Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L26: Quantum Foundations IFocus

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Sponsoring Units: DQI Chair: Ravi Kunjwal, Perimeter Inst for Theo Phys Room: LACC 404A 
Wednesday, March 7, 2018 11:15AM  11:51AM 
L26.00001: Quantum Theory of the Classical Invited Speaker: Wojciech Zurek I will describe three insights into the transition from quantum to classical. After a brief discussion of decoherence I will give (i) a minimalist (and decoherencefree) derivation of preferred states. Such pointer states define events (e.g., measurement outcomes) without appealing to Born's rule. Probabilities and (ii) Born’s rule then follows from the symmetries of entangled quantum states. With probabilities at hand one can analyze information flows from the system to the environment in course of decoherence. They explain how, as a result of selective proliferation of information, robust classical reality arises from the quantum substrate by accounting for all the symptoms of objective existence of preferred pointer states of quantum systems through the redundancy of their records in the environment. Taken together, these three advances (i)(iii) elucidate quantum origins of the classical. 
Wednesday, March 7, 2018 11:51AM  12:03PM 
L26.00002: A NoGo Theorem For Theories That Decohere To Quantum Mechanics Ciarán Lee , John Selby Quantum theory is the most accurately tested theory of Nature in the history of science. Yet it may be the case that quantum theory is only an effective description of our world, in the same way that classical physics is an effective description of the quantum world. This work asks whether there exists an operationallydefined theory that supersedes quantum theory, but that reduces to it via a decoherencelike mechanism. We prove that no such postquantum theory exists if it is demanded that it satisfy two natural physical principles: causality and purification. Causality formalises the statement that information propagates from present to future, and purification that each state of incomplete information arises in an essentially unique way due to lack of information about an environment. In a sense, purification can be thought of as a statement of ``information conservation''; any missing information about the state of a given system can always be accounted for by considering it as part of a larger system. Hence, our result can either be viewed as evidence that the fundamental theory of Nature is quantum, or as highlighting the ways a postquantum theory must radically depart from a quantum description of the world. 
Wednesday, March 7, 2018 12:03PM  12:15PM 
L26.00003: Quantum Mereology : Factorizing Hilbert Space into SubSystems Ashmeet Singh , Sean Carroll How we talk about quantum systems depends crucially on how Hilbert space is factorized, or equivalently on a set of preferred observables. We tackle the question, given a finitedimensional Hilbert space and a Hamiltonian, without any additional structure, how does one decompose the Hilbert space into a tensor factorization of subsystems with quasiclassical behavior? A quasiclassical decomposition has features such as low entropy states resilient to entanglement production, existence of preferred pointer observables robust under evolution, and preserving predictive power while decohering subsystems relatively quickly. We connect these features with properties of the Hamiltonian, in particular locality, and show that arbitrary factorizations will not exhibit quasiclassicality. We make contact with conjugate operators and point out conditions under which they correspond to classical conjugate variables, characterized by classical dynamics. An algorithm which minimizes an entropybased quantity sifting through factorizations of Hilbert space to select the quasiclassical one is outlined. We remark on the application of this formalism to the emergence of spacetime from quantum dynamics. 
Wednesday, March 7, 2018 12:15PM  12:27PM 
L26.00004: The Ironic ManyWorlds Interpretation of Quantum Theory Matthew Leifer Recent results, such as the PuseyBarrettRudolph theorem, imply that a realist interpretation of quantum theory needs to include an objectively real quantum state if it is to fit into the standard ontological models framework. Adopting a more exotic ontology allows one to evade the conclusions of these results and possibly construct a realist interpretation in which the quantum state just represents knowledge or information (known as a ψepistemic interpretation). In this talk, I will describe such an interpretation based on modifying the Everett/manyworlds interpretation. The basic idea is that the branching structure of "worlds" in Everett does not depend on the amplitudes assigned to the branches, so these can be epistemic while retaining the usual decoherence account of the emergence of worlds. One can do this in such a way that there is still enough structure left to derive the Born rule, now interpreted in a subjective Bayesian way similar to how it is viewed in QBism. In addition to providing a concrete example of a realist ψepistemic interpretation, this solves a conceptual puzzle about probability in Everett due to Adrian Kent, known as the "numbers in the sky" objection. 
