Quantum-correlated photon-pair generation via cascaded nonlinearity in an ultra-compact lithium-niobate nano-waveguide

We generate quantum-correlated photon pairs using cascaded χ⁽²⁾ : χ⁽²⁾ traveling-wave interactions for second-harmonic generation (SHG) and spontaneous parametric downconversion (SPDC) in a single periodically-poled thin-film lithium-niobate (TFLN) waveguide. When pulse-pumped at 50 MHz, a 4-mm-long poled region with nearly 300%/Wcm² SHG peak efficiency yields a generated photon-pair probability of 7±0.2 × 10⁻⁴ with corresponding coincidence-to-accidental ratio (CAR) of 13.6±0.7. The CAR is found to be limited by Stokes/anti-Stokes Raman-scattering noise generated primarily in the waveguide. A Raman peak of photon counts at 250 cm⁻¹ Stokes shift from the fundamental-pump wavenumber suggests most of the noise that limits the CAR originates within the lithium niobate material of the waveguide.