估算低强度光源的单光子投影
Estimating the single-photon projection of low-intensity light sources
摘要：
估计单光子源的质量对于其在量子技术中的使用至关重要。半导体源的标准测试是在零时间延迟时低于$1/2$的发射场的二阶关联函数的值。仅该标准并不能提供有关一般量子态单光子贡献幅度的信息。解决这个问题需要了解更多的可观察性。我们推导出一个有效的二阶关联函数，与Mandel- $Q$参数密切相关，并给出了二阶相关和平均光子数，它提供了单光子到多光子投影比的下限。单独使用这两个观测值允许单光子投影的下限和上限。比较我们的边界与文献中的紧密性，我们发现，相对边界可能会更好地描述使用平均光子数，而绝对边界低激发态更紧使用真空投影。我们的研究结果表明，基于附加信息来估计单光子源的质量在很大程度上取决于人们感兴趣的光的量子态的哪个方面。
英文摘要：
Estimating the quality of a single-photon source is crucial for its use in quantum technologies. The standard test for semiconductor sources is a value of the second-order correlation function of the emitted field below $1/2$ at zero time-delay. This criterion alone provides no information regarding the amplitude of the single-photon contribution for general quantum states. Addressing this question requires the knowledge of additional observables. We derive an effective second-order correlation function, strongly connected to the Mandel-$Q$ parameter and given in terms of both the second-order correlation and the average photon number, that provides a lower bound on the single-to-multi-photon projection ratio. Using both observables individually allows for lower and upper bounds for the single-photon projection. Comparing the tightness of our bounds with those in the literature, we find that relative bounds may be better described using the average photon number, while absolute bounds for low excitation states are tighter using the vacuum projection. Our results show that estimating the quality of a single-photon source based on additional information is very much dependent on what aspect of the quantum state of light one is interested in.
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