Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V27: Quantum Foundations IFocus
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Sponsoring Units: DQI Chair: Mordecai Waegell, Chapman University Room: BCEC 160C |
Thursday, March 7, 2019 2:30PM - 3:06PM |
V27.00001: Wigner’s friend as a rational agent Invited Speaker: Caslav Brukner In 1961 the physicist Eugene Wigner proposed the “Wigner's friend” thought experiment in which an observer, Wigner, observes another observer, his friend, who performs a quantum measurement on a physical system. I will first derive a no-go theorem for observer-independent outcomes (directly observable “facts” such as “detector clicks”) which would be common both for Wigner and the friend. The outcomes then are to be understood as relational in the sense that their determinacy is relative to an observer. I will then discuss a situation where the rationale is to force Wigner’s friend to depart from “standard rules of quantum mechanics” when updating her degrees of belief. |
Thursday, March 7, 2019 3:06PM - 3:18PM |
V27.00002: Contextuality without access to a tomographically complete set Matthew F. Pusey, Lidia del Rio, Bettina Meyer The operational approach to contextuality due to Spekkens requires finding operationally equivalent preparation procedures. Previously these have been obtained by demanding indistinguishably under a set of measurements taken to be tomographically complete. That is, it is taken for granted that the experimenter has access to the full dimension of the system of interest. However, there may in fact be other measurements in the true tomographically complete set, which could break the operational equivalences and hence eliminate the putative contextuality. Here we design tests of contextuality that are immune to this effect for a given number of unknown measurements in the tomographically complete set, allowing contextuality to be demonstrated with weaker assumptions. |
Thursday, March 7, 2019 3:18PM - 3:30PM |
V27.00003: Symmetric Informationally Complete Measurements Pinpoint the Essential Difference between Classical and Quantum Probability Theories Blake Stacey I describe a general procedure for crafting a purely probabilistic representation of the Born Rule by means of a reference measurement: a minimal informationally-complete quantum measurement (MIC) and a set of linearly independent post-measurement quantum states. It follows that the Born Rule is a consistency condition between probabilities assigned to the outcomes of different, mutually exclusive experiments. The difference between quantum and classical physics is the way their physical assumptions augment bare probability theory: Classical physics corresponds to a trivial augmentation—one just applies the Law of Total Probability between the scenarios—while quantum theory makes use of the Born Rule expressed in one or another of the forms of our general procedure. To mark the essential difference between quantum and classical, one should seek the representations that minimize the disparity between the expressions. Using a symmetric informationally-complete measurement (SIC) minimizes this disparity, according to a large family of optimality criteria. This work complements recent studies in quantum computation where the deviation of the Born Rule from the LTP is measured in terms of negativity of Wigner functions. |
Thursday, March 7, 2019 3:30PM - 3:42PM |
V27.00004: A Volume-Maximizing Map from Quantum States to the Probability Simplex, with Applications to QBism John DeBrota In this talk, I make use of a recent formulation of quantum theory in terms of minimal informationally complete quantum measurements (MICs) to promote the idea that probability is more central to quantum theory than either state vectors or operators. One advantage of this formulation is that it permits a direct comparison of classical statistical physics with quantum theory via a standard reference measurement. Classically, the possibility of perfect knowledge of a system's phase space point means any vector in the reference probability simplex is an allowable state of knowledge. By contrast, the image of quantum state space under a MIC is necessarily a proper subset of the simplex. This suggests we could take the size of the allowable subset under a given MIC to be a measure of the representation's deviation from classicality. I report work from arxiv:1805.08721, where we prove that this deviation is minimized if and only if the MIC is chosen to be a symmetric informationally complete quantum measurement (SIC)---that is, the SIC representation minimizes the deviation from classicality. Finally, I speculate that this grants the SICs a unique foundational significance. (Joint work with C. A. Fuchs and B. C. Stacey) |
Thursday, March 7, 2019 3:42PM - 3:54PM |
V27.00005: What is nonlocal in counterfactual quantum communication? Daniel Rohrlich, Yakir Aharonov We revisit the “counterfactual quantum communication” of Salih et al., who claim that an observer “Bob” can send one bit of information to a second observer “Alice” without any physical particle traveling between them. We show that a locally conserved, massless current - specifically, a current of modular angular momentum, Lz mod 2$hbar$ - carries the one bit of information. We integrate the flux of Lz mod 2$hbar$ from Bob to Alice and show that it equals one of the two eigenvalues of Lz mod 2$hbar$, either 0 or $hbar$, thus precisely accounting for the one bit of information he sends her. We previously obtained this result using weak values of Lz mod 2$hbar$; here we do not use weak values. |
Thursday, March 7, 2019 3:54PM - 4:06PM |
V27.00006: Tsirelson’s bound and Landauer’s principle in a single-system game Luciana Henaut, Lorenzo Catani, Dan Browne, Shane Mansfield, Anna Pappa We introduce a simple single-system game inspired by the Clauser-Horne-Shimony-Holt (CHSH) |
Thursday, March 7, 2019 4:06PM - 4:18PM |
