Ultra-narrowband photon pair source for solid state quantum memories based on widely non-degenerate cavity-enhanced downconversion

We present a source of ultra-narrow band, widely non degenerate photon pairs source compatible with quantum repeaters architectures. One photon is compatible with a solid state Praseodymium (Pr³⁺) doped crystal spin wave quantum memory, and the other photon is at telecommunication wavelengths, such that it can be transmitted over large distances in optical fibers. Pr³⁺ doped solids absorbing photons at 606 nm have shown very promising properties as QM for light. Despite this fact, so far no experiment demonstrated storage of quantum light in Pr³⁺ doped crystals. The main challenge to realize such a source is that the bandwidth of the memory is limited to a few MHz by the excited state spacing of the ions. To achieve the required narrow linewidth we use widely non-degenerate cavity enhanced spontaneous down conversion (SPDC). We measure a correlation time of 104 ns, the longest reported to date for SPDC emitted photons, corresponding to around 2 MHz bandwidth. In addition, the number of longitudinal modes emitted by the source is strongly reduced, thanks to a clustering effect, which facilitates the filtering to reach single mode operation.