Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session X33: Quantum Foundations IIFocus Live

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Sponsoring Units: DQI Chair: Jayne Thompson, Natl Univ of Singapore 
Friday, March 19, 2021 8:00AM  8:12AM Live 
X33.00001: Certification of Maximum Confidence Quantum Measurements and Their Contextual Advantages Kieran Flatt, Hanwool Lee, Joonwoo Bae

Friday, March 19, 2021 8:12AM  8:24AM Live 
X33.00002: Answering Mermin's Challenge with the Relativity Principle: An Underlying Coherence Between SR and QM William Stuckey In 1981, Mermin published a paper in American Journal of Physics that Feynman called, “One of the most beautiful papers in physics that I know.” Therein, Mermin explained the mystery of quantum entanglement per the Bell spin states to the “general reader” without recourse to the formalism of quantum mechanics. He then challenged the “physicist reader” to resolve the mystery in equally accessible terms. In this talk, I will show how the relativity principle, i.e., “no preferred reference frame” (NPRF), answers Mermin’s challenge. Accordingly, NPRF with the fundamental constants c and h, renders special relativity (SR) and nonrelativistic quantum mechanics (QM) mutually coherent and comprehensive, contrary to what some believe. Essentially, NPRF applied to the speed of light c leads to the counterintuitive aspects (“mysteries”) of time dilation and length contraction for SR, and NPRF applied to Planck’s constant h leads to the “mysteries” of Bell state entanglement and the Tsirelson bound for QM. Thus, we see an underlying coherence and integrity between SR and QM via its “mysteries” stemming from the relativity principle. 
Friday, March 19, 2021 8:24AM  8:36AM Live 
X33.00003: Block Universe Ontological Models: A Framework for Realist Accounts of Quantum Theory That Allows for Retrocausality Matthew Leifer In light of Bell's theorem, it has been suggested that one way of maintaining locality is to violate the measurement independence assumption, i.e. to allow the hidden variables to depend on the choice of measurement settings. This is variously viewed as retrocausality, superdeterminism, or a violation of free choice. However, when we drop measurement independence, it is not clear that the other assumption of Bell's theorem  local causality  still captures the intended sense of locality. I will argue that it does not and that, in this context, it needs to be upgraded to a stronger assumption called Λmediation. I derive how probabilities should decompose in a model that satisfies Λmediation, but not measurement independence, and show that such models can reproduce the correlations in a Bell experiment. The Λmediation assumption can be generalized to arbitrary causal structures, and the corresponding decomposition of probabilities can be derived, leading to a graphical model that has some similarities to Markov Networks and Factor Graphs. 
Friday, March 19, 2021 8:36AM  8:48AM Live 
X33.00004: Agreement between observers, a physical principle? Adam Brandenburger, Patricia Contreras Tejada, Aleksander M. Kubicki, Pierfrancesco La Mura, Giannicola Scarpa Is the world quantum? An active research line in quantum foundations is devoted to exploring what constraints can rule out the postquantum theories that are consistent with experimentally observed results. We explore this question in the context of epistemics, and ask whether agreement between observers can serve as a physical principle that must hold for any theory of the world. The seminal Agreement Theorem by Aumann (Annals of Statistics, 1976) states that two (classical) agents cannot agree to disagree. We examine the extension of this theorem to nosignalling settings. In particular, we establish an Agreement Theorem for quantum agents. We also construct examples of (postquantum) nosignalling boxes where agents can agree to disagree. The PR box is an extremal instance of this phenomenon. These results make it plausible that agreement might be a physical principle, while they also establish links between the fields of epistemics and quantum information that seem worthy of further exploration. 
Friday, March 19, 2021 8:48AM  9:00AM Live 
X33.00005: A nogo theorem for the persistent reality of Wigner's friend's perception Philippe Allard Guerin, Veronika Baumann, Flavio Del Santo, Caslav Brukner The notorious Wigner's friend thought experiment has in recent years received renewed interest especially due to new arguments that force us to question some of the fundamental assumptions of quantum theory. In this work we formulate a nogo theorem for the persistent reality of Wigner's friend's perception, which allows us to conclude that the perceptions that the friend has of her own measurement outcomes at different times cannot "share the same reality", if seemingly natural quantum mechanical assumptions are met. We show that there is no joint probability distribution for the friend's perceived measurement outcomes at two different times, that depends linearly on the initial state of the measured system and whose marginals reproduce the predictions of unitary quantum theory. This theorem entails that one must either (1) propose a nonlinear modification of the Born rule for twotime predictions, (2) sometimes prohibit the use of present information to predict the future thereby reducing the predictive power of quantum theory or (3) deny that unitary quantum mechanics makes valid singletime predictions for all observers. We briefly discuss which of the theorem's assumptions are more likely to be dropped within various popular interpretations of quantum mechanics. 
Friday, March 19, 2021 9:00AM  9:12AM Live 
X33.00006: Geometric tools for quantum information science Fabio Anza Geometric Quantum Mechanics (GQM) is an approach to quantum mechanics which focuses on the geometric aspects of the manifold of states, rather than on the vector properties of the Hilbert space. I will discuss how GQM inspires a novel set of tools and results to study the phenomenology of quantum systems and their informationtheoretic aspects. Particular attention will be devoted to discuss (1) a new notion of statistical state of a quantum system (2) a novel approach to the thermodynamics of quantum systems and (3) a new notion of quantum entropy. 
