Experimental verification of a quantumness criterion for single systems based on a Bell-like inequality

TL;DR

This paper introduces and experimentally verifies a new criterion to distinguish quantum from classical behavior in single systems, based on violating a classical inequality using optical single photons.

Contribution

It presents the first experimental test of a quantumness criterion for single systems based on a Bell-like inequality involving joint probabilities.

Findings

The criterion is successfully violated by single photons in the experiment.

Superposition is identified as the key quantum feature.

The method uses only projective preparations and measurements.

Abstract

In this letter, we propose and experimentally test a quantumness criterion for single systems. The criterion is based on the violation of an already reported classical inequality. This inequality is expressed in terms of joint probabilities that we identify as realizations of preparation-and-measurement schemes. Our analysis points out that superposition is the key notion to grasp the quantum nature of physical systems. We verify our criterion in an all-optical experimental setup with heralded single photons generated via spontaneous parametric down conversion and use exclusively projective preparations and projective measurements.