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

We generate quantum-correlated photon pairs using cascaded chi((2)) : chi((2)) travelingwave 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%/Wcm(2) SHG peak efficiency yields a generated photon-pair probability of 7 +/- 0.2 x 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. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement