Wavelength-tunable and high-heralding-efficiency quantum photon source in a birefringent phase-matched lithium niobate waveguide.

Lithium niobate (LN) is a birefringent material, where the strong birefringence thermo-optic effect is promising for the generation of a quantum photon source with a widely tunable wavelength. Here, we demonstrate birefringent phase matching in a 20-mm-long waveguide fabricated on a 5-μm-thick x-cut lithium niobate on insulator (LNOI). The waveguide is deviated from the optical axis of LN by an angle of 53.5^∘, enabling the phase matching between telecom and visible wavelengths. The phase-matching wavelength of this device can be thermally tuned with rate of 0.6 nm/K. We demonstrate the type-1 spontaneous parametric downconversion to generate photon pairs with a brightness of 4.7 MHz/mW and a coincidence-to-accidental ratio of up to 2.8 × 10⁵. Furthermore, the heralded single photon is obtained from the photon pair with an efficiency of 13.8% and a count rate of up to 37.8 kHz.