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Advanced Quantum Technologies

Generation of Polarization‐Entangled Photon Pairs Based on Beam Displacers

Peng Yan, Zhiyu Wang, Chengjie Zhang

Published Jun 22, 2025• 1 citation
Publisher

Abstract

Abstract The generation of high‐fidelity entangled states is crucial for quantum technologies, including quantum communication, quantum computing, and quantum metrology. Here, a polarization‐entangled photon‐pair source is demonstrated based on beam displacers and a type‐II periodically poled potassium titanyl phosphate crystal, employing a degenerate quasi‐phase‐matched spontaneous parametric down‐conversion process. The configuration features a fully linear architecture with a compact and robust design. Its performance is characterized by high‐visibility quantum interference fringes ( and ), high quantum state fidelity (), and significant violation of Bell inequality (). The system offers excellent stability, ease of alignment, and high integrability, making it suitable for operation in challenging and long‐term environments. In particular, quantum key distribution benefits from entangled photon sources with high fidelity to ensure secure and efficient information transmission.

DOI
10.1002/qute.202500191

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Abstract The generation of high‐fidelity entangled states is crucial for quantum technologies, including quantum communication, quantum computing, and quantum metrology. Here, a polarization‐entangled photon‐pair source is demonstrated based on beam displacers and a type‐II periodically poled potassium titanyl phosphate crystal, employing a degenerate quasi‐phase‐matched spontaneous parametric down‐conversion process. The configuration features a fully linear architecture with a compact and robust design. Its performance is characterized by high‐visibility quantum interference fringes ( and ), high quantum state fidelity (), and significant violation of Bell inequality (). The system offers excellent stability, ease of alignment, and high integrability, making it suitable for operation in challenging and long‐term environments. In particular, quantum key distribution benefits from entangled photon sources with high fidelity to ensure secure and efficient information transmission.
We present a simple but highly efficient source of polarization-entangled photons based on spontaneous parametric down-conversion (SPDC) in bulk periodically poled potassium titanyl phosphate crystals (PPKTP) pumped by a 405 nm laser diode. Utilizing one of the highest available nonlinear coefficients in a non-degenerate, collinear type-0 phase-matching configuration, we generate polarization entanglement via the crossed-crystal scheme and detect 0.64 million photon pair events/s/mW, while maintaining an overlap fidelity with the ideal Bell state of 0.98 at a pump power of 0.025 mW.
We present a simple but highly efficient source of polarization-entangled photons based on spontaneous parametric down-conversion (SPDC) in bulk periodically poled potassium titanyl phosphate crystals (PPKTP) pumped by a 405 nm laser diode. Utilizing one of the highest available nonlinear coefficients in a non-degenerate, collinear type-0 phase-matching configuration, we generate polarization entanglement via the crossed-crystal scheme and detect 0.64 million photon pair events/s/mW, while maintaining an overlap fidelity with the ideal Bell state of 0.98 at a pump power of 0.025 mW.
We present results of a bright polarization-entangled photon source operating at 1552 nm via type-II collinear degenerate spontaneous parametric down-conversion in a periodically poled potassium titanyl phosphate crystal. We report a conservative inferred pair generation rate of 123,000 pairs/s/mW into collection modes. Minimization of spectral and spatial entanglement was achieved by group velocity matching the pump, signal, and idler modes and through properly focusing the pump beam. By utilizing a pair of calcite beam displacers, we are able to overlap photons from adjacent down-conversion processes to obtain polarization-entanglement visibility of 94.7+/-1.1% with accidentals subtracted.
Bell states are fundamental resources in quantum optics, underpinning a range of applications in quantum computation and communication. However, many experimental sources focus on generating a single Bell state, limiting their utility for protocols that require the full set of maximally entangled states. In this work, we present a polarization-entangled photon-pair source based on spontaneous parametric down-conversion in a periodically poled potassium titanyl phosphate crystal integrated into a Sagnac loop interferometer. Our source initially generates one Bell state, which is then transformed into the complete set of Bell states through the application of phase and bit flips using phase plates. We characterize the entanglement quality of all generated states via two-photon interference measurements, quantum state tomography, and evaluation of the Bell parameter. Our results show spectral brightness of 0.15 kHz/mW/nm with interference visibilities of ∼98% (H/V basis) and ∼96% (A/D basis) and state fidelity of ∼0.97 across all Bell states, with all states demonstrating a clear violation of the Clauser–Horne–Shimony–Holt inequality (S≥2.6). This approach offers a practical solution for obtaining all four Bell states with high efficiency from a single source, which could facilitate more versatile implementations in quantum information processing.
The realization of quantum networks requires the development of robust, low-size, weight, and power (SWaP) systems suitable for operation under harsh environments in remote and mobile nodes such as satellites. We present a source of polarization-entangled photon-pairs in a folded linear displacement interferometer based on spontaneous parametric down conversion using a Type-0 periodically poled potassium titanyl phosphate crystal. Featuring a compact and stable double-pass geometry using a corner-cube retroreflector, the source has a detected pair rate of 2.5 M pairs/s/mW with a Bell state fidelity of 94.1% ± 2.1%. The qualities and demonstrated performance of the source make it suitable for deployment in entanglement-based quantum networks.
We present two experiments geared toward the realization of a robust and intense source of polarization-entangled photons. First, we describe a novel source of polarization-entangled pairs that uses periodically-poled potassium titanyl phosphate (PPKTP) and an interferometer based on polarization beam displacers. The source emits an intense flux of high-quality single-mode entangled photons and is stable, robust, and easy to align. Second, we report on sources of correlated photons generated in PPKTP waveguides. Waveguide sources of correlated pairs have been shown to generate high fluxes of pairs: we theoretically and experimentally investigate spontaneous parametric down-conversion generation of photon pairs in waveguides at different wavelengths.