Low loss transmission is an intrinsic and unique property for single photons at 1550 nm in optical fiber. In quantum networks over optical fiber, single photons at 1550 nm are used for virtually all quantum information tasks, such as quantum metrology, quantum computation and quantum cryptography. Spontaneous parametric down-conversion (SPDC) sources are readily available for the production of entangled photon pairs at 1550 nm, and typically yield spectral bandwidths of 300 GHz. Nevertheless, the narrowband photons in the near-visible wavelength possess the most efficient quantum memories and an ability of being easily detected by a silicon avalanche photodiode (APD). Therefore, it is highly expected that a coherent photonic interface is necessary which is capable of spectrum compressing and frequency conversing in the telecom band simultaneously.

In this paper, we experimentally demonstrated that the bandwidth of single photons laser pulse was compressed by a factor of 58 in a periodically poled lithium niobate (PPLN) waveguide chip. A positively chirped single photons laser pulse and a negatively chirped classical laser pulse were employed to produce a narrowband single photon pulse with new frequency through sum-frequency generation. In our experiment, the frequency and bandwidth of single photons at 1550 nm were simultaneously converted. Our results marked a critical step towards the realization of coherent photonic interface between quantum communication at 1550 nm and quantum memory in the near-visible window.

Yuanhua Li, Tong Xiang, Yiyou Nie, Minghuang Sang, and Xianfeng Chen, Spectral compression of single-photon-level laser pulse, Scientific Reports, 7, 43494 (2017)