Chang Liu, Hong Kong University of Science and Technology
As a standard method for producing correlated photon pairs (biphotons), spontaneous parametric down-conversion in nonlinear crystal usually has a wide bandwidth (terahertz) and very short coherence time (pico-seconds). Within spontaneous four-wave mixing process in cold atom ensembles, here we present a method for producing narrow-band (megahertz) Stokes and anti-Stokes paired photons. As detecting a Stokes photon heralds the generation of its paired anti-Stokes photon, the biphoton source is also used as a good heralded single photon source. With narrow-band frequency-tunable entangled biphotons, we observe coalescence interference for both degenerate and nondegenerate photons, and find that the path-exchange symmetry plays a more important role in the Hong-Ou-Mandel interference than the temporal or frequency indistinguishability. Based on a direct modulation of heralded single-photon waveform with an arbitrary phase pattern, we demonstrate the first proof of principle differential phase shift quantum key distribution experiment using heralded single photons. When the single-photon waveform is modulated to exponential growth, we experimentally demonstrate that a single photon can be almost completely loaded into a Fabry-Perot cavity, which has a potential application in cavity Quantum Electrodynamics field.