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Quantum Frontiers

Zero-order Bessel beam pumped type-II PPKTP-Sagnac polarization-entangled photon pair source

H. Nourolahi, S. Momeni, S. Mostajeran, Samaneh Birzhandi, A. Abedi

Published May 27, 2026• 0 citations
Publisher

Abstract

Abstract Tightly focused Bessel beams exhibit significantly higher intensity and non-diffractive three-dimensional micro-scale features compared to conventional fundamental Gaussian beams under the same optical power. These unique characteristics make Bessel beams particularly advantageous for enhancing photon pair generation in periodically poled nonlinear crystals. In this work, we experimentally demonstrate a compact pumping system using an asymmetric convex-convex lens and a plano-convex lens to generate Bessel beams. A broadband continuous-wave (CW) laser diode with a full-width at half-maximum (FWHM) of $0.54~(nm)$ 0.54 ( n m ) is used to pump a periodically poled potassium titanyl phosphate (PPKTP) crystal configured in a type-II phase-matched dual-path Sagnac loop. Our setup achieves a record-high coincidence-to-single-photon (CSP) ratio of 25% for type-II phase-matched spontaneous parametric down-conversion (SPDC) processes using broadband laser diodes. The degenerate polarization-entangled photon pair source, operating at $810~(nm)$ 810 ( n m ) , demonstrates exceptional performance metrics, including a brightness of $9.3~(kcps/mW)$ 9.3 ( k c p s / m W ) and a coincidences-to-accidentals ratio (CAR) of approximately 1091. Furthermore, the entanglement quality is verified through high visibility measurements: 99.2% in the horizontal/vertical (H/V) basis and 94.3% in the diagonal/ anti-diagonal (D/A) basis. In a Bell inequality test, we achieve an S value of $(2.7 \pm 0.04)$ ( 2.7 ± 0.04 ) , corresponding to a state fidelity of 0.95. In comparison, the Gaussian-pumped configuration yields a CSP ratio of 11.27%, a brightness of $4.46~({kcps/mW})$ 4.46 ( k c p s / m W ) , and a CAR of approximately 500. Measured polarization visibilities are 99.1% in the H/V basis and 96.7% in the diagonal (±45 ∘ ) basis, with a Bell parameter of $S = 2.6 \pm 0.03$ S = 2.6 ± 0.03 . These comparative results underscore the substantial advantages of utilizing Bessel beams for high-performance entangled photon pair generation.

DOI
10.1007/s44214-026-00104-z

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Abstract Tightly focused Bessel beams exhibit significantly higher intensity and non-diffractive three-dimensional micro-scale features compared to conventional fundamental Gaussian beams under the same optical power. These unique characteristics make Bessel beams particularly advantageous for enhancing photon pair generation in periodically poled nonlinear crystals. In this work, we experimentally demonstrate a compact pumping system using an asymmetric convex-convex lens and a plano-convex lens to generate Bessel beams. A broadband continuous-wave (CW) laser diode with a full-width at half-maximum (FWHM) of 0.54 (nm)0.54~(nm)0.54 (nm) 0.54 ( n m ) is used to pump a periodically poled potassium titanyl phosphate (PPKTP) crystal configured in a type-II phase-matched dual-path Sagnac loop. Our setup achieves a record-high coincidence-to-single-photon (CSP) ratio of 25% for type-II phase-matched spontaneous parametric down-conversion (SPDC) processes using broadband laser diodes. The degenerate polarization-entangled photon pair source, operating at 810 (nm)810~(nm)810 (nm) 810 ( n m ) , demonstrates exceptional performance metrics, including a brightness of 9.3 (kcps/mW)9.3~(kcps/mW)9.3 (kcps/mW) 9.3 ( k c p s / m W ) and a coincidences-to-accidentals ratio (CAR) of approximately 1091. Furthermore, the entanglement quality is verified through high visibility measurements: 99.2% in the horizontal/vertical (H/V) basis and 94.3% in the diagonal/ anti-diagonal (D/A) basis. In a Bell inequality test, we achieve an S value of (2.7±0.04)(2.7 \pm 0.04)(2.7±0.04) ( 2.7 ± 0.04 ) , corresponding to a state fidelity of 0.95. In comparison, the Gaussian-pumped configuration yields a CSP ratio of 11.27%, a brightness of 4.46 (kcps/mW)4.46~({kcps/mW})4.46 (kcps/mW) 4.46 ( k c p s / m W ) , and a CAR of approximately 500. Measured polarization visibilities are 99.1% in the H/V basis and 96.7% in the diagonal (±45 ∘ ) basis, with a Bell parameter of S=2.6±0.03S = 2.6 \pm 0.03S=2.6±0.03 S = 2.6 ± 0.03 . These comparative results underscore the substantial advantages of utilizing Bessel beams for high-performance entangled photon pair generation.
We experimentally demonstrate a polarization-entangled photon source at 810 nm using a type-II phase-matched periodically poled potassium titanyl phosphate (PPKTP) crystal pumped by a low-cost, broadband laser diode with a central wavelength of 405 nm and a typical bandwidth of 0.53 nm. The PPKTP crystal is placed in a Sagnac loop to realize the compact size and high stability. The downconverted biphotons, the signal and the idler, have typical bandwidths of 5.57 and 7.32 nm. We prepare two Bell states | Ψ + ⟩ and | Ψ − ⟩ with the fidelities of 0.948 ± 0.004 and 0.963 ± 0.002 . In the polarization correlation measurement, the visibilities are all higher than 96.2%, and in the Bell inequality test, the S value can achieve 2.78 ± 0.01 . This high-quality and low-cost entangled photon source may have many practical applications in quantum information processing.
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 report on a compact, simple and robust high brightness entangled photon source at room temperature. Based on a 30-mm-long periodically-poled potassium titanyl phosphate crystal, the source produces non-collinear, type-0, phase-matched, degenerate photons at 810 nm with spectral brightness as high as ~0.41 ± 0.02 (~0.025 ± 0.02) MHz/mW/nm for multi (single) mode fiber coupling. So far, this is the highest number of degenerate photons generated using a continuous-wave laser pumped bulk crystal and detected using multimode fiber. We have studied the dependence of pump focusing on the brightness of the generated photons collected using both multimode, and single mode fibers. For a fixed pump power and crystal parameters, the SPDC source has an optimum pump waist radius producing maximum number of paired photons. Combining the crystal in a novel system architecture comprised with Sagnac interferometer and polarizing optical elements, the source produces polarization entangled photon states with high spectral brightness. Even in the absence of any phase compensation, the entangled photon states detected using single mode fiber have a Bell's parameter, S = 2.63 ± 0.02, violating the Bell's inequality by nearly 32 standard deviations and fidelity of 0.975. The compact footprint, robust design, and room temperature operation, make our source ideal for various quantum communication experiments.
We report a bright source of polarization-entangled photon pairs using spontaneous parametric down-conversion (SPDC) in a 10 mm long type-II PPKTP crystal pumped by a broadband multi-mode diode laser with the coherence length of 330 μm. Ordinarily, the huge mismatch between the pump coherence length and the PPKTP length would degrade the polarization entanglement completely. By employing the universal Bell-state synthesizer scheme, we remove the spectral/temporal distinguishability of the biphoton amplitudes entirely to recover high-visibility and high-fidelity two-photon polarization entanglement. The pair detection rates are 7,000 pairs/mW via single-mode fibers (with 99.2% fidelity) and 90,900 pairs/mW via multi-mode fibers (with 96.8% fidelity). We also analyze the scheme theoretically to show the effect of broadband multi-mode pumping on the phase matching condition of the type-II PPKTP.