Interferometric measurement of the quadrature coherence scale using two replicas of a quantum optical state

TL;DR

This paper introduces an experimental method using a simple interferometer with two copies of a quantum state to directly measure its quadrature coherence scale, bypassing the need for full state tomography.

Contribution

It presents a novel, practical interferometric technique to assess nonclassicality of quantum states without full tomography, extending the Hong-Ou-Mandel effect.

Findings

Demonstrated direct measurement of quadrature coherence scale

Implemented a multicopy interferometric technique in quantum optics

Circumvented the need for full state tomography

Abstract

Assessing whether a quantum state $\hat \rho$ is nonclassical ($\textit{i.e.}$, incompatible with a mixture of coherent states) is a ubiquitous question in quantum optics, yet a nontrivial experimental task because many nonclassicality witnesses are nonlinear in $\hat \rho$. In particular, if we want to witness or measure the nonclassicality of a state by evaluating its quadrature coherence scale, this $\textit{a priori}$ requires full state tomography. Here, we provide an experimental procedure for directly accessing this quantity with a simple linear interferometer involving two replicas (independent and identical copies) of the state $\hat \rho$ supplemented with photon-number-resolving measurements. This finding, which we interpret as an extension of the Hong-Ou-Mandel effect, illustrates the wide applicability of the multicopy interferometric technique in order to circumvent state tomography in quantum optics.