我们利用周期性极化铌酸锂(PPLN)波导模块演示了从铷原子D1线(795 nm)到光通信L波段(1 621 nm)的频率下转换,并将1 621 nm的转换光子通过15 km光纤进行长距离传输。为了降低由泵浦光引起的宽带自发拉曼散射噪声,我们用两个标准具级连方法将噪声带宽压窄至256 MHz。我们研究了下转换光子远距离传输后的信噪比随脉宽和平均输入光子数的变化关系,表明在低的器件外部转换效率(0.84%)下,当脉宽为30 ns,平均输入光子数为2个时,信噪比为1.5。

Quantum communication holds promise for absolutely secure transmission of secret messages and the faithful transfer of unknown quantum states. Photonic channels appear to be very attractive for the physical implementation of quantum communication. To achieve long-distance transmission of quantum states, quantum repeaters are essential, which contain quantum memories. Atomic ensemble, single atom, trapped ions and solid-state systems are all potential candidates for quantum repeater, in which quantum storage has been actively performed. The majority of these quantum memories operate at visible wavelengths. However, the visible wavelength bands photons have a large transmission loss in the optical fiber and cannot be directly transmitted over long distances. To achieve efficient long-distance quantum communication, the visible band needs to be converted to the communication band. Here we demonstrate a frequency conversion from rubidium D1 line (795 nm) to the telecom L-band (1 621 nm) based on difference frequency generation by using a fiber-coupled waveguide module. In quantum frequency conversion process, the strong pumping beam causes spontaneous Raman scattering (SRS) noise, which typically covers a region of several hundreds of nanometers. The noise suppression is important for improving the signal-noise ratio (SNR) of the converted field, especially for the frequency conversion of quantum state light field. To suppress the broadband noise, we use two cascaded etalons to narrow the noise bandwidth to 256 MHz and the transmission of the converted photons through a 15 km optical fiber. When the input light is strongly coherent continuous light, the maximum external conversion efficiency of the device was measured to be 0.95%. We use a single photon detector to measure the noise level after the filtering system. When the pump power is 400mW, the noise count is 1.5×104s-1. When the 795nm input pulse is attenuated to the single-photon level, westudy the signal-to-noise ratio of converted photons under different input pulse widths and the mean number of photons, we show that the SNR of converted field is 1.5 when the input photon number is 2 under the condition of low external device conversionefficiency (0.84%) and short duration of input pulse (30 ns). This work is a meaningful attempt to realize the quantum interface between quantum memory at 87Rb D1 line and telecom band.