Single photons at 1550 nm, which are produced in spontaneous parametric down-conversion (SPDC) sources, are paramount in the development of future long-distance quantum communication. Unfortunately, these generated single photons are broadband, and they are not suitable for long-distance quantum communication. Therefore, it is a problem to be solved that how to use these broadband single photons directly in the future quantum network.

In this paper, we experimentally demonstrate the nonlinear interaction between two chirped broadband single-photon level coherent states. Each chirped coherent state is generated in independent fiber Bragg gratings. They are simultaneously coupled into a high-efficiency nonlinear waveguide where they are converted into a narrowband single photon state with new frequency by the process of sum-frequency generation (SFG). A higher SFG efficiency of   is realized, and this efficiency may achieve heralding entanglement at a distance. This is also made possible to realize long-distance quantum communication, such as device-independent quantum key distribution, by using directly the broadband single photons without filtering.

Yuanhua Li, Tong Xiang, Yiyou Nie, Minghuang Sang, and Xianfeng Chen, Nonlinear interaction between broadband single-photon-level coherent states, Photonics Research, 5(4), 324-328(2017)