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Frontiers in Optics

Increasing the correlation level of polarization entangled photon pairs generated by type-II SPDC in BBO

Csaba Tamás Holló, Gábor Erdei, Tamás Sarkadi

Published Jan 1, 2020• 0 citations
Publisher

Abstract

Entangled photon sources introduce phase shift between polarization states. Exceeding coherence length, coincidence rate drops for certain bases. Correlation is restored by phase compensation. Applying anisotropic retarders coincidence rate doubles.
DOI
10.1364/fio.2020.jth4a.36

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Pumping a BBO crystal by a violet diode laser with a wavelength of (405 nm) output power of (24 mW) and a line width of (3nm) was employed to generate entangled photons with a wavelength of 810 nm by achieving type II phase matching conditions. The coincidence count rate obtained in this experiment was in the range of (18000) counts/s. Two BBO crystals with different thicknesses of (4 mm and 2 mm) were tested, where maximum count rates of about (18000) counts/s was obtained with a (5*5*2) mm BBO crystal where the short coherence time for the pumping source was tolerated by using shorter BBO crystals. Also, the effect of compensating crystal on the walk-off effect was studied. The coincidence count rates were increased by using these crystals when their optic axes were fixed in the opposite direction relative to the optic axis of the original BBO crystal
The study, reliable generation, and application of spectral and polarization-correlated biphotons is a widely researched area in the field of quantum technology. In this Letter, we report a bright narrowband source of spectral and polarization-correlated orthogonal photon-pairs around the telecom wavelength of 1560 nm from a fiber-pigtailed, type-II quasi-phase-matched, MgO-doped periodically poled lithium niobate (MgO:ppLN) ridge waveguide. We achieved a high spectral brightness of ∼5×10 6 photon pairs/s/mW/nm with a coincidence-to-accidental ratio (CAR) value of ∼427 and an emission bandwidth of ∼2.4 nm. Owing to its compact and narrowband nature, this source can reliably be used in commercial quantum communication systems for long-distance transmissions. The generation of such high rates of correlated orthogonal twin photons from a fully guided type-II SPDC-based source has not been thoroughly investigated to the best of our knowledge.