Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T33: Focus Session: Quantum Foundations: Interpretations, Contextuality, and Nonlocality |
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Sponsoring Units: GQI Chair: William Wootters, Williams College Room: 706 |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T33.00001: Contextuality and state-space geometry Raymond Lal We shall explore the connection between state-space geometry and the Abramsky-Brandenburger sheaf-theoretic framework for classifying no-go theorems. The classic example of such a no-go theorem is the Kochen-Specker theorem. No-go results prohibit any theory from the specified class, e.g.~non-contextual theories, from replicating the empirical predictions of quantum theory. The sheaf-theoretic framework allows such no-go results to be generalised according to a certain kind of topology relating to the compatibility of the measurements used. We show that there is a correspondence between a class of no-go results and a class of polygonal state-spaces. The latter is a family of models whose geometric realisation lies in the equatorial plane of the Bloch sphere. This shows that the geometry of the state space used to define a physical theory related in a crucial way to the type of contextuality the theory exhibits. In particular, it also yields an understanding of the quantitative violation that quantum theory yields for the chained Bell inequality. [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T33.00002: Testability of the Pusey-Barrett-Rudolph Theorem Seyyed Mohammad Hassan Halataei Pusey, Barrett, and Rudolph (PBR) proved a mathematically neat theorem which assesses the reality of the quantum state. They proposed a test such that if any pair of quantum states could pass it, then for small deviation in the probabilities of measurement outcomes, $\epsilon$, from the predicted quantum probabilities, one can conclude that the physical state $\lambda$ ``is normally closely associated with only one of the two quantum states.'' While the mathematics of their theorem is correct, the physical conclusion is incomplete. In this talk, I present an argument which greatly limits the conclusion one can draw from even a successful PBR test. Specifically, I show that the physical state can be associated with several quantum states and, thus, the reality of quantum states cannot be deduced. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T33.00003: Amplification of Information by Photons and the Quantum Chernoff Bound Michael Zwolak, C. Jess Riedel, Wojciech H. Zurek Amplification was regarded, since the early days of quantum theory, as a mysterious ingredient that endows quantum microstates with macroscopic consequences, key to the ``collapse of the wavepacket,'' and a way to avoid embarrassing problems exemplified by Schr\"odinger's cat. This bridge between the quantum microworld and the classical world of our experience was postulated ad hoc in the Copenhagen Interpretation. Quantum Darwinism views amplification as replication, in many copies, of information about quantum states. We show that such amplification is a natural consequence of a broad class of models of decoherence, including the photon environment we use to obtain most of our information. The resultant amplification is huge, proportional to ${^{\sharp}}\hspace{-0.5mm}\mathcal{E} \bar{\xi}_{QCB}$. Here, ${^{\sharp}}\hspace{-0.5mm}\mathcal{E}$ is the environment size and $\bar{\xi}_{QCB}$ is the ``typical'' Quantum Chernoff Information, which quantifies the efficiency of the amplification. The information communicated though the environment is imprinted in the states of individual environment subsystems, e.g., in single photons, which document the transfer of information into the environment and result in the emergence of the classical world. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T33.00004: Wavepacket Collapse, Amplification, and Actionable Information Wojciech Zurek An unknown state of a single quantum system cannot be discovered, as it is re-prepared; the system jumps into an eigenstate of the measured observable. As was recently demonstrated [1], this and other symptoms of the wave-packet collapse follow for pure states from unitarity (that does not, of course, allow for a literal collapse) and from repeatability of measurements: Together they impose discreteness underlying quantum jumps. We consider macroscopic, open system (e.g., an apparatus). Its microstates can change when copied/measured, provided coarse-grained macrostate still represent the same measurement record. We show that such repeatably accessible macrostates (e.g. of an apparatus pointer) correspond to orthogonal subspaces [2]. This symmetry breaking yields the discreteness that underlies quantum jumps. It emerges from the core quantum postulates plus repeatability (prerequisite for amplification) in macroscopic, open quantum systems including measuring devices, where (in contrast to microsystems) repeatability is paramount. \\[4pt] [1] WHZ, Quantum