Hong-Ou-Mandel interference of entangled photons generated under pump-tight-focusing condition

Abstract

Hong-Ou-Mandel (HOM) interference is a non-classical effect of photons and plays an important role in quantum optics. <i>β</i>-barium borate (BBO) has high nonlinear efficiency, and is commonly used to generate biphoton states, thereby exhibiting HOM interference. However, in previous experiments, researchers often used band-pass filters, so the resulting spectrum was directly determined by the band-pass filter. As a result, the original spectrum of the BBO crystal, especially the spectrum under tight focusing, was lack of systematic research. In this paper, the biphoton spectral distribution and HOM interference generated by the BBO crystal under the condition of tight focusing were systematically studied for the first time. Theoretical calculations found that using a lens with 50 mm focusing length, the spectral width of the down-converted photons was increased by 7.9 times compared with the non-focused case; the width of the HOM interference fringe decreased by 4 times, and the visibility of the interference fringe was increased from 53.0% to 98.7%. We experimentally prepared the energy-time entanglement state using type II BBO crystal and performed HOM interference, obtaining an interference visibility of <inline-formula><tex-math id="M2">\begin{document}$86.6 \pm 1.0 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20211783_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20211783_M2.png"/></alternatives></inline-formula>%. The increasing of the HOM visibility is due to the improvement of biphoton's spectral symmetry. In addition, the proposed technique that obtaining different spectral distributions at different incident angles was expected to be applied to the preparation of high-dimensional qudits in the future.

Mid-infrared Spectrally Pure Single-Photon States Generation from 22 Nonlinear Optical Crystals

“…Tere are two kinds of HOM interference, the frst one is the HOM interferences using signal and idler photons from the same SPDC source, with a typical setup, as shown in [63]. In this case, the two-fold coincidence probability P 2 (τ) as a function of the time delay τ is given by [64][65][66][67] as follows:…”