Experiments on a compact and robust polarization-entangled photon source

Abstract

We construct a compact polarization-entangled photon source using type-II degenerate spontaneous parametric down-conversion (SPDC) in beta-barium borate (BBO) crystal pumped by a 405 nm violet laser diode. In order to compensate the spatial displacement and the temporal delay due to the birefringence and dispersion effect of signal and idler photons, we make the down-converted photon pairs pass through a half wave plate and an additional BBO crystal with the half thickness of the original one. This improves the visibility of two-photon interference by eliminating the distinguishability of the paired photons. We measure the polarization correlations by two adjustable polarization analyzers in two conjugate bases, H/V and +45°/-45°, respectively. The polarization analyzer consists of a polarization beam splitter cube preceded by a rotatable half wave plate. When rotating one of the half wave plates and keeping the other one at fixed angle, we obtain the expected sin2 dependence of the coincidence counts. The highly visible sinusoidal coincidence indicates the violation of the Bell inequality and demonstrates the high quality of the polarization-entangled photon source. This compact polarization-entangled photon source is easily configurable and robust to demonstrate optical quantum information processing.