Friday, March 19, 2021 9:12AM  9:24AM Live 
X33.00007: A mathematical framework for operational fine tunings Lorenzo Catani, Matthew Leifer In the framework of ontological models, the features of quantum 
Friday, March 19, 2021 9:24AM  10:00AM Live 
X33.00008: Probability arises from entropy in axiomatic information thermodynamics Invited Speaker: Michael Westmoreland Axiomatic information thermodynamics (Entropy 2018, 20(4), 237) is a formal mathematical system that describes thermodynamic processes including those in which information is acquired, transformed or discarded. A central concept in the theory is the "eidostate", which is a simple array of possible states, including both thermodynamic macrostates and information records. Wellmotivated axioms lead to an entropy function that describes the irreversibility of eidostate processes, and this function in turn leads to a unique probability measure for the states and records in a uniform eidostate. Does this development shed light on the kinds of reasoning available to physical agents that observe and intervene in the quantum world? 
Friday, March 19, 2021 10:00AM  10:12AM Live 
X33.00009: Conditions tighter than noncommutation needed for nonclassicality David ArvidssonShukur, Jacob Chevalier Drori, Nicole Yunger Halpern Kirkwood discovered in 1933, and Dirac discovered in 1945, a representation of quantum states that has undergone a renaissance recently. The KirkwoodDirac (KD) distribution has been employed to study nonclassicality across quantum physics, from metrology to chaos to the foundations of quantum theory. The KD distribution is a quasiprobability distribution, a quantum generalization of a probability distribution, which can behave nonclassically by having negative or nonreal elements. Negative KD elements signify quantum information scrambling and potential metrological quantum advantages. Nonreal elements encode measurement disturbance and thermodynamic nonclassicality. KD distributions' nonclassicality has been believed to follow necessarily from noncommutation of operators. We show that noncommutation does not suffice. We prove sufficient conditions for the KD distribution to be nonclassical (equivalently, necessary conditions for it to be classical). We also quantify the KD nonclassicality achievable under various conditions. This work resolves longstanding questions about nonclassicality and may be used to engineer quantum advantages. 
Friday, March 19, 2021 10:12AM  10:24AM Live 
X33.00010: Quantum and Classical Bayesian Agents John DeBrota, Peter J Love The paradigm of multiple interacting agents who each utilize quantum mechanics is central to much of the recent activity in quantum foundations and quantum information. Relevant work in the latter broadly seeks to identify what advantages quantum theory affords us in the multiagent setting, but, as recent foundational work highlights, there remains confusion over how to appropriately work in such a setting so as to avoid inconsistencies. A careful and consistent treatment of multiple users of quantum theory would allow us to properly interpret extant work and may suggest new approaches in other areas such as algorithm design. The natural starting point for an agentbased treatment is given by the Quantum Bayesian (QBist) approach. Adopting this, we outline a general approach to modeling rational decision making agents who adopt a normative constraint on expectations for the consequences of hypothetical actions. Agents who universally adopt quantum or classical mechanics in all of their reasoning are seen to be special cases of this approach. As an illustration, we consider quantum and classical agents in two physical scenarios: agents receiving data from an exogenous, nonagential, source and two or more agents interacting with each other. 
Friday, March 19, 2021 10:24AM  10:36AM Live 
X33.00011: Retrocausal model of reality for quantum fields Margaret Reid, Peter Drummond We show that one may interpret physical reality as random fields in spacetime. These have a probability given by the expectation of a coherent state projection operator, called the Qfunction. The resulting dynamical evolution includes retrocausal effects. This suggests that a physical universe exists without requiring observers, but with a welldefined probability for its field configuration. By including the meter dynamics, we show that field trajectories have quantum measurement properties without wavefunction collapse, including sharp measured 
Friday, March 19, 2021 10:36AM  10:48AM Live 
X33.00012: Quantum Fields from Quantum Cellular Automata Todd Brun, Leonard Mlodinow It has been shown that quantum walks on a lattice can give rise to relativistic wave equations like the Dirac equation in the long wavelength limit. To go to multiple particles and describe a quantum field theory in discrete spacetime, a quantum cellular automaton (QCA) is a natural choice. But can a QCA, with purely local dynamics, give rise to a fermionic field theory at long wavelengths? A straightforward QCA construction can work in one spatial dimension, but we prove that this same approach cannot succeed in two or three dimensions. To evade this nogo result, we construct a QCA in a different way, confining distinguishable particles in a totally antisymmetrized subspace, and show that this recovers Dirac field theory at long wavelengths. We show that an analogous construction for bosons can produce Maxwell's theory, and discuss the issues in going from free to interacting fields. 
Friday, March 19, 2021 10:48AM  11:00AM Live 
X33.00013: The WaveFunction Must Be PsiOntic Mario Hubert The PBRtheorem aimed at proving that the wavefunction has to represent objective features of a single physical system. There have been many attempts to interpret the wavefunction as not representing the objective physical state of a quantum system by abandoning one of the two explicit assumptions of the PBRtheorem: (i) the existence of objective physical states and (ii) preparation independence. I argue that each theory that violates either of these assumptions meets unsurmountable problems. Although these alternative theories are physically possible, they are for several reasons implausible and problematic. I therefore advocate to search for quantum theories that fulfill the assumptions of the PBRtheorem. 
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