We present an approach to engineer the photon correlations emerging from the interference between an input field and the field scattered by a single atom in free space. Nominally, the inefficient atom-light coupling causes the quantum correlations to be dominated by the input field alone. To overcome this issue, we propose the use of separate pump and probe beams, where the former increases the atomic emission to be comparable to the probe. Examining the second-order correlation function $g^{\left(2\right)}\left(\tau \right)$ of the total field in the probe direction, we find that the addition of the pump formally plays the same role as increasing the coupling efficiency, even though the physical atom-light coupling efficiency remains unchanged. We show that one can tune the correlation function $g^{\left(2\right)}\left(0\right)$ from zero (perfect antibunching) to infinite (extreme bunching) by a proper choice of pump amplitude. We further elucidate the origin of these correlations in terms of the transient atomic state following the detection of a photon, and show that these correlations can be observed under realistic conditions.

中文翻译：

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自由空间中单个原子发射光的非常规量子关联

我们提出了一种方法来设计由输入场和自由空间中单个原子散射的场之间的干扰产生的光子相关性。名义上，低效的原子光耦合导致量子相关性仅由输入场主导。为了克服这个问题，我们建议使用单独的泵浦和探测光束，其中前者增加了原子发射以与探针相媲美。检查二阶相关函数$G^{\left(2\right)}\left(\tau \right)$在探测方向的总场中，我们发现泵的加入在形式上起到了与提高耦合效率相同的作用，即使物理原子-光耦合效率保持不变。我们表明可以调整相关函数$G^{\left(2\right)}\left(0\right)$从零（完美的抗闭）到无限（极端束缚）通过适当的泵幅度选择。我们根据光子检测后的瞬态原子状态进一步阐明了这些相关性的起源，并表明可以在现实条件下观察到这些相关性。