All Fiber‐Coupled Noiseless Photon Pair Generation Based on Two Cascaded Periodically Poled LiNbO3${\rm LiNbO}_3$ Waveguides

ABSTRACT All‐fiber telecom‐band photon‐pair sources underpin long‐haul quantum networks. While periodically poled lithium niobate (PPLN) waveguides with fiber pigtails enable spontaneous parametric down‐conversion (SPDC) process, direct pumping fixes the wavelength to scarce, costly tunable lasers. Cascaded second harmonic generation (SHG) and SPDC in a single PPLN waveguide allow the use of enormous standard telecom lasers but suffer from noise photons due to spontaneous Raman scattering (SpRS) and spectral discontinuity when the pump is removed. In this paper, we propose and demonstrate an all‐fiber‐coupled method using two PPLN waveguides, one for SHG and the other for SPDC, to generate tunable, low‐noise photon pairs. Pump and SpRS photons are blocked by several mechanisms, including mode mismatch between waveguides and polarization‐maintaining (PM) fibers, high loss of leaky modes in PM fibers, and the high‐extinction ratio filtering effect of isolators. Photon counting shows that the noise photons have been suppressed to the dark count level. A continuous biphoton bandwidth surpassing 35 nm is observed, and 339 MHz photon‐pair generation rate (PPGR) is obtained when the coincidence‐to‐accidental ratio (CAR) is 370. Two‐photon Mach–Zehnder interference (TP‐MZI) reveals that the paired photons are highly indistinguishable. This source could be helpful for scalable quantum networks.