Condition for macroscopic realism beyond the Leggett-Garg inequalities

TL;DR

This paper introduces a new necessary condition called no-signaling in time for macrorealism, which can be violated by quantum mechanics even when Leggett-Garg inequalities are not, offering a deeper understanding of quantum temporal correlations.

Contribution

It proposes the no-signaling in time condition as an alternative criterion for macrorealism, extending the analysis beyond Leggett-Garg inequalities.

Findings

No-signaling in time can be violated by quantum predictions.

Violations occur even when Leggett-Garg inequalities hold.

Provides a new perspective on quantum temporal correlations.

Abstract

In 1985, Leggett and Garg put forward the concept of macroscopic realism (macrorealism) and, in analogy to Bell's theorem, derived a necessary condition in terms of inequalities, which are now known as the Leggett-Garg inequalities. In this paper, we discuss another necessary condition called no-signaling in time. It solely bases on comparing the probability distribution for a macrovariable at some time for the cases where previously a measurement has or has not been performed. Although the concept is analogous to the no-signaling condition in the case of Bell tests, it can be violated according to quantum mechanical predictions even in situations where no violation of Leggett-Garg inequalities is possible.