Quantum frequency conversion of photons with microsecond duration from the visible to the telecommunication C band

Quantum frequency conversion is a widely used technique to interface atomic systems with the telecom band in order to facilitate propagation over longer distances in fiber. Here, we demonstrate the difference-frequency conversion from 606 nm to 1552 nm of microsecond-long weak coherent pulses at the single-photon level compatible with $\mathrm{Pr}$${}^{3+}$:$\mathrm{Y}$${}_{2}$$\mathrm{Si}\mathrm{O}$${}_{5}$ quantum memories, with a high signal-to-noise ratio (SNR). We use a single-step difference-frequency-generation process with a continuous-wave pump at 994 nm in a periodically poled lithium niobate ($\mathrm{Mg}\mathrm{O}$:ppLN) waveguide and ultranarrow spectral filtering down to a bandwidth of 12.5 MHz. With this setup, we achieve the conversion of weak coherent pulses of duration up to 13.6 \textmu{}s with a device efficiency of about 25 % and an SNR >460 for 10-$\text{\ensuremath{\mu}}\mathrm{s}$-long pulses containing one photon on average. This SNR is large enough to enable a high-fidelity conversion of qubits emitted from an emissive quantum memory based on $\mathrm{Pr}$${}^{3+}$:$\mathrm{Y}$${}_{2}$$\mathrm{Si}\mathrm{O}$${}_{5}$ and to realize an interface with quantum processing nodes based on narrow-linewidth cavity-enhanced trapped ions.