Measuring nonclassical correlations of two-photon states

TL;DR

This paper proposes a practical method to measure quantum discord in two-photon states using linear optics, offering a more efficient alternative to quantum tomography and enabling optical implementation of experiments previously limited to NMR.

Contribution

It introduces a feasible optical setup to measure quantum discord efficiently, surpassing the efficiency of full quantum state tomography.

Findings

The proposed method effectively measures quantum discord in optical systems.

Efficiency of the setup exceeds that of full quantum tomography.

Optical implementation of nonclassical correlation measurements is demonstrated.

Abstract

The threshold between classical and nonclassical two-qubit states is drawn at the place when these states can no longer be described by classical correlations, i.e., quantum discord or entanglement appear. However, to check if the correlations are classical (in terms of quantum discord and entanglement) it is sufficient to witness the lack of quantum discord because its zero value implies the lack of entanglement. We explain how the indicator of quantum discord introduced by Girolami and Adesso [Phys. Rev. Lett. 108, 150403 (2012)] can be practically measured in linear-optical systems using standard beam splitters and photon detectors. We study the efficiency of the setup assuming both ideal and real components and show that the efficiency of the proposed implementation is better than the full two-photon quantum tomography. Thus, we demonstrate that a class of experiments previously available on NMR platform can be implemented in optical systems.