origin of quantum jumps ... PRA 76, 052110 (2007), arXiv:quant-ph/0703160 \\[0pt] [2] WHZ, Wave-packet collapse ... and actionable information PRA 87, 052111 (2013), arXiv:1212.3245 [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T33.00005: Possibilities of a test of the temporal Bell inequalities using a flux qubit coupling to a dcSQUID Mao-Chuang Yeh, Anthony J. Leggett Although then last few years have seen tests of the temporal Bell inequalities (TBI) on microscopic systems with the use of ``ideal negative result'' (INR) measurements [1], and on macroscopic systems using weak measurement [2], to date there have been no tests on macroscopic systems using INR measurements. Moreover, in neither case was the assumption of noninvasiveness explicitly tested in an ancillary experiment [3,4]. Here we propose a complete INR protocol, including the ancillary experiment, for a test of the TBI on a macroscopic system, namely a flux qubit,with the measuring apparatus a dc SQUID. The general setup mirrors that of Knee et al. [1], with the nuclear spins replaced by the flux qubit and the electron spins by the dc SQUID, and we analyze the relation between the theoretical concept of ``venality''" introduced in ref. [1] and the experimental behavior expected in our ancillary test. On the basis of this analysis we assess the current feasibility of the proposed experiment.\\[4pt] [1] G. C.Knee et al., Nature Comm. 3, 606 (2012).\\[0pt] [2] A. Palacios-Laloy et al., Nature Phys. 6, 442(2010).\\[0pt] [3] A. J. Leggett, Found. Phys.18, 939 (1988).\\[0pt] [4] A.Mizel and A.Wilde, Found. Phys. 42, 256-265(2012). [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T33.00006: A robust Bell inequality without two-outcome measurements William Plick, Robert Fickler, Radek Lapkiewicz, Sven Ramelow We present a novel Bell inequality that does not require dichotomic (two-outcome) measurements. It is based on an inequality originally derived by Wigner in 1969, extending it such that no assumptions other than local-realism, fair-sampling, and freedom-of-choice are necessary. It is most useful in situations where there is no direct access to true two-outcome (dichotomic) measurements, like photonic quantum experiments where spatial degrees-of-freedoms are analyzed with spatial light modulators (SLMs), as well as many other experimental scenarios. The only other class of inequalities (CH-type) that has this feature requires coincidence and singles rates to be of the same order of magnitude for violation, ours does not. It thereby enables the stringent verification of entanglement and rejection of local-realism, without any assumptions about the underlying Hilbert-space, such as dimensionality \--- in the most difficult experimental conditions. We also experimentally violate this inequality in a novel setup: entangled states of very high orbital angular momentum. This constitutes a rejection of the hypothesis of local realism (under reasonable assumptions) with the highest quanta to date. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 1:03PM |
T33.00007: Bounds on Epistemic Interpretations of the Quantum State from Contextuality Invited Speaker: Matthew Leifer The status of the quantum state is perhaps the most controversial issue in the foundations of quantum theory. Is it an epistemic state (representing knowledge, information, or belief) or an ontic state (a direct reflection of reality)? In the ontological models framework, quantum states correspond to probability measures over more fundamental states of reality. The quantum state is then ontic if every pair of pure states corresponds to a pair of measures that do not overlap, and is otherwise epistemic. Recently, several authors have derived theorems that aim to show that the quantum state must be ontic in this framework. Each of these theorems involve auxiliary assumptions of varying degrees of plausibility. Without such assumptions, it has been shown that models exist in which the quantum state is epistemic. However, the definition of an epistemic quantum state used in these works is extremely permissive. Only two quantum states need correspond to overlapping measures and furthermore the amount of overlap may be arbitrarily small. In order to provide an explanation of quantum phenomena such as no-cloning and the indistinguishability of pure states, the amount of overlap should be comparable to the inner product of the quantum states. In this talk, I show, without making auxiliary assumptions, that the ratio of overlap to inner product must go to zero exponentially in Hilbert space dimension for some families of states. This is done by connecting the overlap to Kochen-Specker noncontextuality, from which we infer that any contextuality inequality gives a bound on the ratio of overlap to inner product. