定义：
只有一个原子作为增益介质的激光器。

通常来说，激光器的增益介质包含大量的激光活性原子或离子。然而，由于各种原因，研究的兴趣放在单原子激光器上，其单个原子构成了整个增益介质。加州理工学院的Kimble的研究小组早在2003年，第一次实现证明了单原子激光器（或一原子激光器）。 
铯原子是激光冷却并捕获到磁光阱（MOT）中，然后在释放下降。一个铯原子被加载到一个远端谐振光学陷阱（FORT），实现在两个超反射镜之间形成一个高精细谐振器，以此作为激光谐振器。
使用辅助激光使得谐振器波长稳定。铯原子系统（至少在这种体系中有意义）的“反转”也实现了光抽运。激光经过最后镜面后谐振腔，以两个高斯光束的形式发射。
这种单原子层激光器不仅仅是普通激光器的微缩版。由于高质量腔处理和小的模式体，这种设备在不寻常的光场光子和原子跃迁间的强耦合环境下运行。具体来说，拉比频率远高于自发发射速率和腔中光子的衰减率，因此耦合系统可以接受激发衰变前的几个拉比周期。
在这种结构中，从完全地量子动力学描述的理论预测与常用于描述激光运作的半经典的模型（对光场的经典描述）有很大的差异。 
对这类设备研究的基本兴趣点是因为它可以在量子光学领域内进行一些预测的测试。的确，实验证实了单原子激光器应该没有激光阈值（→无阈值激光器），即在很小的泵浦功率下激光也能发射。此外，激光输出不想其它大多数激光器一样，其输出不是相干态，但由非经典光组成。
尤其是重要的光子反群聚和亚泊松光子统计在同时测量中可能观察到，特别是在低泵浦速率情况下。通过比较各种实验观测结果与涉及量子化光物质相互作用的预测激光模型，我们可以获得很多信息。

一些对于有机会对于这些效应的细节研究的努力是很值得的，即使很难确信关于单原子激光器的研究是否能够实际应用。 
注意到，单原子与微波腔相互作用的单原子微量子放大器。其中一个显著差异，就是在这种情况下，通常是处理一个原子束，但即使每次腔体中最多有一个原子，不同原子随后也与光场相互作用。
相比之下，单原子激光器在很长时间真的只有一个原子工作，可以拿例如逆拉比频率相对比。 单原子激光不能与原子激光器混淆，后者发射的是相干物质波而不是光。

Definition: lasers with only a single atom as the gain medium

More general terms: lasers

Usually, the gain medium of a laser contains a huge number of laser-active atoms or ions. However, for various reasons it is of interest to study the behavior of a single-atom laser, where a single atom constitutes the whole gain medium. In 2003, Kimble's research group at the California Institute of Technology demonstrated the first realization of a single-atom laser (or one-atom laser) [3]. Cesium atoms were laser cooled and trapped in a magneto-optical trap (MOT) and then released in order to fall downwards. A single cesium atom was then loaded into a far-off-resonance optical trap (FORT), realized between two supermirrors which formed a high-finesse resonator serving as the laser resonator. The resonator length was actively stabilized using an auxiliary laser. The “inversion” of the cesium system (as far as this term makes sense in this regime) was also achieved with optical pumping. Laser emission occurred in the form of two Gaussian beams exiting the laser resonator at the end mirrors.

Such a one-atom laser is not just a miniaturized version of an ordinary laser. Due to the high cavity finesse and the small mode volume, this device operates in the unusual regime of strong coupling between the photons of the light field and the atomic transition. Specifically, the Rabi frequency is well above both the spontaneous emission rate and the photon decay rate of the cavity, so that the coupled system can undergo several Rabi cycles before the excitation decays. In this regime, the theoretical predictions from a full quantum description of the dynamics differ strongly from those of a semiclassical model (with classical treatment of the light field) as usually used to describe laser operation.

The investigation of such devices is of fundamental interest as it makes it possible to test certain predictions of quantum optics. Indeed, the experiments confirmed the prediction that a single-atom laser should have no laser threshold (→ thresholdless lasers), i.e. the laser emission occurs even for the smallest pump powers. Further, the laser output is not in a coherent state as for most other lasers, but consists of nonclassical light. In particular, significant photon antibunching and sub-Poissonian photon statistics could be observed with coincidence measurements, particularly at low pump rates. Much can be learned by comparing various experimental observations with predictions from laser models involving the quantized light–matter interaction. The opportunity to do detailed studies of all these effects makes such efforts worthwhile, even though it is hardly conceivable that a single-atom laser will find any practical application.

Note that there have been single-atom micromasers, where single atoms interacted with a microwave cavity. A significant difference, however, is that in this case one is usually dealing with an atomic beam where different atoms subsequently interact with the light field, even though at each time there is at most one atom in the cavity. In contrast, the single-atom laser really works with just one atom over longer times, as compared with, e.g., the inverse Rabi frequency.

Single-atom lasers should not be confused with atom lasers, which emit coherent matter waves rather than light.

Bibliography