光谱亮度接近极限的室温双光子源
Room-temperature biphoton source with a spectral brightness near the ultimate limit
摘要：
热原子蒸气通过自发四波混频（SFWM）过程产生的双光子具有频率稳定、线宽可调等优点。这些优点在长距离量子通信的应用中是非常有用的。然而，热原子SFWM双光子源先前具有低得多的每线宽产生率值，即，光谱亮度，与自发参量下转换（SPDC）过程产生的双光子源相比。在这里，我们报告了一个热原子SFWM源的双光子的线宽为960 kHz，产生率为3.7 $\times$ $10^5$对/s。高的产生速率和窄的线宽导致3.8 $\times$ $10^5$ pairs/s/MHz的光谱亮度，这是以前用原子蒸气得到的最佳结果的17倍，也优于所有已知的用各种介质得到的结果。的所有共同传播的计划连同一个大的光学深度（OD）的原子蒸气是关键的改进，使实现光谱亮度是极限的四分之一左右。此外，该双光子源的信号背景比（SBR）为2.7，这违反了经典光的柯西-施瓦茨不等式约3.6倍。虽然增加光谱亮度通常会导致SBR下降，我们的系统研究表明，目前的光谱亮度和SBR可以通过进一步增加OD增强。这项工作展示了一个重大的进步，并提供了有用的知识，在量子技术使用光子。
英文摘要：
The biphotons, generated from a hot atomic vapor via the process of spontaneous four-wave mixing (SFWM), have the following merits: stable and tunable frequencies as well as linewidth. Such merits are very useful in the applications of long-distance quantum communication. However, the hot-atom SFWM biphoton sources previously had far lower values of generation rate per linewidth, i.e., spectral brightness, as compared with the sources of biphotons generated by the spontaneous parametric down conversion (SPDC) process. Here, we report a hot-atom SFWM source of biphotons with a linewidth of 960 kHz and a generation rate of 3.7$\times$ $10^5$ pairs/s. The high generation rate, together with the narrow linewidth, results in a spectral brightness of 3.8$\times$ $10^5$ pairs/s/MHz, which is 17 times of the previous best result with atomic vapors and also better than all known results with all kinds of media. The all-copropagating scheme together with a large optical depth (OD) of the atomic vapor is the key improvement, enabling the achieved spectral brightness to be about one quarter of the ultimate limit. Furthermore, this biphoton source had a signal-to-background ratio (SBR) of 2.7, which violated the Cauchy-Schwartz inequality for classical light by about 3.6 folds. Although an increasing spectral brightness usually leads to a decreasing SBR, our systematic study indicates that both of the present spectral brightness and SBR can be enhanced by further increasing the OD. This work demonstrates a significant advancement and provides useful knowledge in the quantum technology using photons.
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