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T33.00008: Avoiding Loopholes with Hybrid Bell-Leggett-Garg Inequalities Justin Dressel, Alexander Korotkov By combining the postulates of macrorealism with Bell-locality, we derive a qualitatively different hybrid inequality that avoids two loopholes that commonly appear in Leggett-Garg and Bell inequalities. First, locally-invasive measurements can be used, which avoids the ``clumsiness'' Leggett-Garg inequality loophole. Second, a single experimental ensemble with fixed analyzer settings is sampled, which avoids the ``disjoint sampling'' Bell inequality loophole. The derived hybrid inequality has the same form as the Clauser-Horne-Shimony-Holt Bell inequality; however, its quantum violation intriguingly requires weak measurements. A realistic explanation of an observed violation requires either the failure of Bell-locality, or a preparation-conspiracy of finely tuned and nonlocally-correlated noise. Modern superconducting and optical implementations of this test are considered. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T33.00009: Statistical analysis of recent experiments closing the detection loophole with photons, and implications Peter Bierhorst In the past year, two teams [Giustina \textit{et. al; }Christensen \textit{et. al}] report having closed the detection loophole in Bell's inequality using entangled photons. That is, detection efficiencies were high enough to obviate the need for fair-sampling assumptions in the data analysis. Here, we show how to analyze the results if we allow for a more general hidden variable with memory, thus widening the class of hidden variable theories that can be ruled out by the data. We also discuss the issues raised by the block-measurement experimental design that was employed, and show how these issues make it difficult to claim that these experiments are subject \textit{only} to the locality loophole. Finally, we examine the implications for our understanding of the nature of photons. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T33.00010: Quantum communication complexity and the reality of the wave-function Alberto Montina, Stefan Wolf The communication complexity of a quantum channel is the minimal amount of classical communication required for classically simulating the process of preparation, transmission through the channel, and subsequent measurement of a quantum state. At present, only little is known about this quantity. We have recently presented a procedure for systematically evaluating the communication complexity of channels in any general probabilistic theory, in particular quantum theory [A. Montina, M. Pfaffhauser, S. Wolf, Phys. Rev. Lett. 111, 160502 (2013)]. The procedure is particularly important in quantum foundations, as classical simulations of quantum channels employing a finite amount of communication are essentially equivalent to a special class of hidden variable theories where quantum states represent statistical knowledge about the classical state and not an element of reality [A. Montina, Phys. Rev. Lett. 109, 110501 (2012)]. This special class of theories, called psi-epistemic, has attracted strong interest very recently. With our procedure, we are able to build up a psi-epistemic theory that is also the most efficient one in terms of employed communication resources. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T33.00011: Quantum Collapse Requires Pre-and Immediate Post-measurement States to Belong to Disjoint Sets Armin Nikkhah Shirazi We present a simple proof that the orthodox interpretation of quantum mechanics, due to its incorporation of quantum collapse, requires pre- and immediate post-measurement states to belong to disjoint sets of states. This requires a reformulation of the projection postulate as a transformation of the quantum state to one that ``looks like'' an eigenstate, because otherwise such projection implies that the two kinds of states belong to the same set of states. An attempt to render more precise what is meant by a state that ``looks like'' an eigenstate is presented. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T33.00012: The Transactional Interpretation: Still Viable and Still the Best Account of the Born Rule Ruth Kastner It has been widely supposed that the Transactional Interpretation of Quantum Mechanics (TIQM) was shown to fail based on an objection due to Tim Maudlin in 1996. However, that objection has been decisively refuted, and TIQM has recently been expanded into the relativistic domain. This elaboration of TIQM presents the simplest and most elegant way to explain the Born Rule for the probabilities of measurement outcomes, and indeed the measurement process itself. It therefore answer's John Bell's demand for an unambiguous and physically grounded account of the measurement process. TIQM does not need to invoke the consciousness of an observer, a notion not subject to any physically grounded analysis. Rather, it explains measurement by including absorption as a real physical process. This approach naturally lends itself to the relativistic domain, where emission and absorption are fundamental and crucial processes. [Preview Abstract] |
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