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

Abstract

Assessing whether a quantum state $\hat{\rho }$ is nonclassical ($\text{i.e.}$, 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 $\text{a priori}$ 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.

arXiv commentMinor corrections in v2 to match the published version of the paper, 12 pages, 3 figures,

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• Rigorous results on approach to thermal equilibrium, entanglement, and nonclassicality of an optical quantum field mode scattering from the elements of a non-equilibrium quantum reservoir