This is a list of this week’s papers on quantum foundations published in various journals or uploaded to preprint servers such as arxiv.org and PhilSci Archive.

on 2016-11-24 7:50am GMT

Authors: Alessio Belenchia, Dionigi M. T. Benincasa, Stefano Liberati, Francesco Marin, Francesco Marino, Antonello Ortolan

The nonrelativistic limit of nonlocal modifications to the Klein Gordon operator is studied, and the experimental possibilities of casting stringent constraints on the nonlocality scale via planned and/or current optomechanical experiments are discussed. Details of the perturbative analysis and semianalitical simulations leading to the dynamical evolution of a quantum harmonic oscillator in the presence of non locality reported in [1], together with a comprehensive account of the experimental methodology with particular regard to sensitivity limitations related to thermal decoherence time and active cooling of the oscillator, are given. Finally, a strategy for detecting non-locality scales of the order of $10^{- 22} \div 10^{- 26}$ m by means of the spontaneous time periodic squeezing of quantum coherent states is provided.

Thermodynamical cost of some interpretations of quantum theory

on 2016-11-23 3:00pm GMT

Author(s): Adán Cabello, Mile Gu, Otfried Gühne, Jan-Åke Larsson, and Karoline Wiesner

The interpretation of quantum theory is one of the longest-standing debates in physics. Type I interpretations see quantum probabilities as determined by intrinsic properties of the observed system. Type II see them as relational experiences between an observer and the system. It is usually believed…

[Phys. Rev. A 94, 052127] Published Wed Nov 23, 2016

on 2016-11-23 12:50am GMT

Authors: Paweł Horodecki, Ravishankar Ramanathan

The no-signaling constraints state that the probability distribution of the outputs of any subset of parties is independent of the inputs of the complementary set; here we re-examine these to see how they arise from relativistic causality. We show that while the usual no-signaling constraints are sufficient, in general they are not necessary to ensure that a theory does not violate causality. Depending on the exact space-time coordinates of the measurement events in a multi-party Bell experiment, causality only imposes a subset of the usual no-signaling conditions. After revisiting the derivation of the two-party no-signaling constraints, we consider the three-party Bell scenario and characterise a space-time region in which a subset of the no-signaling conditions is sufficient to preserve causality. Secondly, we examine the implications for device-independent cryptography against an eavesdropper constrained only by the laws of relativity. We show an explicit attack in certain measurement configurations on a family of protocols based on the n-party Mermin inequalities that were previously proven secure under the old no-signaling conditions. We then show that security of two-party protocols can also be compromised when the eavesdropper’s measurement configuration is not constrained. The monogamy of non-local correlations that underpin their use in secrecy can also be broken in certain space-time configurations. Thirdly, we inspect the notion of free-choice and propose a definition of free-choice in the multi-party Bell experiment. We then re-examine the question whether quantum correlations may admit explanations by finite speed superluminal influences. Finally, we study genuine multiparty non-locality and present a new class of causal bilocal models. We propose a new Svetlichny-type inequality that is satisfied by the causal bilocal model and show its violation within quantum theory.

on 2016-11-23 12:50am GMT

Authors: Johannes Handsteiner, Andrew S. Friedman, Dominik Rauch, Jason Gallicchio, Bo Liu, Hannes Hosp, Johannes Kofler,David Bricher, Matthias Fink, Calvin Leung, Anthony Mark, Hien T. Nguyen, Isabella Sanders, Fabian Steinlechner, Rupert Ursin,Sören Wengerowsky, Alan H. Guth, David I. Kaiser, Thomas Scheidl, Anton Zeilinger

Bell’s theorem states that some predictions of quantum mechanics cannot be reproduced by a local-realist theory. That conflict is expressed by Bell’s inequality, which is usually derived under the assumption that there are no statistical correlations between the choices of measurement settings and anything else that can causally affect the measurement outcomes. In previous experiments, this “freedom of choice” was addressed by ensuring that selection of measurement settings via conventional “quantum random number generators” (QRNGs) was space-like separated from the entangled particle creation. This, however, left open the possibility that an unknown cause affected both the setting choices and measurement outcomes as recently as mere microseconds before each experimental trial. Here we report on a new experimental test of Bell’s inequality that, for the first time, uses distant astronomical sources as “cosmic setting generators.” In our tests with polarization-entangled photons, measurement settings were chosen using real-time observations of Milky Way stars while simultaneously ensuring locality. We observe statistically significant $\gtrsim 11.7 \sigma$ and $\gtrsim 13.8 \sigma$ violations of Bell’s inequality with estimated $p$-values of $ \lesssim 7.4 \times 10^{-32}$ and $\lesssim 1.1 \times 10^{-43}$, respectively, thereby pushing back by $\sim$600 years the most recent time by which any local-realist influences could have engineered the observed Bell violation.

What path does a tunneling particle follow?. (arXiv:1611.06780v1 [quant-ph])

on 2016-11-23 12:50am GMT

Authors: Charis Anastopoulos, Ntina Savvidou

Attempts to find a quantum-to-classical correspondence in a classically forbidden region leads to non-physical paths, involving, for example, complex time or spatial coordinates. Here, we identify genuine quasi-classical paths for tunneling in terms of probabilistic correlations in sequential time-of-arrival measurements. In particular, we construct the post-selected probability density $P_{p.s.}(x, \tau)$ for a particle to be found at time $\tau$ in position $x$ inside the forbidden region, provided that it later crossed the barrier. The classical paths follow from the maximization of the probability density with respect to $\tau$. For almost monochromatic initial states, the paths correspond to the maxima of the modulus square of the wave-function $|\psi(x,\tau)|^2$ with respect to $\tau$ and for constant $x$ inside the barrier region. The derived paths are expressed in terms of classical equations, but they have no analogues in classical mechanics. Finally, we evaluate the paths explicitly for the case of a square potential barrier.

