Direct observation of the degree of quantum correlations using photon-number resolving detectors

TL;DR

This paper demonstrates direct measurement of quantum correlations in photon pairs generated by parametric down-conversion using photon-number resolving detectors, revealing the correlation degree despite detector imperfections.

Contribution

It introduces a method to measure and control photon-number correlations with high photon counts using photon-number resolving detectors and data-driven criteria.

Findings

Photon-number correlations are clearly observed despite detector inefficiencies.

Two data-derived criteria effectively measure the correlation degree.

A fitting technique links measured distributions to the original quantum state.

Abstract

Optical parametric down-conversion is a common source for the generation of non-classical correlated photonic states. Using a parametric down-conversion source and photon-number resolving detectors, we measure the two-mode photon-number distribution of up to 10 photons. By changing the heralded collection efficiency, we control the level of correlations between the two modes. Clear evidence for photon-number correlations are presented despite detector imperfections such as low detection efficiency and other distorting effects. Two criteria, derived directly from the raw data, are shown to be good measures for the degree of correlation. Additionally, using a fitting technique, we find a connection between the measured photon-number distribution and the degree of correlation of the reconstructed original two-mode state. These observations are only possible as a result of the detection of high photon number events.