Violations of a Leggett-Garg inequality without signalling for a photonic qutrit probed with ambiguous measurements

TL;DR

This paper demonstrates a photonic qutrit system where violations of a Leggett-Garg inequality occur without signalling, using ambiguous measurements to test quantum temporal correlations.

Contribution

It introduces a method to perform Leggett-Garg tests with ambiguous measurements on a photonic qutrit, showing violations without signalling and validating ambiguous measurement use.

Findings

Violations of Leggett-Garg inequality observed without signalling.

Ambiguous and unambiguous measurements are both non-signalling.

The setup confirms the validity of ambiguous measurements in quantum tests.

Abstract

We realise a quantum three-level system with photons distributed among three different spatial and polarization modes. Ambiguous measurement of the state of the qutrit are realised by blocking one out for the three modes at any one time. Using these measurements we construct a test of a Leggett-Garg inequality as well as tests of no-signalling-in-time for the measurements. We observe violations of the Leggett-Garg inequality that can not be accounted for in terms of signalling. Moreover, we tailor the qutrit dynamics such that both ambiguous and unambiguous measurements are simultaneously non-signalling, which is an essential step for the justification of the use of ambiguous measurements in Leggett-Garg tests.