%0 Journal Article
%J Physical Review A
%D 2014
%T Quantum Interactions with Closed Timelike Curves and Superluminal Signaling
%A Jeffrey Bub
%A Allen Stairs
%X There is now a significant body of results on quantum interactions with closed timelike curves (CTCs) in the quantum information literature, for both the Deutsch model of CTC interactions (D-CTCs) and the projective model (P-CTCs). As a consequence, there is a prima facie argument exploiting entanglement that CTC interactions would enable superluminal and, indeed, effectively instantaneous signaling. In cases of spacelike separation between the sender of a signal and the receiver, whether a receiver measures the local part of an entangled state or a disentangled state to access the signal can depend on the reference frame. We propose a consistency condition that gives priority to either an entangled perspective or a disentangled perspective in spacelike separated scenarios. For D-CTC interactions, the consistency condition gives priority to frames of reference in which the state is disentangled, while for P-CTC interactions the condition selects the entangled state. Using the consistency condition, we show that there is a procedure that allows Alice to signal to Bob in the past via relayed superluminal communications between spacelike separated Alice and Clio, and spacelike separated Clio and Bob. This opens the door to time travel paradoxes in the classical domain. Ralph (arXiv:1107.4675) first pointed this out for P-CTCs, but we show that Ralph's procedure for a 'radio to the past' is flawed. Since both D-CTCs and P-CTCs allow classical information to be sent around a spacetime loop, it follows from a result by Aaronson and Watrous (Proc.Roy.Soc.A, 465:631-647 (2009)) for CTC-enhanced classical computation that a quantum computer with access to P-CTCs would have the power of PSPACE, equivalent to a D-CTC-enhanced quantum computer.
%B Physical Review A
%V 89
%8 2014/2/12
%G eng
%U http://arxiv.org/abs/1309.4751v4
%N 2
%! Phys. Rev. A
%R 10.1103/PhysRevA.89.022311
%0 Journal Article
%D 2010
%T Contextuality in Quantum Mechanics: Testing the Klyachko Inequality
%A Jeffrey Bub
%A Allen Stairs
%X The Klyachko inequality is an inequality for the probabiities of the values of five observables of a spin-1 particle, which is satisfied by any noncontextual assignment of values to this set of observables, but is violated by the probabilities defined by a certain quantum state. We describe an experiment between two entangled spin-1 particles to test contextuality via a related inequality. We point out that a test of contextuality by measurements on a single particle to confirm the Klyachko inequality requires an assumption of non-disturbance by the measuring instrument, which is avoided in the two-particle experiment.
%8 2010/06/02
%G eng
%U http://arxiv.org/abs/1006.0500v2
%0 Journal Article
%J Foundations of Physics
%D 2009
%T Contextuality and nonlocality in 'no signaling' theories
%A Jeffrey Bub
%A Allen Stairs
%X We define a family of 'no signaling' bipartite boxes with arbitrary inputs and binary outputs, and with a range of marginal probabilities. The defining correlations are motivated by the Klyachko version of the Kochen-Specker theorem, so we call these boxes Kochen-Specker-Klyachko boxes or, briefly, KS-boxes. The marginals cover a variety of cases, from those that can be simulated classically to the superquantum correlations that saturate the Clauser-Horne-Shimony-Holt inequality, when the KS-box is a generalized PR-box (hence a vertex of the `no signaling' polytope). We show that for certain marginal probabilities a KS-box is classical with respect to nonlocality as measured by the Clauser-Horne-Shimony-Holt correlation, i.e., no better than shared randomness as a resource in simulating a PR-box, even though such KS-boxes cannot be perfectly simulated by classical or quantum resources for all inputs. We comment on the significance of these results for contextuality and nonlocality in 'no signaling' theories.
%B Foundations of Physics
%V 39
%P 690 - 711
%8 2009/4/21
%G eng
%U http://arxiv.org/abs/0903.1462v2
%N 7
%! Found Phys
%R 10.1007/s10701-009-9307-8
%0 Journal Article
%D 2005
%T Conditionalizing and commutativity: a note on Malley
%A Allen Stairs
%A Jeffrey Bub
%X This paper has been withdrawn.
%8 2005/06/19
%G eng
%U http://arxiv.org/abs/quant-ph/0506159v2