Time-resolved second-order autocorrelation function of parametric downconversion

TL;DR

This paper proposes a method to measure the time-resolved second-order autocorrelation function of one beam in parametric downconversion using temporal magnification, enabling direct inference of the beam's coherence and entanglement modes.

Contribution

It introduces a novel measurement technique combining temporal magnification with autocorrelation to analyze coherence and entanglement in parametric downconversion.

Findings

The method can resolve correlation times from picoseconds to nanoseconds.

The autocorrelation function shows a local maximum indicating photon bunching.

The approach links coherence measurement to the number of entanglement modes.

Abstract

We study a possibility of measuring the time-resolved second-order autocorrelation function of one of two beams generated in type-II parametric downconversion by means of temporal magnification of this beam, bringing its correlation time from the picosecond to the nanosecond scale, which can be resolved by modern photodetectors. We show that such a measurement enables one to infer directly the degree of global coherence of that beam, which is linked by a simple relation to the number of modes characterizing the entanglement between the two generated beams. We illustrate the proposed method by an example of photon pairs generated in a periodically poled KTP crystal with a symmetric group velocity matching for various durations of the pump pulse, resulting in different numbers of modes. Our theoretical model also shows that the magnified double-heralded autocorrelation function of one beam exhibits a local maximum around zero delay time, corresponding to photon bunching at a short time scale.