Compact polarization-entangled source near 810 nm and its application to nonlocal retardance measurement

We present direct single-pass generation of polarization-entangled photon pairs near 810 nm in a domain-engineered nonlinear crystal pumped by a continuous-wave 405 nm laser. Despite operating outside the natural group-velocity matching regime, the source produces high-quality polarization entanglement with a Clauser-Horne-Shimony-Holt parameter of S=2.764±0.002. We further exploit the intrinsic phase sensitivity of the entangled state to perform nonlocal, wavelength-dependent retardance measurements of birefringent waveplates through two-photon polarization correlations. Our results establish a high-performance alternative to conventional polarization entanglement sources, combining room-temperature silicon-detector compatibility, compactness, and spectral flexibility, in a practical platform for phase-sensitive quantum metrology.