Weekly Papers on Quantum Foundations (7)

Author(s): L. Czekaj, M. Horodecki, P. Horodecki, and R. Horodecki

To explain the conceptual gap between classical and quantum and other, hypothetical descriptions of the world, several principles have been proposed. So far, all these principles have not explicitly included the uncertainty relation. Here we introduce an information content principle (ICP) which rep…
[Phys. Rev. A 95, 022119] Published Fri Feb 17, 2017

Abstract

A two boundary quantum mechanics without time ordered causal structure is advocated as consistent theory. The apparent causal structure of usual “near future” macroscopic phenomena is attributed to a cosmological asymmetry and to rules governing the transition between microscopic to macroscopic observations. Our interest is a heuristic understanding of the resulting macroscopic physics.

Authors: John R. Klauder

Canonical quantization relies on Cartesian, canonical, phase-space coordinates to promote to Hermitian operators, which also become the principal ingredients in the quantum Hamiltonian. While generally appropriate, this procedure can also fail, e.g., for covariant, quartic, scalar fields in five-and-more spacetime dimensions (and possibly four spacetime dimensions as well), which become trivial; such failures are normally blamed on the `problem' rather than on the `quantization procedure'. In Enhanced Quantization the association of $c$-numbers to $q$-numbers is chosen very differently such that: (i) there is no need to seek classical, Cartesian, phase-space coordinates; (ii) {\it every} classical, contact transformation is applicable and {\it no} change of the quantum operators arises; (iii) a new understanding of the importance of `Cartesian coordinates' is established; and (iv) although discussed elsewhere in detail, the procedures of enhanced quantization offer fully acceptable solutions yielding non-trivial results for quartic scalar fields in four-and-more spacetime dimensions. In early sections, this paper offers a wide-audience approach to the basic principles of Enhanced Quantization using simple examples; later, several significant examples are cited for a deeper understanding. An historical note concludes the paper.

Authors: Robert DickinsonJeff ForshawPeter Millington

We present a novel approach to computing transition probabilities in quantum field theory, which allows them to be written directly in terms of expectation values of nested commutators and anti-commutators of field operators, rather than squared matrix elements. We show that this leads to a diagrammatic expansion in which the retarded propagator plays a dominant role. As a result, one is able to see clearly how faster-than-light signalling is prevented between sources and detectors. Finally, we comment on potential implications of this approach for dealing with infra-red divergences.

<span>I argue that our judgements regarding the locally causal models that are compatible with a given constraint implicitly depend, in part, on the context of inquiry. It follows from this that certain quantum no-go theorems, which are particularly striking in the traditional foundational context, have no force when the context switches to a discussion of the physical systems we are capable of building with the aim of classically reproducing quantum statistics. I close with a general discussion of the possible implications of this for our understanding of the limits of classical description, and for our understanding of the fundamental aim of physical investigation. <ul><li><strong>1</strong><span>Introduction</span></li><li><strong>2</strong><span>No-Go Results</span><ul><li><strong>2.1</strong><span>The CHSH inequality</span></li><li><strong>2.2</strong><span>The GHZ equality</span></li></ul></li><li><strong>3</strong><span>Classically Simulating Quantum Statistics</span><ul><li><strong>3.1</strong><span>GHZ statistics</span></li><li><strong>3.2</strong><span>Singlet statistics</span></li></ul></li><li><strong>4</strong><span>What Is a Classical Computer Simulation?</span></li><li><strong>5</strong><span>Comparing the All-or-Nothing GHZ with Statistical (In)equalities</span></li><li><strong>6</strong><span>General Discussion</span></li><li><strong>7</strong><span>Conclusion</span></li></ul></span>

Author(s): Eli Pollak

A quantum mechanical transition path time probability distribution is formulated and its properties are studied using a parabolic barrier potential model. The average transit time is well defined and readily calculated. It is smaller than the analogous classical mechanical average transit time, vani…
[Phys. Rev. Lett. 118, 070401] Published Wed Feb 15, 2017

Authors: André Großardt

No experimental evidence exists, to date, whether or not the gravitational field must be quantised. Theoretical arguments in favour of quantisation are inconclusive. The most straightforward alternative to quantum gravity, a coupling between classical gravity and quantum matter according to the semi-classical Einstein equations, yields a nonlinear modification of the Schr\"odinger equation. Here, effects of this so-called Schr\"odinger-Newton equation are discussed, which allow for technologically feasible experimental tests.

