Narrowband four-photon states from spontaneous four-wave mixing

TL;DR

This paper demonstrates the generation of narrowband, time-correlated four-photon states from spontaneous four-wave mixing in cold rubidium atoms using continuous-wave pumping, suitable for quantum networking.

Contribution

It introduces a continuous-wave pumped scheme for producing narrowband four-photon states in cold atoms, differing from traditional pulsed nonlinear processes.

Findings

Achieved a four-photon generation rate of 2.5 million counts per second.

Produced photons with MHz bandwidth near atomic resonance.

Verified genuine four-photon correlations beyond accidental coincidences.

Abstract

We observe time-correlated four photons within a correlation window of 20ns from spontaneous four-wave mixing via a double-Lambda scheme in a cold cloud of Rb-87 atoms. In contrast to high-power pulsed pumping of chi^(2) nonlinear processes in crystals, our scheme generates correlated four-photon states by direct continuous-wave pumping at nominal powers. We verify the presence of genuinely correlated four-photon states over accidentals by higher-order intensity cross-correlation measurements and accidental subtraction. We infer a time-correlated four-photon generation rate of 2.5(4)x10^6 counts per second close to saturation. The photons produced are near-resonant with atomic transitions, and have a bandwidth in the order of MHz, making them readily compatible with quantum networking applications involving atoms.