Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W3: Invited Session: Quantum Foundations |
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Sponsoring Units: GQI Chair: Terry Rudolph, Imperial College London Room: Ballroom III |
Thursday, March 21, 2013 2:30PM - 3:06PM |
W3.00001: The freedom of choice assumption and its implications Invited Speaker: Renato Renner The assumption that the parameters of an experiment (e.g., those determining the basis of a quantum measurement) can be chosen freely is implicit to most considerations in physics. One may therefore ask whether it is possible to give a precise meaning to the notion of ``free choice'' and, if yes, study its implications. One natural approach towards defining free choice, considered already by Bell, is to specify a causal structure on the set of all physically relevant parameters and observables. A parameter may then be considered ``free'' if it is statistically independent of all other parameters and observations that do not lie in its causal future. Recently, it has been realized that the assumption of free choice, as defined above, has various interesting consequences. In particular, if defined relative to a causal structure compatible with relativity theory, free choice immediately implies completeness of quantum theory. This means that there cannot exist any additional (hidden) parameters that would improve the statistical predictions that quantum theory makes about the outcomes of future measurements. In this talk, I motivate and explain this definition of free choice and give an overview of the most important implications of the free choice assumption. [Preview Abstract] |
Thursday, March 21, 2013 3:06PM - 3:42PM |
W3.00002: Quantum correlations in Newtonian space and time: arbitrarily fast communication or nonlocality Invited Speaker: Nicolas Gisin Experimental violations of Bell inequalities using space-like separated measurements precludes the explanation of quantum correlations through causal influences propagating at subluminal speed. Yet, ``everything looks as if the two parties somehow communicate behind the scene.'' We investigate the assumption that they do so at a speed faster than light, though finite. Such an assumption doesn't respect the spirit of Einstein relativity. However, it is not crystal clear that such ``communication behind the scene'' would contradict relativity. Indeed, one could imagine that this communication remains for ever hidden to humans, i.e. that it could not be controlled by humans, only Nature exploits it to produce correlations that can't be explained by usual common causes. To define faster than light hidden communication requires a universal privileged reference frame in which this faster than light speed is defined. Again, such a universal privileged frame is not in the spirit of relativity, but it is also clearly not in contradiction: for example the reference frame in which the cosmic microwave background radiation is isotropic defines such a privileged frame. Hence, a priori, a hidden communication explanation is not more surprising than nonlocality. We prove that for \textit{any} finite speed, such models predict correlations that can be exploited for faster-than-light communication. This superluminal communication doesn't require access to any hidden physical quantities, but only the manipulation of measurement devices at the level of our present-day description of quantum experiments. Consequently, all possible explanations of quantum correlations that satisfy the principle of continuity, which states that everything propagates gradually and continuously through space and time, or in other words, all combination of local common causes and direct causes that reproduce quantum correlations, lead to faster than light communication. Accordingly, either there is superluminal communication or the conclusion that Nature is nonlocal (i.e. discontinuous) is unavoidable [Nature Physics DOI: 10.1038/NPHYS2460 (2012); arXiv:1210.7308]. [Preview Abstract] |
Thursday, March 21, 2013 3:42PM - 4:18PM |
W3.00003: Three-dimensionality of space and the quantum bit: an information-theoretic approach Invited Speaker: Markus Mueller It is sometimes pointed out as a curiosity that the state space of quantum two-level systems, i.e. the qubit, and actual physical space are both three-dimensional and Euclidean. In this talk, I report on joint work with Lluis Masanes [1], where we attempt an information-theoretic analysis of this relationship, by proving a particular mathematical result: suppose that physics takes place in d spatial dimensions, and that some events happen probabilistically (not assuming quantum theory in any way). Furthermore, suppose there are systems that behave in some sense as ``units of direction information,'' interacting continuously and reversibly in time. We prove that this uniquely determines spatial dimension d=3 and quantum theory on two qubits (that is, the complex Hilbert space formalism and unitary time evolution). Moreover, we prove that it allows observers to infer local spatial geometry from probability measurements. This applies and generalizes results obtained earlier with further collaborators [2,3]. \\[4pt] [1] M. P. Mueller and Ll. Masanes, Three-dimensionality of space and the quantum bit: how to derive both from information-theoretic postulates, arXiv:1206.0630\\[0pt] [2] G. de la Torre, Ll. Masanes, A. J. Short, and M. P. Mueller, Deriving quantum theory from its local structure and reversibility, Phys. Rev. Lett. 109, 090403 (2012)\\[0pt] [3] Ll. Masanes, M. P. Mueller, D. Perez-Garcia, and R. Augusiak, Entangling dynamics beyond quantum theory, arXiv:1111.4060 [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:54PM |
W3.00004: Quantum correlations with indefinite causal order Invited Speaker: Caslav Brukner Quantum mechanics differs from classical physics in that no definite values can be attributed to unobserved physical quantities. However, the notion of time and of causal order preserves such an objective status in the theory: all operations are assumed to be ordered such that every operation is either in the future, in the past or space-like separated from any other operation. Consequently, the correlations between operations respect definite causal order: they are either signalling correlations for the time-like or no-signalling correlations for the space-like separated operations. I will present a framework that assumes only that operations in local laboratories are described by quantum mechanics (i.e. are completely-positive maps), but relax the assumption that they are causally connected. Remarkably, we find situations where two operations are neither causally ordered nor in a probabilistic mixture of definite causal orders, i.e. one cannot say that one operations is before or after the other. The correlations between the operations are shown to enable performing a communication task (``causal game'') that is impossible if the operations are ordered according to a fixed background time. I will discuss experimental perspectives for observing such correlations in nature. [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:30PM |
W3.00005: Realism and the epistemic view of quantum states Invited Speaker: Terry Rudolph The idea that quantum states reflect only an observers knowledge/beliefs/information about the world has a long history, with a wide variety of strong arguments having been proffered in its favour. The challenge for an advocate of this position, however, is to identify what we can deduce is ``really going on'' out there. There seem to three main paths proponents of the epistemic view have followed in trying to extract such a narrative from quantum theory. I will explain how the most naive such path--that quantum states can be associated with standard (probabilistic) uncertainty about some (arbitrary) real states of the world--is not tenable under some extremely mild assumptions about how any theory of reality must treat independent experiments. I will then overview the other two paths and what I see as the challenges they face. [Preview Abstract] |
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