V27.00007: Consecutive measurements on the same quantum system reveal that wavefunctions do not collapse Chris Adami, Davor Curic, Jennifer Glick, Lambert Giner, Jeff Lundeen Whether or not wavefunctions collapse in quantum measurements is often viewed as a metaphysical question because interpretations of quantum theory that differ in their assessment of the reality of wavefunction collapse have not made testably different predictions. We show that the standard collapse interpretation, and a formalism that uses the relative-state description of measurement in terms of von Neumann operations, differ in their predictions of the outcome of three or more consecutive measurements on the same system (while they agree about the outcome of one or two consecutive measurements). An experiment to distinguish between the two theories has been carried out using three consecutive (strong) quantum measurements of photon polarization, and shows unequivocally that the results are incompatible with wavefunction collapse, but perfectly in line with predictions using von Neumann (non-collapsing) measurements. |
Thursday, March 7, 2019 4:18PM - 4:30PM |
V27.00008: Noncontextuality Inequalities from Antidistinguishability Cristhiano Silva, Jose Raul Gonzalez Alonso, Matthew Leifer Noncontextuality inequalities are usually derived from the distinguishability properties of quantum states, i.e. their orthogonality. Here, we show that antidistinguishability can also be used to derive noncontextuality inequalities. Briefly, a set of states can be antidistinguished if there exists a measurement on the basis of which one can exclude one of the states as definitely not having been prepared. The Yu-Oh 13 ray contextuality inequality can be rederived and generalized as an instance of our antidistinguishability method. For some sets of states, the antidistinguishability method gives tighter bounds on noncontextual models than just considering orthogonality, and the Hadamard states provide an example of this. Antidistiguishability-based inequalities were initially discovered as overlap bounds for the reality of the quantum state. Our main contribution here is to show that they are also noncontextuality inequalities. |
Thursday, March 7, 2019 4:30PM - 4:42PM |
V27.00009: Contextuality and the Fundamental Theorems in Quantum Mechanics Markus Frembs Contextuality is a key feature of quantum mechanics, as was first properly brought to light by the Kochen-Specker theorem. Isham and Butterfield put contextuality at the heart of their topos-based formalism and gave a reformulation of the Kochen-Specker theorem in the language of presheaves. Here, we broaden this perspective considerably (partly drawing on existing, but scattered results) and show that apart from the KS theorem, also Wigner’s theorem, Gleason’s theorem and Bell’s theorem relate fundamentally to contextuality. The language of presheaves serves as a useful tool in this and allows to give reformulations of the theorems with a topological-geometric flavour. Moreover, we show in a mathematically exact way how much of the structure of a quantum system is encoded by contextuality, and also what is missing. |
Thursday, March 7, 2019 4:42PM - 4:54PM |
V27.00010: Rank of contextuality Karol Horodecki, Jingfang Zhou, Pawel Horodecki, Robert Raussendorf, Ryszard Horodecki, Ravishankar Ramanathan, Emily Tyhurst We propose a new measure of statistical Kochen-Specker contextuality, called rank of contextuality. The rank of contextuality is the minimal number of noncontextual boxes (input-output devices admitting a non-contextual hidden variable model) that are needed to switch between in order to simulate a contextual box. We show that the logarithm of the rank of contextuality is additive, faithful measure of contextuality, monotonous under simple wirings. We also provide a construction of contextual boxes with arbitrary high rank of contextuality, exhibiting thereby extremely high logical contradiction. |
Thursday, March 7, 2019 4:54PM - 5:06PM |
V27.00011: Perturbative expansion of entanglement negativity Jesse Cresswell, Ilan Tzitrin, Aaron Goldberg A common way to quantify entanglement in bipartite systems is through entanglement negativity. Because negativity is a non-analytic function of a density matrix, existing methods used in physics literature are insufficient to compute its derivatives. To this end we develop novel techniques in the calculus of complex, patterned matrices and use them to conduct a perturbative analysis of negativity in terms of arbitrary variations of the density operator. The result is an easy-to-implement expansion that can be carried out to all orders. On the way we provide new and convenient representations of the partial transposition map appearing in the definition of negativity. Our methods are well-suited to study the growth and decay of entanglement in a wide range of physical systems, including the generic linear growth of entanglement in many-body systems, and have broad relevance to many functions of quantum states and observables. |
Thursday, March 7, 2019 5:06PM - 5:18PM |
V27.00012: Near-100% two-photon-like coincidence-visibility dip with classical light and the role of complementarity Simanraj Sadana, Debadrita Ghosh, Kaushik Joarder, Naga Lakshmi A, Barry Sanders, Urbasi Sinha The famous Hong-Ou-Mandel two-photon coincidence-visibility dip (TPCVD), which accepts one photon into each port of a balanced beam splitter and yields an equal superposition of a biphoton from one output port and vacuum from the other port, has numerous applications in photon-source characterization and to quantum metrology and quantum computing. |
Thursday, March 7, 2019 5:18PM - 5:30PM |
V27.00013: Out-of-time-ordered-correlator quasi-probabilities for the quantum kicked top Jose Raul Gonzalez Alonso, Nathan Shammah, Shahnawaz Ahmed, Franco Nori, Justin G. Dressel The cumulative nonclassicality of the quasi-probability distribution (QPD) behind the out-of-time-ordered correlator (OTOC) exhibits different time scales that have been conjectured to be useful for distinguishing integrable and nonintegrable Hamiltonians (arXiv: 1806.09637). We further investigate the QPD for a quantum kicked top, and use the time scales of its nonclassicality to understand the relationship between entanglement and chaos for different parameter regimes. |
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