Room-temperature biphoton source with a spectral brightness near the ultimate limit

TL;DR

This paper reports a hot-atom biphoton source with near-ultimate spectral brightness at room temperature, achieved through an all-copropagating SFWM scheme in atomic vapor, surpassing previous results and demonstrating potential for quantum communication.

Contribution

The study introduces a highly efficient hot-atom SFWM biphoton source with record spectral brightness, utilizing an all-copropagating scheme and high optical depth to approach the ultimate brightness limit.

Findings

Spectral brightness of 3.8×10^5 pairs/s/MHz, 17 times higher than previous atomic vapor sources.

Achieved linewidth of 960 kHz and a generation rate of 3.7×10^5 pairs/s.

Signal-to-background ratio of 2.7, violating classical bounds by 3.6 times.

Abstract

The biphotons, generated from a hot atomic vapor via the process of spontaneous four-wave mixing (SFWM), have the following merits: stable and tunable frequencies as well as linewidth. Such merits are very useful in the applications of long-distance quantum communication. However, the hot-atom SFWM biphoton sources previously had far lower values of generation rate per linewidth, i.e., spectral brightness, as compared with the sources of biphotons generated by the spontaneous parametric down conversion (SPDC) process. Here, we report a hot-atom SFWM source of biphotons with a linewidth of 960 kHz and a generation rate of 3.7$\times$ $10^5$ pairs/s. The high generation rate, together with the narrow linewidth, results in a spectral brightness of 3.8$\times$ $10^5$ pairs/s/MHz, which is 17 times of the previous best result with atomic vapors and also better than all known results with all kinds of media. The all-copropagating scheme together with a large optical depth (OD) of the atomic vapor is the key improvement, enabling the achieved spectral brightness to be about one quarter of the ultimate limit. Furthermore, this biphoton source had a signal-to-background ratio (SBR) of 2.7, which violated the Cauchy-Schwartz inequality for classical light by about 3.6 folds. Although an increasing spectral brightness usually leads to a decreasing SBR, our systematic study indicates that both of the present spectral brightness and SBR can be enhanced by further increasing the OD. This work demonstrates a significant advancement and provides useful knowledge in the quantum technology using photons.