We demonstrate the generation of non-diffracting heralded single photons, i.e. which are characterized by a single-photon transverse intensity distribution which remains essentially unchanged over a significant propagation distance. For this purpose we have relied on the process of spontaneous parametric downconversion (SPDC) for the generation of signal and idler photon pairs, where our SPDC crystal is pumped by a Bessel-Gauss (BG) beam. Our experiment shows that the well-understood non-diffracting behavior of a BG beam may be directly mapped to the signal-mode, single photons heralded by the detection of a single idler photon. In our experiment, the heralded single photon is thus arranged to be non-diffracting without the need for projecting its single-photon transverse amplitude, post-generation, in any manner.
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±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±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.
The purpose of this project is to set up a system for creating entangled photon pairs for use in various quantum mechanical experiments, including the testing of Bell’s inequalities which demonstrates the fundamentally indeterministic nature of quantum mechanics. The photons are created by pumping a beta barium borate crystal with 405 nm laser light then, due to a process called type 1 spontaneous parametric down conversion, a single pump photon can be converted into two 810 nm photons in an entangled state. In the same lab, a similar set up has already been achieved using a 474 nm pump laser, but by using a shorter wave length, a higher detection rate can be achieved. Two collimators are set up to collect the down converted photons and send them to two photon counting modules. The collimators need to be positioned precisely to detect any down converted photons because they emerge from the crystal in a narrow beam that is at an angle with respect to the pump beam and only a miniscule fraction of pump photons are down converted. The angle at which the down converted photons emerge is a function of how the crystal is prepared and the polarization orientation of the pump beam. It was calculated using a program by National Institute of Standards and Technology.