observation are discussed with reference to Schrodinger's 'cat paradox' and the paradox of Wigner's friend. A simple version of the theory of measurement is presented, which does not completely resolve these paradoxes but suggests the need for an objective quantal description of the process of observation, and the formation of memory, of an event originating at the microscopic level, by an animal or artificial intelligence. A quantised model is then developed to simulate the function of the cerebral cortex in the formation of memory of sensory impressions, with macroscopic observables expressed in terms of parafermion operators of very large order. A letter from Schrodinger, which corrects some published versions of his paradox, is presented as well as a short account of the simulated formation of long-term memory by the model in an appendix.

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25

Lennox, John. "Science and faith: Friendly allies, not hostile enemies." Theofilos 12, no. 1 (December 15, 2020): 162–65. http://dx.doi.org/10.48032/theo/12/1/11.

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Contemporary science is a wonderfully collaborative activity. It knows no barriers of geography, race, or creed. At its best, it enables us to wrestle with the problems that beset humanity, and we rightly celebrate when an advance is made that brings relief to millions. I have spent my life as a pure mathematician, and I often reflect on what physics Nobel Prize–winner Eugene Wigner called “the unreasonable effectiveness of mathematics.” How is it that equations created in the head of a mathematician can relate to the universe outside that head? This question prompted Albert Einstein to say, “The only incomprehensible thing about the universe is that it is comprehensible.” The very fact that we believe that science can be done is a thing to be wondered at.

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26

Seitz, Frederick. "John von Neumann and Materials Science." MRS Bulletin 19, no. 3 (March 1994): 60–62. http://dx.doi.org/10.1557/s0883769400039762.

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I presume that many of you will be surprised to learn that John von Neumann, a great mathematician, and a remarkable man, had even a passing interest in materials science since his name is usually associated with work in function theory, mathematical logic, the mathematical foundations of quantum mechanics, game theory, and of course with the development of computer logic. Actually, he was deeply interested in the evolution of materials science and technology, and he played an important role in giving prominence to materials science at a critical time in its evolution. But first let me tell you a little about his life.John von Neumann was born in Budapest, Hungary, in 1903 into a prominent business family. His mathematical genius was recognized early by his high school teacher, Lázsló Ratz, who insisted that he receive special tutoring since mathematical geniuses tend to flower early. He became a close and, indeed, a lifetime friend of a slightly older fellow student, Eugene Wigner, who was inspired by the same mathematics teacher. The two of them frequently wandered home together after school, with von Neumann providing a tutorial on some aspect of mathematics while Wigner, who had a comparably brilliant mind, absorbed everything. The two students had different personalities, but shared a great love of mathematics.Von Neumann was never a narrow genius. He soaked up knowledge of all kinds rapidly and was exposed to much because the von Neumann family dinner gatherings were devoted to discussions of technical, historical and cultural affairs, as well as business. This great versatility in interests was a characteristic trademark of von Neumann‘s entire life.

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27

de la Hamette, Anne-Catherine, and Thomas D. Galley. "Quantum reference frames for general symmetry groups." Quantum 4 (November 30, 2020): 367. http://dx.doi.org/10.22331/q-2020-11-30-367.

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A fully relational quantum theory necessarily requires an account of changes of quantum reference frames, where quantum reference frames are quantum systems relative to which other systems are described. By introducing a relational formalism which identifies coordinate systems with elements of a symmetry group G, we define a general operator for reversibly changing between quantum reference frames associated to a group G. This generalises the known operator for translations and boosts to arbitrary finite and locally compact groups, including non-Abelian groups. We show under which conditions one can uniquely assign coordinate choices to physical systems (to form reference frames) and how to reversibly transform between them, providing transformations between coordinate systems which are `in a superposition' of other coordinate systems. We obtain the change of quantum reference frame from the principles of relational physics and of coherent change of reference frame. We prove a theorem stating that the change of quantum reference frame consistent with these principles is unitary if and only if the reference systems carry the left and right regular representations of G. We also define irreversible changes of reference frame for classical and quantum systems in the case where the symmetry group G is a semi-direct product G=N⋊P or a direct product G=N×P, providing multiple examples of both reversible and irreversible changes of quantum reference system along the way. Finally, we apply the relational formalism and changes of reference frame developed in this work to the Wigner's friend scenario, finding similar conclusions to those in relational quantum mechanics using an explicit change of reference frame as opposed to indirect reasoning using measurement operators.