Wednesday, March 7, 2018 12:27PM  12:39PM 
L26.00005: Algorithmic Probability as a Common Foundation for Quantum Interpretation Allan Randall The Born rule has remained a perpetual puzzle in the quantum foundations community. The GleasonBusch Theorem proves the rule, given the assumption of a certain type of noncontextuality, an assumption that only some are willing to grant as obvious. I analyze these issues in terms of two thought experiments from the literature: the Sleeping Beauty problem and the Replicator Copies experiment, showing that differing interpretations can be understood in terms of how they interpret these two (nonquantum) thought experiments, the major division being between those who count ontic (metaphysical) entities, and those who count subjective states. For instance, I argue that wave function realists (such as Everettians) need to be ontic counters to be consistent, yet Everett himself was not. I argue further that algorithmic probability is our best way forward towards resolving the interpretation of quantum probabilty, by providing a common foundation for all four of the above schools, each of which differs in the nature of the computing language used to formulate its algorithms. I would argue that, by using algorithmic probability as a common foundation, those debating quantum interpretations stand a better chance of understanding each other, and may perhaps avoid talking past each other. 
Wednesday, March 7, 2018 12:39PM  12:51PM 
L26.00006: Stochastic Mechanics as a Possible Foundation for Quantum Mechanics Maaneli Derakhshani Stochastic mechanics aims to provide a measurementproblemfree foundation for quantum mechanics in terms of a more fundamental theory involving classical particles interacting with a classicallike ether, where the interaction causes the particles to undergo a diffusion process on configuration space that conserves their average energy. From 1966 and onwards, it was argued by Edward Nelson and others that stochastic mechanics succeeds in this aim. However, Wallstrom showed in 1989 that stochastic mechanics runs into the technical problem that it cannot recover the Schroedinger equation unless an ad hoc quantization condition is imposed on the “current velocity” of the diffusing particles. Wallstrom’s criticism was one of the key causes of diminished research interest in stochastic mechanics from the '90's onward. Recently, however, I have shown [1] that it is possible to reformulate stochastic mechanics so that the aforementioned quantization condition arises as a natural consequence of classical zitterbewegung particles interacting with the posited classicallike ether. In this talk, I will sketch the basic idea of stochastic mechanics, Wallstrom’s criticism thereof, and the basic idea behind my proposed reformulation. 
Wednesday, March 7, 2018 12:51PM  1:03PM 
L26.00007: Operator spreading and the emergence of dissipation in unitary dynamics with conservation laws Vedika Khemani , Ashvin Vishwanath , David Huse I will discuss the “scrambling” of local quantum information in chaotic quantum manybody systems in the presence of a locally conserved quantity like charge or energy that moves diffusively. I will show how the interplay between conservation laws and scrambling provides a mechanism by which unitary quantum dynamics, which is reversible, gives rise to diffusive hydrodynamics, which is a dissipative process. Our results are obtained by examining the dynamics of operator spreading in a random quantum circuit model that is constrained to have a conservation law. The structure of spreading operators in this setting implies that the outoftimeorder commutator (OTOC) between two initially spatially separated operators grows sharply upon the arrival of a ballistic operator front but, in contrast to systems with no conservation laws, it develops a diffusive tail and approaches its asymptotic latetime value only as a power of time instead of exponentially. I will also present an effective hydrodynamic description which contains multiple coupled diffusion equations. 
