Generation of sub-MHz and spectrally-bright biphotons from hot atomic vapors with a phase mismatch-free scheme

TL;DR

This paper presents a phase mismatch-free scheme for generating high-purity, spectrally-bright biphotons from hot atomic vapors with tunable sub-MHz linewidths, outperforming previous hot-atom sources and rivaling cold-atom and cavity-based methods.

Contribution

The authors introduce an all-copropagating, phase-mismatch-free scheme for biphoton generation in hot vapors, achieving record brightness and narrow linewidths.

Findings

Maximum $g_{s,as}^{(2)}$ of 42 at 610 kHz linewidth

Generation rate per linewidth of 1,500 pairs/(s·MHz)

Narrowest linewidth of 290±20 kHz among single-mode biphotons

Abstract

We utilized the all-copropagating scheme, which maintains the phase-match condition, in the spontaneous four-wave mixing (SFWM) process to generate biphotons from a hot atomic vapor. The scheme enables our biphotons not only to surpass those in the previous works of hot-atom SFWM, but also to compete with the biphotons that are generated by either the cold-atom SFWM or the cavity-assisted spontaneous parametric down conversion. The biphoton linewidth in this work is tunable for an order of magnitude. As we tuned the linewidth to 610 kHz, the maximum two-photon correlation function, $g_{s,as}^{(2)}$, of the biphotons is 42. This $g_{s,as}^{(2)}$ violates the Cauchy-Schwartz inequality for classical light by 440 folds, and demonstrates that the biphotons have a high purity. The generation rate per linewidth of the 610-kHz biphoton source is 1,500 pairs/(s$\cdot$MHz), which is the best result of all the sub-MHz biphoton sources in the literature. By increasing the pump power by 16 folds, we further enhanced the generation rate per linewidth to 2.3$\times$10$^4$ pairs/(s$\cdot$MHz), while the maximum $g_{s,as}^{(2)}$ became 6.7. In addition, we are able to tune the linewidth down to 290$\pm$20 kHz. This is the narrowest linewidth to date, among all the various kinds of single-mode biphotons.