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 Y33: Quantum Foundations IIILive

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Sponsoring Units: DQI Chair: Flaminia Giacomini, Perimeter Inst for Theo Phys 
Friday, March 19, 2021 11:30AM  11:42AM Live 
Y33.00001: Experimental quantum interference of distinguishable sets of indistinguishable photons Julian Münzberg, Christoph Dittel, Maxime Lebugle, Andreas Buchleitner, Alexander Szameit, Gregor Weihs, Robert Keil Quantum interference of indistinguishable bosonic particles is indispensable for the description of many quantum optical experiments. As in the famous HongOuMandel effect, symmetry of the input state as well as symmetries in the scattering scenario can lead to destructive interference with the suppression of a large number of output events. The rules specifying which inputoutput combinations interfere totally destructively are summarized in socalled suppression laws. Here, we experimentally investigate the suppression law of the J_{x} unitary in a femtosecond laserwritten waveguide structure with 4 photons emitted from a SPDC source. We show that totally destructive interference does not require mutual indistinguishability between all particles but only between symmetrically paired ones, which is in agreement with recent theoretical predictions. The experimental results are compared to a theoretical model of the experiment, where we consider higher order emission of the source, partial distinguishability, and photon loss. 
Friday, March 19, 2021 11:42AM  11:54AM Live 
Y33.00002: Jumptime unraveling of open quantum systems Clemens Gneiting, Alexander Rozhkov, Franco Nori In contrast to the standard unraveling of quantum master equations, where the stochastically evolving quantum trajectories are ensembleaveraged at specific times, we argue that quantum trajectories can as well be averaged at specific jump counts. The resulting jumptimeaveraged quantum state then follows a discrete, deterministic evolution equation, with time replaced by the jump count. This jumptime evolution represents a tracepreserving quantum dynamical map if and only if the underlying quantum master equation does not exhibit dark states. In the presence of dark states, on the other hand, the jumptimeaveraged state decays into the dark state and the jumptime evolution eventually terminates. Jumptimeaveraged quantum states are operationally accessible in continuous measurement schemes, where quantum jumps are registered as “clicks" in the detector. Monitoring combined with the readout at jumptimes thus allows one to realize quantum channels that are characterized by the jumptime evolution. 
Friday, March 19, 2021 11:54AM  12:06PM Live 
Y33.00003: Stochastic Action Functionals for Diffusive Quantum Trajectories Philippe Lewalle, Kurt Callaghan Cylke, Tanawut Noungneaw, Howard Wiseman, Andrew N Jordan, Areeya Chantasri There has recently been considerable interest in applying a stochastic action formalism to describe diffusive continuous quantum measurement processes, following [Chantasri, Dressel, and Jordan (2013)]. We compare this method to similar approaches based on the OnsagerMachlup (OM) functional, that were developed in the context of classical stochastic processes. Such a comparison of action functionals reveals surprising similarities and differences between distinct approaches, including some severe limitations of a covariant class of OM functionals with respect to quantum measurement problems. Emphasis is given to the interpretation of optimal trajectories derived from each type of action we consider. 
Friday, March 19, 2021 12:06PM  12:18PM Live 
Y33.00004: On Interchangeability of Probe–Object Roles in Quantum–Quantum InteractionFree Measurement Stanislav Filatov, Marcis Auzinsh We examine Interactionfree measurement (IFM) where both the probe and the object are quantum particles. We argue that in this case the description of measurement must be symmetrical with respect to the interchange of the roles of probe and object. A thought experiment is being suggested that helps to determine what does and what doesn’t happen to the state of the particles during the process of noninteraction. It becomes evident that unlike the case of classical object, here the state of both the probe and the object must change. A possible explanation of this might be that the probe and the object form an entangled pair as a result of noninteraction. In other words, we look at the case of two qubits whose degrees of freedom are coupled through a potential interaction (i.e. some potentially observable event happens if the state is │1〉│1〉or │0〉│0〉, but doesn't happen if the state is │1〉│0〉or │0〉│1〉). We show that in the case when they could have interacted, but due to degreeoffreedom mismatch (i.e. state │1〉│0〉) did not, the state of both of them still must chage. The change being creation of entanglement. 
