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

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 $0.54~(nm)$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mn>0.54</mml:mn> <mml:mspace/> <mml:mo>(</mml:mo> <mml:mi>n</mml:mi> <mml:mi>m</mml:mi> <mml:mo>)</mml:mo> </mml:math> 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 $810~(nm)$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mn>810</mml:mn> <mml:mspace/> <mml:mo>(</mml:mo> <mml:mi>n</mml:mi> <mml:mi>m</mml:mi> <mml:mo>)</mml:mo> </mml:math> , demonstrates exceptional performance metrics, including a brightness of $9.3~(kcps/mW)$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mn>9.3</mml:mn> <mml:mspace/> <mml:mo>(</mml:mo> <mml:mi>k</mml:mi> <mml:mi>c</mml:mi> <mml:mi>p</mml:mi> <mml:mi>s</mml:mi> <mml:mo>/</mml:mo> <mml:mi>m</mml:mi> <mml:mi>W</mml:mi> <mml:mo>)</mml:mo> </mml:math> 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 $(2.7 \pm 0.04)$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>(</mml:mo> <mml:mn>2.7</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.04</mml:mn> <mml:mo>)</mml:mo> </mml:math> , corresponding to a state fidelity of 0.95. In comparison, the Gaussian-pumped configuration yields a CSP ratio of 11.27%, a brightness of $4.46~({kcps/mW})$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mn>4.46</mml:mn> <mml:mspace/> <mml:mo>(</mml:mo> <mml:mi>k</mml:mi> <mml:mi>c</mml:mi> <mml:mi>p</mml:mi> <mml:mi>s</mml:mi> <mml:mo>/</mml:mo> <mml:mi>m</mml:mi> <mml:mi>W</mml:mi> <mml:mo>)</mml:mo> </mml:math> , 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 $S = 2.6 \pm 0.03$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>S</mml:mi> <mml:mo>=</mml:mo> <mml:mn>2.6</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.03</mml:mn> </mml:math> . These comparative results underscore the substantial advantages of utilizing Bessel beams for high-performance entangled photon pair generation.