Quantum Frequency Conversion of $μs$-long Photons from the Visible to the Telecom-C-Band

Abstract: Quantum Frequency Conversion (QFC) 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 Pr$^{3+}$:Y$_2$SiO$_5\,$ quantum memories, with high-signal to noise ratio. We use a single step difference frequency generation process with a continuous-wave pump at 994 nm in a MgO:ppLN-waveguide and ultra-narrow 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 $\mu s$ with a device efficiency of about 25% and a signal-to-noise ratio >460 for 10 $\mu s$-long pulses containing one photon on average. This signal-to-noise ratio is large enough to enable a high-fidelity conversion of qubits emitted from an emissive quantum memory based on Pr$^{3+}$:Y$_2$SiO$_5\,$ and to realize an interface with quantum processing nodes based on narrow-linewidth cavity-enhanced trapped ions.