Fig. 1. Schematic view of the experiment. (a) The atomic energy level of Cs133 for generating heralded single photons, with |g⟩=|6S1/2,F=3⟩, |s⟩=|6S1/2,F=4⟩, and the excited state |e⟩ representing the manifold of the 6P3/2 state. (b) The experimental setup. To generate the write and read pulses, an electro-optical modulator is utilized to chop the continuous laser into a pulse sequence. An automatic feedback circuit monitors and locks the operating point of the electro-optical modulator, achieving stable operation over the long term. According to the response of the detector in the Stokes channel, the time tag module varies the drive electric signal of the electro-optical modulator, implementing conditional control of single-photon generation. The Stokes photon and anti-Stokes photon are co-propagating in the coaxial write-read scheme. To separate the Stokes photons and anti-Stokes photons into different paths, we placed a 45° oriented quarter-wave plate in front of the Stokes-resonant cavity, rotating both photons into right circular polarization. The Stokes photons go through the cavity while the anti-Stokes photons are reflected by this cavity at its front surface, and then pass the quarter-wave plate again. This makes the anti-Stokes photons vertically polarized, which enables the anti-Stokes photons to be reflected by the PBS and finally directed to the anti-Stokes-resonant cavity. Stokes photons are detected by APD1 and APD2, while anti-Stokes photons are detected by APD3 and APD4. FPC, Fabry–Perot cavity; WP, Wollaston prism; QWP, quarter-wave plate; FBS, fiber beam splitter; PBS, polarization beam splitter; HWP, half-wave plate; EOM, electro-optical modulator; and APD, avalanche photodiode.

Fig. 2. Heralded single photon with a different time delay in quantum memory. The solid curves in the main part refer to the temporal shape of the generated single photon with a different time delay for retrieval, without reduction of any background noise. The red dots on the right side denote the corresponding cross-correlation function between the Stokes and anti-Stokes photons.

Fig. 3. Characterization of the two single-photon sources from independent room-temperature quantum memories. (a) The counts of the heralded anti-Stokes photons as a function of the write pulse power with 566,037,735 trials performed. The red pentagon refers to the chosen pulse energy in this experiment. (b) The temporal shape of the heralded anti-Stokes photons generated by two photon sources. The blue squares (purple rhombuses) mark the heralded anti-Stokes photons from one photon source detected by APD3 (APD4), while the red dots (orange triangles) represent the heralded anti-Stokes photons that are generated by another photon source and detected by APD3 (APD4). These data are obtained with a pulse energy of 330 pJ and 566,037,735 repeated trials.

Fig. 4. Four-fold coincidence as a function of the time delay between two photons from independent room-temperature quantum memories. The blue dots correspond to the experimental data with an effective measurement of 11,886,792,452 trials, and the solid line represents the theoretical curve. The error bars denote one standard deviation.