Ultrabright Sagnac-type source of non-degenerate polarization-entangled photon pairs using only off-the-shelf optical components

We develop a Sagnac-type source of ultrabright, non-degenerate, polarization-entangled photon pair that is highly stable and compact simultaneously. We use a \(20~\text{mm}\) long PPLN bulk crystal which, upon pumping continuously with \(532~\text{nm}\) wavelength, produces polarization-entangled photon-pairs of \(785~\text{nm}\) signal and \(1651~\text{nm}\) idler wavelengths via the type-0 spontaneous parametric down-conversion (SPDC) process. All optical components used in the setup are off-the-shelf components, readily available commercially; hence, no custom-designed or costly multi-wavelength polarization optics are required. At the same time, long-term phase stability is achieved without any additional active stabilization; due to the geometry of our Sagnac-type design. We also report one of the highest brightness of non-degenerate polarization-entangled photon pairs available in the literature. Even with a very low pump power of \(0.034~\text{mW}\), we detect a coincidence rate of \((6.96\pm 0.03)\times10^{4}\) entangled pairs/sec/mW (averaged over three polarization-basis measurements: H/V, D/A, and R/L basis). The source's brightness is calculated to be \((6.17\pm 0.04)\times10^{6}\) entangled pairs/sec/mW for the signal line width of \(0.4~\text{nm}\). From the raw coincidence counts (without any background coincidence correction), the fidelity of the entangled state is measured to be \((96.72\pm 0.01)\%\) with a concurrence of \((94.68\pm 0.20)\%\). Bell-CHSH inequality violation is reported as \(S=2.71\pm 0.06\).