Abstract

Svensson (Found Phys 45: 1645, 2015) argued that the concept of the weak value of an observable of a pre- and post-selected quantum system cannot be applied when the expectation value of the observable in the initial state vanishes. Svensson’s argument is analyzed and shown to be inconsistent using several examples.

Kastner, Ruth (2017) On Quantum Non-Unitarity as a Basis for the Second Law of Thermodynamics. [Preprint]
Kastner, Ruth (2017) Demystifying weak measurements. [Preprint]

Author(s): Samir Kunkri, Manik Banik, and Sibasish Ghosh

Nonlocality is one of the main characteristic features of quantum systems involving more than one spatially separated subsystem. It is manifested theoretically as well as experimentally through violation of some local realistic inequality. On the other hand, classical behavior of all physical phenom…
[Phys. Rev. A 95, 022116] Published Tue Feb 14, 2017

Author(s): Martí Perarnau-Llobet, Elisa Bäumer, Karen V. Hovhannisyan, Marcus Huber, and Antonio Acin

The operator of work in quantum thermodynamics is incompatible with quantum mechanics, which is why the correspondence principle has to be critically examined whenever work is involved.


[Phys. Rev. Lett. 118, 070601] Published Tue Feb 14, 2017

Authors: Diego A. QuinonesTeodora OnigaBenjamin T. H. VarcoeCharles H.-T. Wang

We carry out a theoretical investigation on the collective dynamics of an ensemble of correlated atoms, subject to both vacuum fluctuations of spacetime and stochastic gravitational waves. A general approach is taken with the derivation of a quantum master equation capable of describing arbitrary confined nonrelativistic matter systems in an open quantum gravitational environment. It enables us to relate the spectral function for gravitational waves and the distribution function for quantum gravitational fluctuations and to indeed introduce a new spectral function for the zero-point fluctuations of spacetime. The formulation is applied to two-level Rydberg-like identical bosonic atoms in a cavity, leading to a gravitational transition mechanism through certain quadrupole moment operators. For a large number $N$ of such atoms, we find their equilibrium state to satisfy the Boltzmann distribution. The overall relaxation rate before reaching equilibrium is found to scale collectively with $N$. However, we are able to identify certain states whose decay and excitation rates with stochastic gravitational waves and vacuum spacetime fluctuations amplify more significantly with a factor of $N^2$. Using such favourable states as a means of measuring both conventional stochastic gravitational waves and novel zero-point spacetime fluctuations, we determine the theoretical lower bounds for the respective spectral functions. Finally, we discuss the implications of our findings on future observations of gravitational waves of a wider spectral window than currently accessible. Especially, the possible sensing of the zero-point fluctuations of spacetime could provide an opportunity to generate initial evidence and further guidance of quantum gravity.

Authors: Armin TavakoliMarc Olivier RenouNicolas GisinNicolas Brunner

The problem of characterizing classical and quantum correlations in networks is considered. Contrary to the usual Bell scenario, where distant observers share a physical system emitted by one common source, a network features several independent sources, each distributing a physical system to a subset of observers. In the quantum setting, the observers can perform joint measurements on initially independent systems, which may lead to strong correlations across the whole network. In this work, we introduce a technique to systematically map a Bell inequality to a family of Bell-type inequalities bounding classical correlations on networks in a star-configuration. Also, we show that whenever a given Bell inequality can be violated by some entangled state $\rho$, then all the corresponding network inequalities can be violated by considering many copies of $\rho$ distributed in the star network. The relevance of these ideas is illustrated by applying our method to a specific multi-setting Bell inequality. We derive the corresponding network inequalities, and study their quantum violations.

Cuffaro, Michael E. (2017) Universality, Invariance, and the Foundations of Computational Complexity in the light of the Quantum Computer. [Preprint]
Szabo, Laszlo E. (2017) Meaning, Truth, and Physics. [Preprint]
Dorato, Mauro and Rossanese, Emanuele (2017) Feynman's Diagrams, Pictorial Representations and Styles of Scientific Thinking. [Preprint]
Dawid, Richard (2017) A Philosophical Look at the Discovery of the Higgs Boson. [Published Article or Volume]

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