Phase-stable source of polarization-entangled photons in a linear double-pass configuration

We demonstrate a compact, robust, and highly efficient source of polarization-entangled photons, based on linear bi-directional downconversion in a novel 'folded sandwich' configuration.Bi-directionally pumping a single periodically poled KTiOPO 4 (ppKTP) crystal with a 405-nm laser diode, we generate entangled photon pairs at the nondegenerate wavelengths 784 nm (signal) and 839 nm (idler), and achieve an unprecedented detection rate of 11.8 kcps for 10.4 µW of pump power (1.1 million pairs / mW), in a 2.9-nm bandwidth, while maintaining a very high two-photon entanglement quality, with a Bell-state fidelity of 99.3 ± 0.3%.