Photon statistics divides light sources into three different categories, characterized by bunched, antibunched, or uncorrelated photon arrival times. Single atoms, ions, molecules, or solid state emitters display antibunching of photons, while classical thermal sources exhibit photon bunching. Here we demonstrate a light source in free space, where the photon statistics depends on the direction of observation, undergoing a continuous crossover between photon bunching and antibunching. We employ two trapped ions, observe their fluorescence under continuous laser light excitation, and record spatially resolved the autocorrelation function g^{(2)}(τ) with a movable Hanbury Brown and Twiss detector. Varying the detector position we find a minimum value for antibunching, g^{(2)}(0)=0.60(5) and a maximum of g^{(2)}(0)=1.46(8) for bunching, demonstrating that this source radiates fundamentally different types of light alike. The observed variation of the autocorrelation function is understood in the Dicke model from which the observed maximum and minimum values can be modeled, taking independently measured experimental parameters into account.

中文翻译：

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自由空间中的两个原子之光：束缚还是反束缚？

光子统计将光源分为三个不同的类别，其特征是束状，反束状或不相关的光子到达时间。单个原子，离子，分子或固态发射器显示出光子的反聚束，而传统的热源则显示出光子聚束。在这里，我们演示了一个自由空间中的光源，其中光子的统计信息取决于观察的方向，并且在光子聚集和反聚集之间经历了连续的转换。我们采用两个被捕获的离子，在连续的激光激发下观察其荧光，并使用可移动的Hanbury Brown和Twiss检测器记录空间分辨的自相关函数g ^ {（2）}（τ）。改变检测器的位置，我们发现反聚束的最小值g ^ {（2）}（0）= 0.60（5），而最大值g ^ {（2）}（0）= 1.46（8），表明该光源辐射的是根本不同类型的光。在Dicke模型中可以理解所观察到的自相关函数的变化，从中可以模拟观察到的最大值和最小值，并考虑到独立测量的实验参数。