Joint Temporal Density Measurements for Two-Photon State Characterization

TL;DR

This paper introduces a novel method for characterizing two-photon quantum states through joint temporal density measurements, enabling detailed analysis of entanglement and state manipulation in quantum optics.

Contribution

The authors develop a new time-resolved detection technique using femtosecond upconversion to measure joint temporal density of entangled photons, advancing quantum state characterization methods.

Findings

First measurement of joint temporal density of entangled photon pairs.

Demonstrated control of frequency entanglement by adjusting pump bandwidth.

Achieved a nearly unentangled two-photon state with high purity.

Abstract

We demonstrate a new technique for characterizing two-photon quantum states based on joint temporal correlation measurements using time resolved single photon detection by femtosecond upconversion. We measure for the first time the joint temporal density of a two-photon entangled state, showing clearly the time anti-correlation of the coincident-frequency entangled photon pair generated by ultrafast spontaneous parametric down-conversion under extended phase-matching conditions. The new technique enables us to manipulate the frequency entanglement by varying the down-conversion pump bandwidth to produce a nearly unentangled two-photon state that is expected to yield a heralded single-photon state with a purity of 0.88. The time-domain correlation technique complements existing frequency-domain measurement methods for a more complete characterization of photonic entanglement in quantum information processing.