Second-order correlations and purity of unheralded single photons from spontaneous parametric down-conversion

TL;DR

This paper presents a new way to calculate the second-order correlation function and experimentally characterizes the purity of unheralded and heralded single photons from SPDC, showing high-purity photon generation without heralding.

Contribution

It introduces a revised expression for $g^{(2)}$ and demonstrates high-rate, high-purity unheralded single photon generation using optimized time bins.

Findings

Higher rate of single photons with $g^{(2)}(0) = 0$ achieved without heralding.

Revised $g^{(2)}$ expression applicable to any fixed time window.

Experimental validation of photon purity from SPDC.

Abstract

Various quantum technology applications require high-purity single photons with high generation rate. Although different methods are employed to generate such photons, heralded single photons from spontaneous parametric down-conversion (SPDC) is the most commonly used approach. Photon generation rate from the heralded single-photon sources are limited by the efficiency of the detectors to record coincidence detection of the photon pairs which are lower than the single-photon counts recorded separately on each detector. In this paper we present a revised expression to calculate second-order temporal correlation function, $g^{(2)}$ for any fixed time window (bin) and report the experimental characterization of purity of unheralded and heralded single photons from the SPDC process. With an appropriate choice of time bin for a given pump power, without heralding we show that higher rate of single photons with $g^{(2)}(0) = 0$ can be generated with very high probability.