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28

Brukner, Časlav. "Wigner’s friend and relational objectivity." Nature Reviews Physics, September 6, 2022. http://dx.doi.org/10.1038/s42254-022-00505-8.

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29

Weststeijn, Nikki. "Wigner’s friend and Relational Quantum Mechanics: A Reply to Laudisa." Foundations of Physics 51, no. 4 (August 2021). http://dx.doi.org/10.1007/s10701-021-00487-0.

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AbstractRelational Quantum Mechanics is an interpretation of quantum mechanics proposed by Carlo Rovelli. Rovelli argues that, in the same spirit as Einstein’s theory of relativity, physical quantities can only have definite values relative to an observer. Relational Quantum Mechanics is hereby able to offer a principled explanation of the problem of nested measurement, also known as Wigner’s friend. Since quantum states are taken to be relative states that depend on both the system and the observer, there is no inconsistency in the descriptions of the observers. Federico Laudisa has recently argued, however, that Rovelli’s description of Wigner’s friend is ambiguous, because it does not take into account the correlation between the observer and the quantum system. He argues that if this correlation is taken into account, the problem with Wigner’s friend disappears and, therefore, a relativization of quantum states is not necessary. I will show that Laudisa’s criticism is not justified. To the extent that the correlation can be accurately reflected, the problem of Wigner’s friend remains. An interpretation of quantum mechanics that provides a solution to it, like Relational Quantum Mechanics, is therefore a welcome one.

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30

Lostaglio, Matteo, and Joseph Bowles. "The original Wigner’s friend paradox within a realist toy model." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 477, no. 2254 (October 2021). http://dx.doi.org/10.1098/rspa.2021.0273.

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The original Wigner’s friend paradox is a gedankenexperiment involving an observer described by an external agent. The paradox highlights the tension between unitary evolution and collapse in quantum theory, and is sometimes taken as requiring a reassessment of the notion of objective reality. In this note, however, we present a classical toy model in which (i) the contradicting predictions at the heart of the thought experiment are reproduced (ii) every system is in a well-defined state at all times. The toy model shows how puzzles such as Wigner’s friend’s experience of being in a superposition, conflicts between different agents’ descriptions of the experiment, the positioning of the Heisenberg’s cut and the apparent lack of objectivity of measurement outcomes can be explained within a classical model where there exists an objective state of affairs about every physical system at all times. Within the model, the debate surrounding the original Wigner’s friend thought experiment and its resolution have striking similarities with arguments concerning the nature of the second law of thermodynamics. The same conclusion however does not apply to more recent extensions of the gedankenexperiment featuring multiple encapsulated observers, and shows that such extensions are indeed necessary avoid simple classical explanations.

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31

Leegwater, Gijs. "When Greenberger, Horne and Zeilinger Meet Wigner’s Friend." Foundations of Physics 52, no. 4 (June 27, 2022). http://dx.doi.org/10.1007/s10701-022-00586-6.

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AbstractA general argument is presented against relativistic, unitary, single-outcome quantum mechanics. This is achieved by combining the Wigner’s Friend thought experiment with measurements on a Greenberger–Horne–Zeilinger (GHZ) state, and describing the evolution of the quantum state in various inertial frames. Assuming unitary quantum mechanics and single outcomes, the result is that the Born rule must be violated in some inertial frame: in that frame, outcomes are obtained for which no corresponding term exists in the pre-measurement wavefunction.

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32

Kastner, R. E. "Unitary Interactions Do Not Yield Outcomes: Attempting to Model “Wigner’s Friend”." Foundations of Physics 51, no. 4 (August 2021). http://dx.doi.org/10.1007/s10701-021-00492-3.

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33

Ramakrishna, Satish. "A microscopic model of wave-function dephasing and decoherence in the double-slit experiment." Scientific Reports 11, no. 1 (October 25, 2021). http://dx.doi.org/10.1038/s41598-021-99995-2.