Scheme of a Derivation of Collapse from Quantum Dynamics II. (arXiv:1611.06731v1 [quant-ph])

on 2016-11-23 12:50am GMT

Authors: Roland Omnès

Is wave function collapse a prediction of the Schr\”odinger equation? This unusual problem is explored in an enlarged framework of interpretation, where quantum dynamics is considered exact and its interpretation is extended to include local entanglement of two systems, including a macroscopic one. This property of local entanglement, which results directly from the Schr\”odinger equation but is unrelated with observables, is measured by local probabilities, fundamentally distinct from quantum probabilities and evolving nonlinearly. When applied to a macroscopic system and the fluctuations in its environment, local entanglement can also inject a formerly ignored species of incoherence into the quantum state of this system,. When applied to a quantum measurement, the conjunction of these two effects suggests a self-consistent mechanism of collapse, which would directly derive from the Schr\”odinger equation. (This work develops and improves significantly a previously circulated version with the same title [23])

on 2016-11-23 12:50am GMT

Authors: Arkady Bolotin

The existence of higher than pairwise quantum interference in the set-up, in which there are more than two slits, is currently under experimental investigation. However, it is still unclear what the confirmation of existence of such interference would mean for quantum theory — whether that usual quantum mechanics is merely a limiting case of some more general theory or whether that some assumption of quantum theory taken as a fundamental one does not actually hold true. The present paper tries to understand why quantum theory is limited only to a certain kind of interference.

on 2016-11-23 12:50am GMT

Authors: R. Flack, B. J. Hiley

In a recent paper Mahler {\em et al.} have argued that the experiments of Kocsis {\em et al.} provide experimental evidence for Bohmian mechanics. Unfortunately these experiments used relativistic, zero rest mass photons whereas Bohmian mechanics is based on non-relativistic Schr\”{o}dinger particles having non-zero rest mass. The experimental results can be correctly understood in terms of a different approach based on the electromagnetic field that was already outlined by Bohm in an appendix of the second of his 1952 papers. A subsequent development of this approach by Bohm, Hiley, Kaloyerou and Holland, show in detail how this theory accounts for the experimental results. We are led to the conclusion that the experiments have constructed mean momentum flow lines by measuring the real part of the weak Poynting vector. These results support and clarify the analysis of Bliokh {\em et al}. The experimental flow lines can be constructed independently of the number of photons in the beam leading to the conclusion that flow lines cannot be interpreted as `photon trajectories’. We discuss exactly how the notion of a photon arises in the field approach.

Entanglement in Macroscopic Systems. (arXiv:1611.06028v1 [quant-ph])

on 2016-11-21 9:28am GMT

Authors: J. Sperling, I. A. Walmsley

In the macroscopic reality of our daily live, quantum phenomena such as entanglement are not observable. However, the constituent parts of this classical realm, e.g., atoms or photons, are almost completely determined by the laws of quantum physics. Yet, theory and experiments cannot predict a sharp borderline between the classical and the quantum domain. So, at which point does one lose the information of the quantum correlations in many-particle systems? For addressing this question, we explore the theory of entangled quantum systems of harmonic oscillators consisting of an arbitrary number of interacting particles. We introduce the operational measure of the entanglement visibility to access the measurable entanglement, which will be applied especially to energy measurements. Moreover, the properties of the energy spectrum of entangled and separable states are compared. Our analytical analysis suggest that the entanglement visibility decays with the number of particles without the need of any decoherence process. To strengthen this claim, we also derive and apply a method to detect entanglement in systems without a fixed, but a fluctuating number of particles. In this context, we further study the thermal states of the given correlated system together with the temperature dependence of entanglement.

WAY beyond conservation laws. (arXiv:1611.05905v1 [quant-ph])

on 2016-11-21 9:28am GMT

Authors: Mikko Tukiainen

The ability to measure every quantum observable is ensured by a fundamental result in quantum measurement theory. Nevertheless, additive conservation laws associated with physical symmetries, such as the angular momentum conservation, may lead to restrictions on the measurability of the observables. Such limitations are imposed by the theorem of Wigner, Araki and Yanase (WAY). In this paper a new formulation of the WAY-theorem is presented rephrasing the measurability limitations in terms of quantum incompatibility. This broader mathematical basis enables us to both capture and generalise the WAY-theorem by allowing to drop the assumptions of additivity and even conservation of the involved quantities. Moreover, we extend the WAY-theorem to the general level of positive operator valued measures.

The physical salience of non-fundamental local beables

on 2016-11-20 6:40pm GMT

Publication date: Available online 19 November 2016

**Source:**Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics

Author(s): Matthias Egg

I defend the idea that objects and events in three-dimensional space (so-called local beables) are part of the derivative ontology of quantum mechanics, rather than its fundamental ontology. The main objection to this idea stems from the question of how it can endow local beables with physical salience, as opposed to mere mathematical definability. I show that the responses to this objection in the previous literature are insufficient, and I provide the necessary arguments to render them successful. This includes demonstrating the legitimacy of dynamical considerations in the derivation of local beables and responding to the threat stemming from the availability of different sets of local beables in the context of the GRW theory.