Narrow-Band Biphoton Generation near Atomic Resonance

TL;DR

This paper reviews the generation of narrow-band biphotons using cold atomic gases, highlighting their advantages, theoretical modeling of the entangled photon states, and agreement with recent experimental results.

Contribution

It provides a theoretical framework for understanding biphoton generation in atomic ensembles, emphasizing the roles of linear and nonlinear responses and regimes of coherence time.

Findings

Long coherence time and high spectral brightness of biphotons from cold atoms.

Theoretical model aligns well with recent experimental data.

Identification of regimes dominated by linear or nonlinear coherence times.

Abstract

Generating nonclassical light offers a benchmark tool for the fundamental research and potential applications in quantum optics. Conventionally, it has become a standard technique to produce the nonclassical light through the nonlinear optical processes occurring in nonlinear crystals. In this review we describe using cold atomic-gas media to generate such nonclassical light, especially focusing on narrow-band biphoton generation. Compared with the standard procedure, the new biphoton source has such properties as long coherence time, long coherence length, high spectral brightness, and high conversion efficiency. In this paper we concentrate on the theoretical aspect of the entangled two-photon state produced from the four-wave mixing in a multilevel atomic ensemble. We show that both linear and nonlinear optical responses to the generated fields play an important role in determining the biphoton waveform and, consequently on the two-photon temporal correlation. There are two characteristic regimes determined by whether the linear or nonlinear coherence time is dominant. In addition, our model provides a clear physical picture that brings insight into understanding biphoton optics with this new source. We apply our model to recent work on generating narrow-band (and even subnatural linewidth) paired photons using the technique of electromagnetically induced transparency and slow-light effect in cold atoms, and find good agreements with experimental results.