Phys. Rev. Lett. 112, 243602 (2014)

To achieve efficient conversion of a quantum state between photons and atoms, the bandwidth of the photons involved must be narrower than the natural linewidth of the relevant atomic transitions. So far, this requirement has been difficult to satisfy. Now, Kaiyu Liao and colleagues from China have demonstrated an experimental scheme for producing subnatural-linewidth photon pairs with polarization entanglement. The polarization-entangled photons are generated from a laser-cooled 85Rb atomic ensemble by the quantum interference of two spontaneous four-wave mixing (SFWM) processes driven by two counter-propagating pump-coupling beams in a Mach–Zehnder interferometer. The phase difference between the two SFWM paths is stabilized by locking the reference laser in the interferometer. The Chinese scientists find that the photon bandwidth can be reduced by lowering the coupling laser power. When the powers of the pump and coupling beams are 8 μW and 0.13 mW respectively, the photon bandwidth is about 0.8 MHz — much narrower than the natural linewidth of the Rb atomic transition (6 MHz).