Friday, March 19, 2021 12:18PM  12:30PM Live 
Y33.00005: Classical model of delayedchoice quantum eraser Brian La Cour, Thomas Yudichak Wheeler's delayedchoice experiment was conceived to illustrate the paradoxical nature of waveparticle duality in quantum mechanics. In the experiment, quantum light can exhibit either wavelike interference patterns or particlelike anticorrelations, depending upon the (possibly delayed) choice of the experimenter. A variant known as the quantum eraser uses entangled light to recover the lost interference in a seemingly nonlocal and retrocausal manner. Although it is believed that this behavior is incompatible with classical mechanics, here we show that the observed quantum phenomena can be reproduced by adopting a simple deterministic detector model and supposing the existence of a random zeropoint electromagnetic field. 
Friday, March 19, 2021 12:30PM  12:42PM Live 
Y33.00006: Particles, Fields, and the Measurement of Electron Spin Charles Sebens In this talk, I will compare treatments of the SternGerlach experiment across different physical theories, building up to a novel analysis of electron spin measurement in the context of classical Dirac field theory. Analyzing the experiment in this context is helpful for clarifying the nature of electron spin and the relationship between classical and quantum field theories. In classical Dirac field theory, the electron can be modeled as a wave packet of rotating charge that (depending on the axis of rotation) might simply be deflected in a SternGerlach experiment or might split into two pieces. In this classical context, we can explain a feature of electron spin that is often presented as distinctively quantum (that there are only two locations where electrons hit the detector) but we cannot explain another important feature (that each electron hits the detector at just one location). Modeling the electron as a classical rigid body or point particle, we can explain the second feature but not the first. Of course, both features can be explained within quantum field theory, relativistic quantum mechanics, or nonrelativistic quantum mechanics. 
Friday, March 19, 2021 12:42PM  12:54PM Live 
Y33.00007: The Cost of Quantum Locality Charles Alexandre Bédard It has been more than 20 years since Deutsch and Hayden^{[1]} demonstrated that quantum systems can be completely described locally — notwithstanding Bell's theorem. More recently, RaymondRobichaud^{[2] }proposed another approach to the same conclusion. The cost of such local descriptions is quantified by the dimensionality of their space. The central result is that the dimension of a single qubit's description grows exponentially with the size of the total system considered, in sharp contrast with the mere three dimensionality of the reduced density matrix. However, the apparently unreasonable cost is shown to be expected of any local and complete description of quantum systems. 
Friday, March 19, 2021 12:54PM  1:06PM Live 
Y33.00008: Modeling Generalized Squeezed States Skylar Turner, Brian La Cour We consider the general problem of modeling an nphoton entangled state by a squeezed vacuum state using a squeezing operator given by a squeezing tensor of rank n. We derived a simple recursion relation for generating the Bogoliubov transformed creation and annihilation operators and use this result to express the generalized squeezed state as an equivalent complex Gaussian random vector. Finally, we compare loworder approximations of the squeezed state to entangled multiphoton GreenbergerHorneZeilinger (GHZ) states using postselection and a deterministic detector model. We find that this classical model is capable of exhibiting violations of the Mermin inequality consistent with experimental observations. 
Friday, March 19, 2021 1:06PM  1:18PM Live 
Y33.00009: Measurementinduced phase transitions in deterministic nonlocal quantum circuits Tomohiro Hashizume, Gregory Bentsen, Andrew Daley

Friday, March 19, 2021 1:18PM  1:30PM Live 
Y33.00010: A QuantumClassical Isomorphic Interpretation of Quantum Foundations Based on Density Functional Theory and Polymer SelfConsistent Field Theory Russell Thompson The Feynman quantumclassical isomorphism between classical statistical mechanics in 3+1 dimensions and quantum statistical mechanics in 3 dimensions is used to relate classical polymer selfconsistent field theory to quantum density functional theory. This allows the theorems of density functional theory, which connect single particle density descriptions of quantum systems to wave function descriptions, to relate nonrelativistic quantum mechanics back to a classical statistical mechanical derivation of polymer selfconsistent field theory for ring polymers. In turn, this allows for a quantumclassical isomorphic interpretation of quantum foundations which may require fewer postulates than standard approaches to quantum mechanics, while preserving all quantum predictions. 
Friday, March 19, 2021 1:30PM  1:42PM Live 
Y33.00011: Ruling out Bipartite Nonsignaling Nonlocal Models for Tripartite Correlations Peter Bierhorst Many threeparty correlations, including some that are commonly described as genuinely tripartite nonlocal, can be simulated by a network of underlying subsystems that display only bipartite nonsignaling nonlocal behavior. Quantum mechanics predicts threeparty correlations that admit no such simulation, suggesting there are versions of nonlocality in nature transcending the phenomenon of bipartite nonsignaling nonlocality. This paper introduces a rigorous framework for analyzing tripartite correlations that can be simulated by such bipartiteonly networks. We confirm that expected properties of soobtained correlations such as nosignaling indeed hold, and show how to use the framework to derive Bellinequalitytype constraints on these correlations that can be robustly violated by tripartite quantum systems, obtaining a new proof for one such constraint previously described in by Chao and Reichardt in arXiv:1706.02008. 
