Fig. 2
Experimental setup. a Our experimental setup for entanglement between Rb atoms and visible/telecom photons without/with QFC. When mirror M is flipped up, AS photon is detected by Das without QFC. When mirror M is flipped down, AS photon is input to polarization-insensitive QFC (PIQFC) as shown in Fig. 1d and then the converted photon is detected by Dast. In order to stabilize the interferometer in optical paths of Stokes and anti-Stokes photons, we use a conventional diode laser at a center wavelength of 850 nm (not shown). The light enters into the interferometer from the vacuum port of PBSs, and then after passing along the same four optical paths for Stokes and anti-Stokes photons, it comes from the vacuum port of PBSas. The light is detected by a photo detector, and its detection signal is used for feedback control of the interferometer by a mirror on a Piezo stage (PZT). b Λ-type energy levels of D2 line in 87Rb used in our experiment. The ground levels of ga and gb correspond to the levels of 52S1/2, F = 2 and 52S1/2, F = 1, respectively. The excited level is 52P3/2, F′ = 2. Δ is set to ~10 MHz. The magnetic sublevel is degenerated in our experiment due to the absence of the magnetic field, and the polarization selection of write, read, AS and S light is to extract larger amounts of the emitted photons under an assumption that the initialized atomic states in ga are fully mixed over the magnetic sublevels (Supplemental Material in ref. 29). c Time sequence of the experiment. The quantum experiment is performed within 1 ms during the MOT is turned off. The period of the two initialization pulses is 1 μs and the injection of them is repeated 990 times within 1 ms
