Jakub Szlachetka, Kaushik Joarder, Piotr Kolenderski
Abstract
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 mm long PPLN bulk crystal which, upon pumping continuously with 532 nm wavelength, produces polarization-entangled photon-pairs of 785 nm signal and 1651 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 mW, we detect a coincidence rate of (6.96±0.03)×104 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±0.04)×106 entangled pairs/sec/mW for the signal line width of 0.4 nm. From the raw coincidence counts (without any background coincidence correction), the fidelity of the entangled state is measured to be (96.72±0.01)% with a concurrence of (94.68±0.20)%. Bell-CHSH inequality violation is reported as S=2.71±0.06.
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