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

## TL;DR

This paper introduces a highly efficient, stable, and tunable narrow-band photon pair source at the $^{87}$Rb D1 line, suitable for quantum communication and quantum information applications.

## Contribution

The work demonstrates a monolithic, cavity-enhanced SPDC source with high heralding efficiency, narrow bandwidth, and tunability, advancing practical quantum photonics technologies.

## Key findings

- Heralding efficiency of 45% achieved.
- Narrow bandwidth of 226 MHz demonstrated.
- Frequency tuning range exceeds 2 GHz via mechanical strain.

## Abstract

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 $\eta_{\mathrm{heralded}}=45(5)$ % is achieved, and the uncorrected number of detected photon pairs is $3.8 \times 10^{3}/(\textrm{s mW})$. For pair generation rates up to $5\times 10^{5}/$s, the source emits heralded single photons with a normalized, heralded, second-order correlation function $g^{(2)}_{c}<0.01$. The source is intrinsically stable due to the monolithic configuration. Frequency drifts are on the order of $\delta/20$ 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.