Ultra-wide single-photon frequency conversion and entanglement via constructive interference

We investigate theoretically a single-photon frequency conversion with a Sagnac waveguide loop by input-output formalism. The results show that an ultra-wide tunable frequency conversion can be achieved via constructive interference and a flexible adjustment of parameters. For instance, we discuss the generation of ultraviolet photons in detail, which corresponds to the up-conversion process of frequency. We also can obtain entangled states of photons and atoms under suitable parameter conditions. Our calculations prove that a single-photon frequency conversion with high efficiency can be realized, even if effected by spontaneous decay and deviation of parameters. (paper)