Subnatural-linewidth biphotons from a Doppler-broadened hot atomic vapor   cell
  We report the efficient generation of subnatural-linewidth (< 6 MHz, for Rb D1/D2 lines) biphotons from a Doppler-broadened (530 MHz) hot atomic vapor cell. We use on-resonance spontaneous four-wave mixing in a hot paraffin-coated 87Rb vapor cell at 63 C to produce time-frequency entangled photon pairs with controllable bandwidth (1.9 - 3.2 MHz) and coherence time (47 - 94 ns). Our backward phase-matching scheme with optical pumping is the key to minimize the generation of uncorrelated photons from resonance fluorescence. The result paves the way toward miniature narrowband biphoton source based on atomic cells 
Chi Shu, Peng Chen, Tsz Kiu Aaron Chow, Lingbang Zhu, Yanhong Xiao, M.   M. T. Loy, and Shengwang Du
quant-ph