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AbstractThe act of measurement on a quantum state is supposed to “dephase” (dephasing refers to the phenomenon that the states lose phase coherence; then the phases get randomized in interaction with a bath of other oscillators, which is referred to as “decoherence”), then “decohere” and “collapse” (or more precisely “register” and “reduce”) the state into one of several eigenstates of the operator corresponding to the observable being measured. This measurement process is sometimes described as outside standard quantum-mechanical evolution and not calculable from Schrödinger’s equation. Progress has, however, been made in studying this problem with two main calculation tools—one uses a time-independent Hamiltonian, while a rather more general approach proving that decoherence occurs under some generic conditions. The two general approaches to the study of wave-function collapse are as follows. The first approach, called the “consistent” or “decoherent”’ histories approach, studies microscopic histories that diverge probabilistically and explains collapse as an event in our particular history. The other, referred to as the “environmental decoherence” approach studies the effect of the environment upon the quantum system, to explain wave-function decoherence which is produced by irreversible effects of various sorts. However, as we know, wave-function collapse is not related to thermal connection with the environment, rather, it is inherent to how measurements are performed by macroscopic apparata. In the “environmental decoherence” approach, one studies decoherence using a Markovian-approximated Master equation to study the time-evolution of the reduced density matrix (post dephasing) and obtains the long-time dependence of the off-diagonal elements of this matrix. The calculation in this paper studies the evolution of a quantum system starting with “dephasing” followed by the effects of the environment with some differences from prior analyses. We start from the Schrödinger equation for the state of the system, with a time-dependent Hamiltonian that reflects the actual microscopic interactions that are occurring. Then we systematically solve (exactly) for the time-evolved state, without invoking a Markovian approximation when writing out the effective time-evolution equation, i.e., keeping the evolution unitary until the end. This approach is useful, and it shows that the system wave-function will explicitly “un-collapse” if the measurement apparatus is sufficiently small. However, in the limit of a macroscopic system, this “dephasing” quickly leads to “decoherence”—collapse is a temporary state that will simply take extremely long (of the order of multiple universe lifetimes) to reverse. This has been attempted previously and our calculation is particularly simple and calculable. We make some connections to the work by Linden et al. while doing so. The calculation in this paper has interesting implications for the interpretation of the Wigner’s friend experiment, as well as the Mott experiment, which is explored in “Connection to some general theoretical results” and “Recurrence times” (especially the enumerated points in “Recurrence times”). The upshot is that as long as Wigner’s friend is macroscopically large (or uses a macroscopically large measuring instrument), no one needs to worry that Wigner would see something different from his friend. Indeed, Wigner’s friend does not even need to be conscious during the measurement that she conducts. It also allows one to reasonably interpret some of the more recent thought experiments proposed. In particular, as a result of the mathematical analysis, the short-time behavior of a collapsing system, at least the one considered in this paper, is not exponential. Instead, it is the usual Fermi-golden rule result. The long-term behavior is, of course, still exponential. This is a second novel feature of the paper—we connect the short-term Fermi-golden rule (quadratic-in-time behavior) transition probability to the exponential long-time behavior of a collapsing wave-function in one continuous mathematical formulation.

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34

Hausmann, Ladina, Nuriya Nurgalieva, and Lidia del Rio. "Toys can’t play: physical agents in Spekkens' theory." New Journal of Physics, January 17, 2023. http://dx.doi.org/10.1088/1367-2630/acb3ef.

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Abstract Information is physical, and for a physical theory to be universal, it should model observers as physical systems, with concrete memories where they store the information acquired through experiments and reasoning. Here we address these issues in Spekkens’ toy theory, a non-contextual epistemically restricted model that partially mimics the behaviour of quantum mechanics. We propose a way to model physical implementations of agents, memories, measurements, conditional actions and information processing. We find that the actions of toy agents are severely limited: although there are non-orthogonal states in the theory, there is no way for physical agents to consciously prepare them. Their memories are also constrained: agents cannot forget in which of two arbitrary states a system is. Finally, we formalize the process of making inferences about other agents’ experiments and model multi-agent experiments like Wigner’s friend. Unlike quantum theory or box world, in the toy theory there are no inconsistencies when physical agents reason about each other’s knowledge.