Friday, March 19, 2021 1:42PM  1:54PM Live 
Y33.00012: Quantum erasing the memory of Wigner's friend Cyril Elouard, Philippe Lewalle, Sreenath Kizhakkumpurath Manikandan, Spencer Rogers, Adam Frank, Andrew N Jordan Wigner's friend scenarios, in which observers measure one another, have generated considerable debate over the years. Most notable in recent times was the article by Frauchiger and Renner (Nature Communications 9, 3711 (2018)), which introduced a situation whereby different agents, applying quantum theory, arrive at inconsistent conclusions. We argue that the agents' differing lines of thought can be traced to distinct experimental contexts, which can never be realized simultaneously in an experiment, since they correspond to observables that do not commute. One of these contexts involves quantum erasing the memory of Wigner's friend. We provide a singlephoton interferometer that maps onto the FrauchigerRenner scenario and illustrates our point. 
Friday, March 19, 2021 1:54PM  2:06PM Live 
Y33.00013: Quantum reference frame transformations as symmetries and the paradox of the third particle
Marius Krumm, coauthors: Philipp A. Hoehn and Markus P. Mueller Marius Krumm, Philipp Höhn, Markus P. Müller In a quantum world, reference frames are ultimately quantum systems too — but what does it mean to “jump into the perspective of a quantum particle”? We show that quantum reference frame (QRF) transformations appear naturally as symmetries of simple physical systems. This allows us to rederive and generalize known QRF transformations within an alternative, operational framework, and to shed new light on their structure and interpretation. We give an explicit description of the observables that are measurable by agents constrained by such quantum symmetries, and apply our results to the ‘paradox of the third particle’. We argue that it can be reduced to the question of how to relationally embed fewer into more particles. This leads us to a generalization of the partial trace which arguably resolves the paradox, and it uncovers important structures of constraint quantization within a simple quantum information setting, such as relational observables which are key in this resolution. 
Friday, March 19, 2021 2:06PM  2:18PM Live 
Y33.00014: Entanglement entropy of energy eigenstates follows a universal scaling function Qiang Miao, Thomas Barthel We consider the entanglement entropies of energy eigenstates in quantum manybody systems. For the typical models that allow for a fieldtheoretical description of the longrange physics, we find that the entanglement entropy of (almost) all eigenstates is described by a single scaling function. This is predicated on the validity of the weak or strong eigenstate thermalization hypothesis (ETH), which then implies that the scaling functions can be deduced from subsystem entropies of thermal ensembles. The scaling functions describe the crossover from the groundstate entanglement regime for low energies and small subsystem size (area or logarea law) to the volumelaw regime for high energies or large subsystem size. For critical 1d systems, the scaling function follows from conformal field theory (CFT). We use it to also deduce the scaling function for Fermi liquids in d>1. These analytical results are complemented by numerics for large noninteracting systems of fermions in d=1,2,3 and the harmonic lattice model in d=1,2. Lastly, we demonstrate ETH for entanglement entropies and the validity of the scaling arguments in integrable and nonintegrable interacting spin chains. 
Friday, March 19, 2021 2:18PM  2:30PM On Demand 
Y33.00015: Quantifying nonMarkovianity: a quantum resourcetheoretic approach Namit Anand, Todd Brun We study quantum nonMarkovianity as a resource theory and introduce the robustness of nonMarkovianity: an operationallymotivated, optimizationfree measure that quantifies the minimum amount of Markovian noise that can be mixed with a nonMarkovian evolution before it becomes Markovian. We show that this quantity is a bonafide nonMarkovianity measure since it is faithful, convex, and monotonic under composition with Markovian maps. A twofold operational interpretation of this measure is provided, with the robustness measure quantifying an advantage in both state and channel discrimination tasks. Moreover, we connect the robustness measure to singleshot information theory by using it to upper bound the minaccessible information of a nonMarkovian map. Furthermore, we provide a closedform analytical expression for this measure and show that, quite remarkably, the robustness measure is exactly equal to half the RivasHuelgaPlenio (RHP) measure [Phys. Rev. Lett. 105, 050403 (2010)]. As a result, we provide a direct operational meaning to the RHP measure while endowing the robustness measure with the physical characterizations of the RHP measure. 
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