Wednesday, March 7, 2018 1:03PM  1:15PM 
L26.00008: Quantization from Clifford Algebra Justin Dressel I review orthogonal and symplectic geometries and their Clifford algebraic representations, and highlight their nontrivial connection to the quantization procedure. I emphasize the role of nontrivial idempotents for constructing left, right, and twosided algebraic ideals that replace Hilbert space state representations. I support the discussion with several concrete examples. 
Wednesday, March 7, 2018 1:15PM  1:27PM 
L26.00009: The Purcell effect in an accelerated cavity containing an UnruhDeWitt detector Kacpar Kozdon , Ian Durham , Andrzej Dragan We show that a twolevel atom resonantly coupled to one of the modes of a cavity field can be used as a sensitive tool to measure the proper acceleration of a combined atomcavity system. To achieve it we investigate the relation between the transition probability of a twolevel atom placed within an ideal cavity and study how it is affected by the acceleration of the whole. We indicate how to choose the position of the atom as well as its characteristic frequency in order to maximize the sensitivity to acceleration. 
Wednesday, March 7, 2018 1:27PM  1:39PM 
L26.00010: Quantum physics of black holes: A superconducting perspective Sreenath Kizhakkumpurath Manikandan , Andrew Jordan We establish an analogy between superconductormetal interfaces and the quantum physics of a black hole. We show that the metalsuperconductor interface can be thought of as an event horizon and Andreev reflection from the interface is analogous to the Hawking radiation in black holes, giving a unitary description of black hole dynamics. Quantum information transfer can be described with Andreev reflection and a final state projection model similar to HorowitzMaldacena, and Hayden and Preskill's description of a black hole final state, where the black hole is described as an information mirror. The analogy between Crossed Andreev Reflections and EinsteinRosen bridges is also discussed. Given these established connections, we conjecture that the final quantum state of a black hole is the BardeenCooperSchrieffer ground state wavefunction. 
Wednesday, March 7, 2018 1:39PM  1:51PM 
L26.00011: Parsing the Path Integral Kenneth Wharton The Feynman path integral offers some promise towards an understanding of quantum phenomena in terms of histories in ordinary spacetime (rather than instantaneous states in configuration space). The path integral's symmetries are also arguably aligned against the standard division of quantum processes into dynamical timeevolution followed by a probabilistic collapse. This talk will discuss the option of an alternate parsing of the singleparticle path integral into special groups of paths. Each group can be assigned a positive probability, and there is no interference between groups (only paths in a given group interfere with each other). If each special group of particle paths is then reinterpreted as flow lines of a realistic field, this indicates a possible onerealfieldhistory interpretation of the path integral. See K.B. Wharton, Quanta, v5, pp. 111 (2016). 
Wednesday, March 7, 2018 1:51PM  2:03PM 
L26.00012: Beyond the Dynamical Universe William Stuckey I will outline arguments from the book "Beyond the Dynamical Universe" (forthcoming with Oxford University Press) that mysteries from quantum mechanics (e.g., measurement problem, no counterfactual definiteness) and general relativity (e.g., paradoxes of closed timelike curves) result from dynamical explanation in the mechanical universe and are easily resolved using adynamical explanation in the block universe. As an example of adynamical explanation, the Relational Blockworld (RBW) interpretation of quantum mechanics based on spatiotemporal ontological contextuality will be presented. RBW provides a realist psiepistemic account of quantum mechanics as called for by Leifer. I will survey the RBW explanations of entanglement, environmental decoherence, quantum noncommutivity, quantum versus classical behavior, and the Born rule, and review new approaches to unification, quantum gravity and cosmology (no nonbaryonic dark matter and no cosmological constant) suggested by RBW. 
Wednesday, March 7, 2018 2:03PM  2:15PM 
L26.00013: Entanglement, Entropy, Erasure, and Error: the Multiple Facets of SternGerlach JeanFrancois Van Huele The SternGerlach effect has a long history of revealing fundamental physics in unexpected ways. In this presentation, we explore the role accurate SternGerlach dynamics can play in the physics of quantum information, from selecting measures of entanglement to retrieving quantum coherence. 
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