A single-crystal source of path-polarization entangled photons at non-degenerate wavelengths

We demonstrate a bright, narrowband, compact, quasi-phase-matched single-crystal source generating path-polarization-entangled photon pairs at 810 nm and 1550 nm at a maximum rate of 3 × 10⁶ s^-1 THz^-1 mW^-1 after coupling to single-mode fiber, and with two-photon interference visibility above 90%. While the source can already be used to implement quantum communication protocols such as quantum key distribution, this work is also instrumental for narrowband applications such as entanglement transfer from photonic to atomic qubits, or entanglement of photons from independent sources.