Subnatural-linewidth biphotons from a Doppler-broadened hot atomic vapour cell

Abstract

Entangled photon pairs, termed as biphotons, have been the benchmark tool for experimental quantum optics. The quantum-network protocols based on photon–atom interfaces have stimulated a great demand for single photons with bandwidth comparable to or narrower than the atomic natural linewidth. In the past decade, laser-cooled atoms have often been used for producing such biphotons, but the apparatus is too large and complicated for engineering. Here we report the generation of subnatural-linewidth (<6 MHz) biphotons from a Doppler-broadened (530 MHz) hot atomic vapour cell. We use on-resonance spontaneous four-wave mixing in a hot paraffin-coated 87Rb vapour cell at 63 °C to produce biphotons with controllable bandwidth (1.9–3.2 MHz) and coherence time (47–94 ns). Our backward phase-matching scheme with spatially separated optical pumping is the key to suppress uncorrelated photons from resonance fluorescence. The result may lead towards miniature narrowband biphoton sources.

Bright narrowband biphoton generation from a hot rubidium atomic vapor cell

“…The high biphoton signal contrast ratio in our system indicates a strong violation of the Cauchy-Schwarz inequality ½g s;as m , 56, is about five times higher than 12, the value that appeared in the previous research. 8 Besides, the overall photon pair rate ($40 000 s À1 ) is also about more than 20 times higher than that obtained in Ref. 8.…”

“…j5P 3=2 ; F ¼ 1i. In the first case, the same as the previous experiment, 8 Published by AIP Publishing. 110, 161101-1 Fig.…”

“…Recently, we demonstrated the generation of subnaturallinewidth (2 MHz) biphotons from a Doppler-broadened hot 87 Rb vapor cell, 8 which provides a possibility of miniaturized narrowband biphoton source based on free-space atomic vapor cell. The paraffin coating in increasing the groundstate coherence time and the spatially separated optical pumping were the two keys in suppressing on-resonance Raman scattering noise photons.…”

“…However, the scheme demonstrated in Ref. 8 can still be further optimized. First, the double-K atomic energy levels can be carefully chosen to maximize the third-order nonlinearity for enhancing the photon pair generation efficiency and the brightness.…”

“…In the previous experiment, the cross-sectional beam profile was Gaussian shape, 8 which was located aside from the SFWM volume. To increase the interaction area of optical pumping, we use a hollow-shaped beam in this work, as shown in the inset of Fig.…”