The cavity-enhanced spontaneous parametric down-conversion far below threshold can be used to generate a narrow-band photon pair efficiently. Previous experiments on the cavity-enhanced spontaneous parametric down- conversion almost always utilize continuous wave pump light, but the pulse pumped case is rarely reported. One disadvantage of the continuous wave case is that the photon pair is produced randomly within the coherence time of the pump, which limits its application in the quantum information realm. However, a pulse pump can help to solve this problem. In this paper, we theoretically analyze pulse pumped cavity-enhanced spontaneous parametric down- conversion in detail and show how the pump pulse affects the multi-photon interference visibility, two-photon waveform, joint spectrum and spectral brightness.
We report low pump power high-efficiency frequency doubling of a fundamental laser beam at 795 nm, corresponding to the rubidium D1 line, to generate UV light at 397.5 nm using a periodically poled KTi OPO4（PPKTP） crystal in a ring cavity. We obtain maximum stable output power of 49 m W for mode-matching pump power of 110 m W, corresponding to 45% raw efficiency（56% net efficiency when considering the output coupling mirror’s 80% transmission）. This is the highest efficiency obtained at this wavelength in PPKTP with such low pump power. We obtain 80% beam coupling efficiency to single-mode fiber, demonstrating high beam quality.
This paper proposes two simple and robust schemes to generate an atomic-ensemble Greenberger-Horne--Zeilinger-type （GHZ-type） entangled state via linear optics and single photon detection. These schemes are based on two-photon Hong-Ou-Mandel-type interference, therefore they are insensitive to the phase fluctuation. This advantage will make the realizations of these two schemes easier. One scheme can scale efficiently with the number of ensembles because of the used quantum memory. Both schemes are also robust to the noise and within the reach of current technology.
We experimentally establish a non-classical correlation between a single Stokes photon and the collective spin excited state of a cold atomic ensemble by using a spontaneous Raman scattering process. The correlation between them can be proved by transferring the spin excited state of the atomic ensemble into an anti-Stokes photon and checking the Cauchy-Schwarz inequality between the Stokes and the anti-Stokes photons. The non-classical correlation can be kept for at least 300 ns.
