强聚焦泵浦产生纠缠光子的Hong-Ou-Mandel干涉

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

Hong-Ou-Mandel(HOM)干涉是光子的一种非经典效应,在量子光学中起到重要作用.偏硼酸钡(β-barium borate,BBO)具有较高的非线性效率,常被用来产生双光子态,进而展示HOM干涉.然而,在以前的实验中,人们往往使用带通滤光片对双光子的频谱进行过滤,所得光谱由带通滤光片直接决定,而对BBO晶体自身的原始光谱,特别是泵浦光强聚焦下的原始光谱,缺乏系统性研究.本文对泵浦光强聚焦条件下BBO晶体产生的双光子纠缠态光谱分布和HOM干涉进行了深入研究.理论计算发现,使用50 mm透镜聚焦的情况和无聚焦情况相比,下转换光的光谱宽度会增加7.4倍,HOM干涉条纹的宽度会减少为无聚焦情况的1/8,干涉条纹可见度会从53.0%提高到98.7%.实验上使用II型BBO晶体制备了能量-时间纠缠态,并进行了HOM干涉,获得了(86.6±1.0)%的干涉可见度.干涉可见度极大提高的原因在于强聚焦改善了光谱的对称性.此外,本文提出的不同入射角获得不同光谱分布的技术方案有望在未来应用于高维量子纠缠态的制备.

Hong-Ou-Mandel(HOM) interference is a non-classical effect of photons and plays an important role in quantum optics. The β-barium borate(BBO) has a 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 are systematically studied for the first time. Theoretical calculations show that using a lens with 50-mm focusing length, the spectral width of the down-converted photons is increased by7.9 times that of the non-focused case;the width of the HOM interference fringe is reduced to 1/8, and the visibility of the interference fringe increases from 53.0% to 98.7%. We experimentally prepare the energy-time entanglement state by using type-II BBO crystal and perform HOM interference, thereby obtaining the interference visibility of(86.6 ± 1.0)%. The increasing of the HOM visibility is due to the improvement of biphoton’s spectral symmetry. In addition, the proposed technique by which different spectral distributions are obtained at different incident angles is expected to be applied to the preparation of high-dimensional qudits in the future.