Spectrally entangled biphoton state of cascade emissions from a Doppler-broadened atomic ensemble

TL;DR

This paper theoretically analyzes the spectral and entanglement properties of biphoton states generated via cascade emissions from Doppler-broadened atomic ensembles, highlighting how temperature, decay rates, and pulse duration influence entanglement for quantum communication.

Contribution

It introduces a detailed theoretical model linking superradiant decay, Doppler broadening, and excitation parameters to biphoton spectral entanglement, enabling spectral shaping for quantum communication.

Findings

Longer excitation pulses increase entanglement.

Enhanced decay rates lead to more entangled biphotons.

Optimal temperature can minimize entanglement for specific applications.

Abstract

We theoretically investigate the spectral property of biphoton state from the cascade emissions from a Doppler-broadened atomic ensemble. This biphoton state is spontaneously created in the four-wave-mixing process. The upper transition of the emissions lies in telecom bandwidth, which prevails in fiber-based quantum communication for low-loss transmission. We obtain the spectral property in terms of superradiant decay rates of the lower transition, excitation pulse durations, and temperature of the medium. We quantify their frequency entanglement by Schmidt decomposition and find that more entangled source can be generated with longer excitation pulses, enhanced decay rates, and significant Doppler broadening. A minimally entangled biphoton source can also be located at some optimal temperature of the atoms. This allows spectral shaping of continuous frequency entanglement, which is useful in multimode long-distance quantum communication.