We report an experiment demonstrating quantum frequency conversion of weak light pulses compatible with atomic quantum memories to telecommunication wavelengths. We use a PPLN nonlinear waveguide to convert weak coherent states at the single photon level with a duration of 30 ns from a wavelength of 780 nm to 1552 nm. We measure a maximal waveguide internal (external) conversion efficiency ηγ_int = 0.41 (η_ext = 0.25), and we show that the signal to noise ratio (SNR) is good enough to reduce the input photon number below 1. In addition, we show that the noise generated by the pump beam in the crystal is proportional to the spectral bandwidth of the device, suggesting that narrower filtering could significantly increase the SNR. Finally, we demonstrate that the quantum frequency converter can operate in the quantum regime by converting a time-bin qubit and measuring the qubit fidelity after conversion.

Ref: Quantum frequency conversion of quantum memory compatible photons to telecommunication wavelengths
Xavier Fernandez-Gonzalvo, Giacomo Corrielli, Boris Albrecht, Marcel.li Grimau, Matteo Cristiani, and Hugues de Riedmatten
Optics Express, Vol. 21, Issue 17, pp. 19473-19487 (2013)