Temporally-ultralong biphotons with a linewidth of 50 kHz

TL;DR

This paper reports the generation of ultralong biphotons with an unprecedented narrow spectral linewidth of 50 kHz, achieved through spontaneous four-wave mixing, enabling advancements in quantum technology with heralded single photons.

Contribution

First demonstration of biphotons with linewidth below 100 kHz, reaching 50 kHz, and showing controllable width with minimal impact on generation rate.

Findings

Longest biphoton temporal width of 13.4 μs

Spectral linewidth of 50 kHz, the narrowest to date

High spectral brightness of 1.2×10^6 pairs/(s·mW·MHz)

Abstract

We report the generation of biphotons, with a temporal full width at the half maximum (FWHM) of 13.4$\pm$0.3 $\mu$s and a spectral FWHM of 50$\pm$1 kHz, via the process of spontaneous four-wave mixing. The temporal width is the longest, and the spectral linewidth is the narrowest up to date. This is also the first biphoton result that obtains a linewidth below 100 kHz, reaching a new milestone. The very long biphoton wave packet has a signal-to-background ratio of 3.4, which violates the Cauchy-Schwarz inequality for classical light by 4.8 folds. Furthermore, we demonstrated a highly-tunable-linewidth biphoton source and showed that while the biphoton source's temporal and spectral width were controllably varied by about 24 folds, its generation rate only changed by less than 15\%. A spectral brightness or generation rate per pump power per linewidth of 1.2$\times$10$^6$ pairs/(s$\cdot$mW$\cdot$MHz) was achieved at the temporal width of 13.4 $\mu$s. The above results were made possible by the low decoherence rate and high optical depth of the experimental system, as well as the nearly phase-mismatch-free scheme employed in the experiment. This work has demonstrated a high-efficiency ultranarrow-linewidth biphoton source, and has made a substantial advancement in the quantum technology utilizing heralded single photons.