Bright source of narrowband polarization-entangled photons from a thick type-II ppKTP crystal

We demonstrate a high brightness (similar to 2.36 x 10(5) pairs/s/mW) polarization-entangled photon-pair source at 800-nm via spontaneous parametric down-conversion (SPDC) in a 3-cm long type-II ppKTP crystal pumped unidirectionally in a single-pass geometry. A high coincidences-to-accidentals ratio (CAR similar to 1200) depicted by our source indicates a strong temporal correlation between the generated photon pairs. This correlated photon source is tunable from collinear to non-collinear emission of the photons and over a range of signal/idler wavelengths similar to 8 - 9 nm corresponding to a temperature range of 20-60degree celsius. We measure a quantum state fidelity F>95% with the singlet entangled state |psi(-)> = 1 root 2(|HV >-|VH >) along with a violation of the CHSH-Bell's inequality by similar to 485 standard deviations (S = 2.68 +/- 0.0014). (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement