The coherent manipulation of the frequency of single photons is an important requirement for future quantum network technologies. It allows, for instance, quantum systems emitting in the visible range to be connected to the telecommunication wavelengths, thus extending the communication distances. Here we report on quantum frequency conversion of memory-compatible narrow-bandwidth photons at 606 nm to the telecom C-band at 1552 nm. The 200 ns long photons, compatible with praseodymium-based solid-state quantum memories, are frequency converted using a single-step difference frequency-generation process in a periodically poled lithium niobate waveguide. We characterize the noise processes involved in the conversion and, by applying strong spectral filtering of the noise, we demonstrate high signal-to-noise ratio conversion at the single-photon level ($\mathrm{SNR}>100$, for a mean input photon number per pulse of 1). We finally observe that a memory-compatible heralded single photon with a bandwidth of 1.8 MHz, obtained from a spontaneous parametric down-conversion pair source, still shows a strong non-classical behavior after conversion. We first demonstrate that correlations between heralding and converted heralded photons stay in the non-classical regime. Moreover, we measure the heralded autocorrelation function of the heralded photon using the converter device as a frequency-domain beam splitter, yielding a value of $0.19\pm 0.07$. The presented work represents a step towards the connection of several quantum memory systems emitting narrowband visible photons to the telecommunication wavelengths.

中文翻译：

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存储器兼容单光子从606 nm到电信C波段的量子频率转换

单光子频率的相干操纵是未来量子网络技术的重要要求。例如，它允许在可见光范围内发射的量子系统连接到电信波长，从而延长了通信距离。在这里，我们报告了存储器兼容的窄带宽光子在606 nm到1552 nm的电信C波段的量子频率转换。与200纳秒长的photo原子固态量子存储器兼容的200 ns长光子在周期极化的铌酸锂波导中使用单步差分频率生成过程进行了频率转换。我们表征了转换中涉及的噪声过程，并通过对噪声进行强频谱过滤，$\mathrm{信噪比}>100$，每个脉冲的平均输入光子数为1）。我们最终观察到，从自发参量下转换对源获得的，具有存储器兼容性的预言单光子具有1.8 MHz的带宽，在转换后仍显示出很强的非经典行为。我们首先证明先驱和已转换先驱光子之间的相关性仍处于非经典状态。此外，我们使用转换器设备作为频域分束器，测量了先导光子的先导自相关函数，得出的值为$0.19\pm 0.07$。提出的工作代表了将几个发射窄带可见光子到电信波长的量子存储系统连接的一步。