Interfering single photons retreived from collective atomic excitations in two dense cold-atom clouds

Abstract

We report the Hong-Ou-Mandel (HOM) interference, with visibility of 91%, produced from two independent single photons retrieved from collective atomic excitations in two separate cold-atom clouds with high optical depths of 90. The high visibility of the HOM dip is ascribed to the pure single photon in the Fock state that was generated from a dense-cold-atom cloud pumping by a short pulse. The visibility is always the same regardless of the time response of the single-photon detectors. This result experimentally shows that the single photons retrieved are in a separable temporal state with their idler photons.OCIS codes: 020. 4180, 190.4223, 020.3320, 270.5565. doi: 10.3788/COL201614.080201.The perfect destructive two-photon interference, first observed by Hong, Ou, and Mandel (HOM) with paired photons generated from spontaneous parametric down conversion (SPDC) in χð2Þ non-linear crystal media [1] , is one of the most well-known phenomena revealing the quantum nature of photons. HOM interference of independent single photons from separate sources is the basis of quantum swapping [2,3] and hence realistic linear optical realization of quantum repeaters [4,5] . To have complete destructive interference, each of the two independent photons must be in a pure single-photon Fock state, and they must be indistinguishable in every aspect, including polarization and spatial and spectral modes. Interfering with the single photons generated from the SPDC process in non-linear crystals has been experimentally demonstrated [6][7][8] . The single photons are constituted from the signal photons heralded by the correlated idler photons. In fact, two-photon interference from independent SPDC crystals is widely used in experimental demonstration of quantum swapping [9] , multiphoton entanglement, etc. [10] , because of the high generation rate of SPDC photon pairs.However, SPDC schemes produce broadband photon pairs, typically with terahertz of linewidths and hence femtoseconds of coherence time. This implies a very strict synchronization between two independent SPDC processes, with which the temporal wave functions of two independent photons overlap in time at the beam splitter (BS). Passive spectral filtering can increase the coherence time, but waste a great amount of photons and thus seriously lower the generation rate. Active filtering through putting the SPDC crystal into a cavity is an efficient approach, but complicated cavity stabilization limits its further application [11,12] . With laser cooling and trapping technology developing fast, cold-atom clouds have become a new non-linear media to generate photon pairs and herald single photons. A continuous spontaneous four-wave mixing (SFWM) process in a cold-atom cloud can produce non-classical paired photons with sub-natural linewidth, determined by the transmission window caused by electromagnetically induced transparency (EIT) [13][14][15][16][17][18] . Especially with high optical depths, SFWM in an atomic ensemble can produce photon pairs with higher generation rate...