Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session Y27: Quantum Foundations IIIFocus

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Sponsoring Units: DQI Chair: John DeBrota, University of Massachusetts Boston Room: BCEC 160C 
Friday, March 8, 2019 11:15AM  11:51AM 
Y27.00001: Reconstructing quantum spacetime from agents' subjective experiences Invited Speaker: Lidia del Rio Within a global physical theory, a notion of locality allows us to find and justify informationprocessing primitives, like nonsignalling between distant agents. Here, we propose exploring the opposite direction: to take agents as the basic building blocks through which we test a physical theory, and recover operational notions of locality from signalling conditions. 
Friday, March 8, 2019 11:51AM  12:03PM 
Y27.00002: Locally Causal Quantum Mechanics in SpaceTime Mordecai Waegell I introduce a reformulation of quantum mechanics as a theory of physical fields in spacetime which obey the constraints of special relativity. Instead of being described by a single delocalized wavefunction in configuration space, the fundamental subsystems within a composite system are each described by a continuum of local relative wavefunctions in space. Each relative wavefunction is defined on a particular worldline through spacetime, using only the past states and coupling unitaries of the systems within the pastinteractioncone, and thus obeying explicit local causality. As a result, the narrative and causal structures of this reformulation are Lorentz covariant  unlike singlewavefunction descriptions. This model explains violations of Bell inequalities while obeying local causality, but entanglement correlations are not physically obeyed until Alice's and Bob's measurement results are compared locally in spacetime. Explaining their experiences in this experiment then necessitates the existence of multiple Alices and multiple Bobs who got different measurement outcomes, and thus we arrive at a locallybranching manyworlds interpretation of quantum mechanics in spacetime which obeys local causality  unlike globalbranching manyworlds models. 
Friday, March 8, 2019 12:03PM  12:15PM 
Y27.00003: Quantum Clocks: Gravitation and Relativity Alexander R. H. Smith, Mehdi Ahmadi Time enters quantum theory through its appearance as an external classical parameter in the Schrödinger equation. In general relativity, time is defined operationally as what is indicated by a clock and the spacetime metric encodes the relationship between different clocks. Reconciling these two different notions of time in a quantum theory of gravity leads to the problem of time, one aspect of which is the disappearance of time in the WheelerDeWitt equation. 
Friday, March 8, 2019 12:15PM  12:27PM 
Y27.00004: Quantized ElectromagneticField Propagation in General NonLocal and NonStationary Dispersive and Absorbing Media Verne Jacobs We develop dynamical descriptions for the propagation of quantized electromagnetic fields, in the presence of environmental interactions. These descriptions are systematically and selfconsistently developed in the complimentary Schrödinger and Heisenberg pictures. An opensystems (nonequilibrium) quantumelectrodynamics and quantumstatistical description is thereby provided for electromagneticfield propagation in general nonlocal and nonstationary dispersive and absorbing optical media, including a fundamental microscopic treatment of decoherence and relaxation processes due to environmental collisional and electromagnetic interactions. Particular interest is centered on entangled states and other nonclassical states of electromagnetic fields, which may be created by nonlinear electromagnetic interactions and detected by the measurement of various electromagneticfield correlation functions. Accordingly, we develop dynamical descriptions based on general forms of electromagneticfield correlation functions involving both the electricfield and the magneticfield components of the electromagnetic field, which are treated on an equal footing. 
Friday, March 8, 2019 12:27PM  12:39PM 
Y27.00005: Spacetime formulation for time crystals and continuous variables Tian Zhang, Oscar Dahlsten In ordinary, nonrelativistic quantum theory, especially in quantum information, space and time are treated differently. For example, time is a parameter rather than an operator; states are defined across the whole space but only at one time. These go against our intuition from relativity as well as classical probability theory. Thus, spacetime density matrices have been introduced and here we discuss one of these formulations called pseudodensity matrix (PDM) which treats space and time indiscriminately. As a first application, we use this formulation to analyse time crystals via quantum error correction, which is the first time to discuss time crystals in the language of quantum information. We achieve the same result as the absence of continuous time crystals and formulate discrete time crystals in terms of quantum error correction. NMR experiments have been conducted to verify the results. We also extend this spacetime formulation to the continuous variables via a generalisation for Wigner function, and discuss Gaussian case in particular. This is the first step we take to understand temporal vs spatial modes in quantum fields and black holes. We want to analyse temporal correlations in black hole information paradox. 
Friday, March 8, 2019 12:39PM  12:51PM 
Y27.00006: Using Fisher Information to Analyse Intermeasurement Quantum Particles David Arvidsson Shukur, Crispin Barnes, Nicole Yunger Halpern A debated question in quantum mechanics is how to describe and interpret a quantum particle between observations. Previous discussions have mainly been centred on controversial and interpretationdependent weak values. In this talk we provide an operational methodology for the analysis of intermeasurement quantum particles. We assume that absolutely perfect quantum evolutions are impossible, and that all interactions include some disturbances. By calculating the quantum Fisher information with respect to these disturbances we obtain a quantitative tool to map out the past path of quantum particles and to investigate the “quantumness” of postselected experiments. We relate the Fisher information to the KirkwoodDirac quasiprobability distribution by decomposing the density matrix of the quantum state in terms of the projectors of the final measurement and the generator eigenbasis of the relevant interaction. We then show that when the Fisher information of a postselected experiment exceeds its standard maximum, there exist negative quasiprobabilities in the representation, revealing violations of noncontextuality. Our methodology provides a new approach to investigating the foundations of quantum mechanics. 
