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

We demonstrate a high brightness (∼2.36 × 10⁵ 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 ∼ 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 ∼8 - 9 nm corresponding to a temperature range of 20-60°C. We measure a quantum state fidelity F>95% with the singlet entangled state |ψ ^-⟩=12(|H V⟩-|V H⟩) along with a violation of the CHSH-Bell's inequality by ∼485 standard deviations (S = 2.68 ± 0.0014).