How focused pumping affects type-II spontaneous parametric down-conversion
How focused pumping affects type-II spontaneous parametric down-conversion
复制标题
聚焦泵浦如何影响 II 型自发参数下转换
DOI：
10.1103/physreva.72.033803
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发表时间：
2005
影响因子：
2.9
通讯作者：
J. Woerdman
中科院分区：
文献类型：
--
作者：
P. Lee;M. V. Exter;J. Woerdman
自发参量下转换SPDC已经成为产生纠缠光子对的常用方法，用于量子力学基本特征的实验研究1-3。虽然对于第二类SPDC，这些光子对可以同时在能量、动量和偏振方面纠缠，但偏振纠缠的使用由于其简单而最受欢迎。第二类SPDC中双光子纠缠的一般理论方面已在4，5中广为人知并得到了深入的研究。更具体地说，泵浦的光谱特性对下转换光的影响也是几篇论文研究的主题，包括光谱泵浦宽度对下转换光束6的空间相干性的影响以及在第二类SPDC中宽带脉冲7，8泵浦的光谱后果。然而，泵浦的空间特性在第二类SPDC中的作用，特别是聚焦泵浦9，10的作用，是一个较少被探索的制度。当利用光纤耦合的光子计数器11、12检测到纠缠的光子对时，泵浦激光的适当聚焦当然是必要的。为了优化纠缠光子对的收集，晶体中反向传播光纤模的大小和横向光束走离的大小都必须与泵浦光斑11的大小相匹配。当使用孔后的“桶”探测器进行配对检测时，也可能出现聚焦泵浦的潜在有益效果。一个表明这种效应的简单论点是，与聚焦泵浦相关的大波矢扩散通常会加宽组成通常的SPDC图案的两个环。因此，环交叉面积的增加可能会允许我们使用更大的光圈，并提高偏振纠缠光子对的产额。为了研究这一方案的可行性，并检查桶和光纤耦合检测方案中的任何副作用，需要更好地理解聚焦泵浦在SPDC中的作用。在这篇文章中，我们研究了聚焦泵浦对由II型SPDC产生的单光子图像的影响，而不是专门讨论它对符合成像9，10的影响。特别是，我们从理论和实验上证明了从平面波到聚焦泵浦的跃迁导致两个下转换环的不对称展宽相同。我们的理论描述遵循了Grice和Walmsley 8的方法，用来分析从连续泵浦到宽带脉冲泵浦过渡过程中普通光谱和非常光谱之间的差异，这可以松散地称为时间而不是空间上的聚焦效应。我们还研究了聚焦泵浦对偏振纠缠光子对测量的光子产额和纠缠质量的影响。我们提供的实验数据仅支持桶检测的这些结果，并在展望讨论中包括光纤耦合检测的情况。
Spontaneous parametric down-conversion SPDC has become the common method to generate entangled photon pairs for experimental studies on fundamental features of quantum mechanics 1–3 . Though these photon pairs can be simultaneously entangled in energy, momentum, and polarization for type-II SPDC , the use of polarization entanglement is most popular due to its simplicity. The general theoretical aspects of two-photon entanglement in type-II SPDC are well known and thoroughly studied in 4,5 . More specifically, also the effect of the spectral properties of the pump on the down-converted light has been the topic of investigation in several papers, including the effect of the spectral pump width on the spatial coherence of the downconverted beams 6 and the spectral consequences of broadband pulsed 7,8 pumping in type-II SPDC. The role of the spatial properties of the pump in type-II SPDC, and particularly that of focused pumping 9,10 , is a less explored regime, though. Proper focusing of the pump laser is certainly necessary when the entangled photon pairs are detected with fiber-coupled photon counters 11,12 . In order to optimize the collection of entangled photon pairs, both the size of the backward-propagated fiber mode and the transverse beam walk-off in the crystal have to match the size of the pump spot 11 . A potentially beneficial effect of focused pumping may also arise when using “bucket” detectors behind apertures for pair detection. A simple argument that suggests such effect is that the large wave-vector spread associated with focused pumping will generally broaden the two rings that comprise the usual SPDC pattern. The increased area of the ring crossings might thus allow us to work with larger apertures and enhance the yield of polarization-entangled photon pairs. To investigate the feasibility of this scheme and check for any side effects in both the bucket and fiber-coupled detection scheme, a better understanding of the role of focused pumping in SPDC is needed. In this paper, we study the effect of focused pumping on the single-photon image generated via type-II SPDC, contrary to papers that specifically treat the effect on coincidence imaging 9,10 . In particular, we theoretically and experimentally demonstrate that the transition from plane-wave to focused pumping leads to the same asymmetric broadening of both down-converted rings. Our theoretical description follows the approach that Grice and Walmsley 8 use to analyze the difference between the ordinary and extraordinary spectrum in the transition from cw pumping to broadband pulsed pumping, which could be loosely called “the effects of focusing in time” instead of space . We also study the consequences of focused pumping for the measured photon yield and entanglement quality of the polarizationentangled photon pairs. We present the experimental data that support these consequences for bucket detection only and include the case of fiber-coupled detection in an outlook discussion.