Friday, March 8, 2019 12:51PM  1:03PM 
Y27.00007: Noninformative prior of the quantum statistical model in the qubit system Fuyuhiko Tanaka In quantum process tomography, we often encounter a parametric family of qubits with less symmetry and estimate the parameter from experimental data. If we use Bayes estimates, then we have to choose a default probability distribution over the parameter space, which is called a noninformative prior. While the effect of the prior becomes smaller with the large amount of data, the choice of the noninformative prior based on a certain criterion has been of theoretical interest. In this talk, we consider a suitable definition of a noninformative prior in a general parametric family of qubits. 
Friday, March 8, 2019 1:03PM  1:15PM 
Y27.00008: Uncertainty relations for time averaged weak values Eli Pollak Time averaging of weak values using the quantum transition path time probability distribution leads to the establishment of a general uncertainty principle for the weak values of two not necessarily Hermitian operators. This new principle is a weak value analog of the HeisenbergRobertson strong value uncertainty principle. It shows that complementarity does not prevent the simultaneous determination 
Friday, March 8, 2019 1:15PM  1:27PM 
Y27.00009: ThreeParticles AharonovBohm Effect Yutaka Shikano Recently, we showed the successful experiment on the threeparticles AharonovBohm effect using the tapped ion [A. Noguchi, YS, K. Toyoda, S. Urabe, Nature Communications 5, 3868 (2014)]. In this experiment, there still remain several fundamental questions on the identical particles. Inspired by our experiment setup, we can discuss the properties of the identical particles from the AharonovBohm effect perspectives. The phase shift by chaning the particle status leads to give an answer to the following question when the AharonovBohm phase can be aquired. As the minimum setup, three particle setting is considered. Maybe, it is possible to be genralized to the Nbody particles. Furthermore, the dynamical nonlocality aspects will be discussed if possible. 
Friday, March 8, 2019 1:27PM  1:39PM 
Y27.00010: Topological barriers for charged quantum systems Ismael Paiva, Yakir Aharonov, Mordecai Waegell, Jeff M Tollaksen The AharonovBohm effect introduced a surprising influence of topology on the dynamics of quantum systems. The discovery of such a phenomenon unveiled that the evolution of a quantum system with charge traveling around a solenoid is affected by the magnetic flux on it, even though the solenoid only produces electromagnetic field on its interior. Here, making use of the concept of modular variables, which evidence the dynamical nonlocality presented in quantum mechanics, we argue that this effect allows solenoids to be used as barriers of energy for charged quantum systems. Moreover, we present and discuss a thought experiment, as well as its numerical simulation, where a configuration of solenoids inside a cavity can trap some quantum states in a certain subregion of it. 
Friday, March 8, 2019 1:39PM  1:51PM 
Y27.00011: Quantum simulation of a general PTsymmetric twolevel system Chao Zheng The evolution of a twolevel quantum system with a general PTsymmetric Hamiltonian is simulated as a subsystem in a fourdimensional Hilbert space, which makes it possible to simulate any twolevel PTsymmetric quantum system using a conventional quantum mechanics system. We studied the ability to simulate and investigate novel quantum systems and phenomena using a conventional Hermitian quantum computer. The quantum circuits are given for a qubit and two qudit systems. 
Friday, March 8, 2019 1:51PM  2:03PM 
Y27.00012: Computing with a single qubit faster than the computation quantum speed limit Nikolai Sinitsyn The possibility to save and process information in fundamentally indistinguishable states is the quantum mechanical resource that is not encountered in classical computing. I demonstrate that, if energy constraints are imposed, this resource can be used to accelerate informationprocessing without relying on entanglement or any other type of quantum correlations. In fact, there are computational problems that can be solved much faster, in comparison to currently used classical schemes, by saving intermediate information in nonorthogonal states of just a single qubit. There are also error correction strategies that protect such computations. 
Friday, March 8, 2019 2:03PM  2:15PM 
Y27.00013: Problems with Decoherence in Quantum Geometrodynamics Maaneli Derakhshani I illustrate some severe conceptual and technical problems that arise in attempts to apply quantum decoherence concepts to "quantum geometrodynamics" (i.e. Dirac constraint quantization of classical general relativity in the Hamiltonian formulation). In particular, I show that the Problem of Time, the Hilbert Space Problem, and the conceptual dependence of decoherence on time evolution and Hilbert space structure implies: 1) suppression of interference cannot take place between components of a superposition of quantumgravitational (i.e. WheelerDeWitt) wave functionals; 2) approximate diagonalization of the reduced density matrix (associated with said superposition) in the 3geometry basis cannot take place, and such a reduced density matrix is mathematically illdefined; 3) timedependent functional Schroedinger equations for the matter components of said superposition cannot be derived in a semiclassical approximation. I conclude, contrary to extant claims in the quantum gravity literature, that quantum decoherence concepts cannot be consistently applied to quantum geometrodynamics. I then suggest that quantum decoherence concepts can only be sensibly applied to quantum gravity theories that posit classical time parameters or matterclock variables at a fundamental level. 
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