Operational definition of quantum correlations of light

TL;DR

This paper introduces a practical method to define and measure the core quantum correlations in two-mode light fields using simple interferometric setups and photon-number-resolving detectors, even with imperfect devices.

Contribution

It establishes the concept of essential quantum correlations and provides a reconstruction method and nonclassicality criteria based on accessible measurements.

Findings

Quantum correlations can be characterized without global phase reference.

The method is robust to detector imperfections.

Essential quantum correlations are observable with simple interferometric setups.

Abstract

Quantum features of correlated optical modes define a major aspect of the nonclassicality in quantized radiation fields. However, the phase-sensitive detection of a two-mode light field is restricted to interferometric setups and local intensity measurements. Even the full reconstruction of the quantum state of a single radiation mode relies on such detection layouts and the preparation of a well-defined reference light field. In this work, we establish the notion of the essential quantum correlations of two-mode light fields. It refers to those quantum correlations which are measurable by a given device, i.e., the accessible part of a nonclassical Glauber-Sudarshan phase-space distribution, which does not depend on a global phase. Assuming a simple four-port interferometer and photon-number-resolving detectors, we derive the reconstruction method and nonclassicality criteria based on the Laplace-transformed moment-generating function of the essential quasiprobability. With this technique, we demonstrate that the essential quantum correlations of a polarization tomography scheme are observable even if the detectors are imperfect and cannot truly resolve the photon statistics.