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35

Dieks, Dennis. "Perspectival Quantum Realism." Foundations of Physics 52, no. 4 (August 2022). http://dx.doi.org/10.1007/s10701-022-00611-8.

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AbstractThe theories of pre-quantum physics are standardly seen as representing physical systems and their properties. Quantum mechanics in its standard form is a more problematic case: here, interpretational problems have led to doubts about the tenability of realist views. Thus, QBists and Quantum Pragmatists maintain that quantum mechanics should not be thought of as representing physical systems, but rather as an agent-centered tool for updating beliefs about such systems. It is part and parcel of such views that different agents may have different beliefs and may assign different quantum states. What results is a collection of agent-centered perspectives rather than a unique representation of the physical world. In this paper we argue that the problems identified by QBism and Quantum Pragmatism do not necessitate abandoning the ideal of representing the physical world. We can avail ourselves of the same puzzle-solving strategies as employed by QBists and pragmatists by adopting a perspectival quantum realism. According to this perspectivalism (close to the relational interpretation of quantum mechanics) objects may possess different, but equally objective properties with respect to different physically defined perspectives. We discuss two options for such a perspectivalism, a local and a nonlocal one, and apply them to Wigner’s friend and EPR scenarios. Finally, we connect quantum perspectivalism to the recently proposed philosophical position of fragmentalism.

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36

Relaño, Armando. "Decoherence framework for Wigner's-friend experiments." Physical Review A 101, no. 3 (March 16, 2020). http://dx.doi.org/10.1103/physreva.101.032107.

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Rossi, Vinicius P., and Diogo O. Soares-Pinto. "Wigner's friend and the quasi-ideal clock." Physical Review A 103, no. 5 (May 14, 2021). http://dx.doi.org/10.1103/physreva.103.052206.

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38

Belinsky, Aleksandr V. "Wigner's friend paradox: does objective reality not exist?" Physics-Uspekhi 63, no. 12 (May 2020). http://dx.doi.org/10.3367/ufne.2020.05.038767.

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Matzkin, A., and D. Sokolovski. "Wigner-friend scenarios with noninvasive weak measurements." Physical Review A 102, no. 6 (December 4, 2020). http://dx.doi.org/10.1103/physreva.102.062204.

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40

Trassinelli, M. "Conditional probabilities of measurements, quantum time, and the Wigner's-friend case." Physical Review A 105, no. 3 (March 28, 2022). http://dx.doi.org/10.1103/physreva.105.032213.

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41

Żukowski, Marek, and Marcin Markiewicz. "Physics and Metaphysics of Wigner’s Friends: Even Performed Premeasurements Have No Results." Physical Review Letters 126, no. 13 (April 2, 2021). http://dx.doi.org/10.1103/physrevlett.126.130402.

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Hance, Jonte R., and Sabine Hossenfelder. "The wave function as a true ensemble." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 478, no. 2262 (June 2022). http://dx.doi.org/10.1098/rspa.2021.0705.

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In quantum mechanics, the wave function predicts probabilities of possible measurement outcomes, but not which individual outcome is realized in each run of an experiment. This suggests that it describes an ensemble of states with different values of a hidden variable. Here, we analyse this idea with reference to currently known theorems and experiments. We argue that the ψ-ontic/epistemic distinction fails to properly identify ensemble interpretations and propose a more useful definition. We then show that all local ψ-ensemble interpretations which reproduce quantum mechanics violate statistical independence. Theories with this property are commonly referred to as superdeterministic or retrocausal. Finally, we explain how this interpretation helps make sense of some otherwise puzzling phenomena in quantum mechanics, such as the delayed choice experiment, the Elitzur–Vaidman bomb detector and the extended Wigner’s friends scenario.

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Xu, Zhen-Peng, Jonathan Steinberg, H. Chau Nguyen, and Otfried Gühne. "No-go theorem based on incomplete information of Wigner about his friend." Physical Review A 107, no. 2 (February 14, 2023). http://dx.doi.org/10.1103/physreva.107.022424.

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