Demonstration of narrowband biphotons with sub-MHz bandwidth from low-optical-depth atomic ensemble

Biphotons of narrow bandwidth and long temporal length have potential applications in realizing efficient light-matter interface and quantum communication. However, generation of these photons usually requires atomic ensembles with high optical depth or spontaneous parametric down-conversion with delicate optical cavities. We propose and demonstrate narrowband biphotons with sub-MHz bandwidth using detuned four-wave mixing in low-optical-depth atomic ensemble. The bandwidth of the biphotons is only limited by the ground-state decoherence rate. We also demonstrate the potential of shaping these photons.