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arXiv:2212.02461

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

Jakub Szlachetka, Kaushik Joarder, Piotr Leszek Kolenderski

Published Dec 5, 2022• 0 citations
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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 $20~\text{mm}$ 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 $S=2.71\pm 0.06$.

DOI
10.48550/arxiv.2212.02461

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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 mm20~\text{mm}20 mm long PPLN bulk crystal which, upon pumping continuously with 532 nm532~\text{nm}532 nm wavelength, produces polarization-entangled photon-pairs of 785 nm785~\text{nm}785 nm signal and 1651 nm1651~\text{nm}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 mW0.034~\text{mW}0.034 mW, we detect a coincidence rate of (6.96±0.03)×104(6.96\pm 0.03)\times10^{4}(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(6.17\pm 0.04)\times10^{6}(6.17±0.04)×106 entangled pairs/sec/mW for the signal line width of 0.4 nm0.4~\text{nm}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)%(96.72\pm 0.01)\%(96.72±0.01)% with a concurrence of (94.68±0.20)%(94.68\pm 0.20)\%(94.68±0.20)%. Bell-CHSH inequality violation is reported as S=2.71±0.06S=2.71\pm 0.06S=2.71±0.06.
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We demonstrate a compact and stable source of polarization-entangled pairs of photons, one at 810 nm wavelength for high detection efficiency and the other at 1550 nm for long-distance fiber communication networks. Due to a novel Sagnac-based design of the interferometer no active stabilization is needed. Using only one 30 mm ppKTP bulk crystal the source produces photons with a spectral brightness of 1.13 x 10(6) pairs/s/mW/THz with an entanglement fidelity of 98.2%. Both photons are single-mode fiber coupled and ready to be used in quantum key distribution (QKD) or transmission of photonic quantum states over large distances.
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We present an ultrabright polarization-entangled photon source that is optimally coupled into single-mode fibers (SMFs). This study theoretically and experimentally examines the characteristics of spontaneous parametric down-conversion (SPDC) photons, including their spectrum, bandwidth, emission angle, and intensity, as functions of crystal length, temperature and beam waist condition. Notably, we measure the collinear spatial modes of photon-pairs and collection optics under various beam waist conditions and analyze them using a collinear Gaussian approximation model. By employing a simple mode-matching optical setup, we optimize the SMF coupling and heralding efficiencies of the photon-pairs. Consequently, we achieve a spectral brightness exceeding 2.0 MHz/mW/nm from a fiber-coupled entangled photon source, utilizing a 30-mm ppKTP crystal inside a polarization Sagnac interferometer. This represents the highest spectral brightness of SPDC photons generated using a CW laser pumped bulk crystal to date. Polarization entanglement was verified by a quantum state tomography and a polarization-correlation measurement. The fidelity of the entangled state is measured to be 97.8 % and the Bell-CHSH value S = 2.782 +- 0.04. The results obtained here provide practical insights for designing high-performance SPDC sources for satellite-based communication and long-distance optical links with extremely high-photon loss.
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