Direct Sampling of Negative Quasiprobabilities of a Squeezed State

TL;DR

This paper introduces a method to directly measure nonclassicality quasiprobabilities of squeezed states, enabling clear demonstration of their nonclassical nature through negativity detection.

Contribution

The authors derive pattern functions for direct experimental determination of nonclassicality quasiprobabilities, providing a universal test for nonclassicality in quantum states.

Findings

Negativities of quasiprobabilities are necessary and sufficient for nonclassicality.

Method successfully applied to a squeezed vacuum state generated by parametric down-conversion.

Enables direct experimental verification of nonclassicality without state reconstruction.

Abstract

Although squeezed states are nonclassical states, so far, their nonclassicality could not be demonstrated by negative quasiprobabilities. In this work we derive pattern functions for the direct experimental determination of so-called nonclassicality quasiprobabilities. The negativities of these quantities turn out to be necessary and sufficient for the nonclassicality of an arbitrary quantum state and are therefore suitable for a direct and general test of nonclassicality. We apply the method to a squeezed vacuum state of light that was generated by parametric down-conversion in a second-order nonlinear crystal.