Observation of time correlation function of multimode two-photon pairs on a rubidium D$_2$ line

TL;DR

This paper demonstrates the generation and measurement of multimode two-photon states on a rubidium D2 line, revealing their time correlation functions and potential for quantum communication applications.

Contribution

It reports the first observation of oscillatory time correlation functions of multimode two-photon states on a rubidium D2 line using a PPKTP crystal and cavity filtering.

Findings

Cavity bandwidth of about 7.8 MHz observed

Time correlation functions match theoretical predictions

Potential for quantum storage and retrieval in Rb ensembles

Abstract

We report the generation of a type-I multimode two-photon state on a rubidium D$_2$ line (780 nm) using periodically poled KTiOPO$_4$ crystals. With a degenerate optical parametric oscillator far below threshold, we observe an oscillatory correlation function, the cross-correlation between two photons shows a cavity bandwidth of about 7.8 MHz. We also use a Fabry-P$\acute{\rm{e}}$rot etalon to filter its most longitudinal modes and observe its time correlation function. The experimental data are well fitted to theoretical curves. This system could be utilized for demonstrating storage and retrieval of narrowband photons in Rb atomic ensembles, which is important for long-distance quantum communication.