Compact and stable source of polarization-entangled photon-pairs based on a folded linear displacement interferometer

The realization of quantum networks requires the development of robust, low-size, weight, and power (SWaP) systems suitable for operation under harsh environments in remote and mobile nodes such as satellites. We present a source of polarization-entangled photon-pairs in a folded linear displacement interferometer based on spontaneous parametric down conversion using a Type-0 periodically poled potassium titanyl phosphate crystal. Featuring a compact and stable double-pass geometry using a corner-cube retroreflector, the source has a detected pair rate of 2.5 M pairs/s/mW with a Bell state fidelity of 94.1% ± 2.1%. The qualities and demonstrated performance of the source make it suitable for deployment in entanglement-based quantum networks.