Leggett-Garg inequalities and no-signaling in time: A quasiprobability approach

J. J. Halliwell
Phys. Rev. A 93, 022123 – Published 26 February 2016

Abstract

The Leggett-Garg (LG) inequalities were proposed in order to assess whether sets of pairs of sequential measurements on a single quantum system can be consistent with an underlying notion of macrorealism. Here, the LG inequalities are explored using a simple quasiprobability linear in the projection operators to describe the properties of the system at two times. We show that this quasiprobability is measurable, has the same correlation function as the usual two-time measurement probability (for the bivalent variables considered here) and has the key property that the probabilities for the later time are independent of whether an earlier measurement was made, a generalization of the no-signaling in time condition of Kofler and Brukner. We argue that this quasiprobability, appropriately measured, provides a noninvasive measure of macrorealism per se at the two-time level. This measure, when combined with the LG inequalities, provides a characterization of macrorealism more detailed than that provided by the LG inequalities alone. When the quasiprobability is non-negative, the LG system has a natural parallel with the Einstein-Podolsky-Rosen-Bohm system and Fine's theorem. A simple spin model illustrating key features of the approach is exhibited.

  • Received 13 August 2015
  • Revised 16 December 2015

DOI:https://doi.org/10.1103/PhysRevA.93.022123

©2016 American Physical Society

Physics Subject Headings (PhySH)

General Physics

Authors & Affiliations

J. J. Halliwell*

  • Blackett Laboratory, Imperial College, London SW7 2BZ, United Kingdom

  • *j.halliwell@imperial.ac.uk

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Issue

Vol. 93, Iss. 2 — February 2016

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