An efficient, tunable, and robust source of narrow-band photon pairs at the ${}^{87}$Rb D1 line

We present an efficient and robust source of photons at the ${}^{87}$Rb D1-line (795 nm) with a narrow bandwidth of $\delta =226(1)$ MHz. The source is based on non-degenerate, cavity-enhanced spontaneous parametric down-conversion in a monolithic optical parametric oscillator far below threshold. The setup allows for efficient coupling to single mode fibers. A heralding efficiency of ${\eta }_{\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{l}\mathrm{d}\mathrm{e}\mathrm{d}}=45(5)$ % is achieved, and the uncorrected number of detected photon pairs is $3.8\times {10}^3\mathrm{/}(\text{s mW})$. For pair generation rates up to $5\times {10}^5\mathrm{/}$s, the source emits heralded single photons with a normalized, heralded, second-order correlation function $g_c^{(2)}<0.01$. The source is intrinsically stable due to the monolithic configuration. Frequency drifts are on the order of $\delta \mathrm{/}20$ per hour without active feedback on the emission frequency. We achieved fine-tuning of the source frequency within a range of $>2$ GHz by applying mechanical strain.