JILA, NIST and University of Colorado, Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
Nature. 2012 Apr 4;484(7392):78-81. doi: 10.1038/nature10920.
The spectral purity of an oscillator is central to many applications, such as detecting gravity waves, defining the second, ground-state cooling and quantum manipulation of nanomechanical objects, and quantum computation. Recent proposals suggest that laser oscillators which use very narrow optical transitions in atoms can be orders of magnitude more spectrally pure than present lasers. Lasers of this high spectral purity are predicted to operate deep in the 'bad-cavity', or superradiant, regime, where the bare atomic linewidth is much less than the cavity linewidth. Here we demonstrate a Raman superradiant laser source in which spontaneous synchronization of more than one million rubidium-87 atomic dipoles is continuously sustained by less than 0.2 photons on average inside the optical cavity. By operating at low intracavity photon number, we demonstrate isolation of the collective atomic dipole from the environment by a factor of more than ten thousand, as characterized by cavity frequency pulling measurements. The emitted light has a frequency linewidth, measured relative to the Raman dressing laser, that is less than that of single-particle decoherence linewidths and more than ten thousand times less than the quantum linewidth limit typically applied to 'good-cavity' optical lasers, for which the cavity linewidth is much less than the atomic linewidth. These results demonstrate several key predictions for future superradiant lasers, which could be used to improve the stability of passive atomic clocks and which may lead to new searches for physics beyond the standard model.
振荡器的光谱纯度对许多应用至关重要，例如探测引力波、定义秒、基态冷却和纳米机械物体的量子操纵以及量子计算。最近的一些提议表明，使用原子中非常窄的光学跃迁的激光振荡器可以在光谱纯度方面比现有激光高出几个数量级。这种高光谱纯度的激光预计将在“糟糕腔”或超辐射状态下运行，在这种状态下，原子线宽比腔线宽窄得多。在这里，我们展示了一种拉曼超辐射激光源，其中超过一百万个铷-87 原子偶极子的自发同步通过腔内平均不到 0.2 个光子持续维持。通过在低腔内光子数下操作，我们通过腔频率牵引测量证明了集体原子偶极子与环境的隔离超过一万倍，这是由环境的隔离超过一万倍来描述的。发射光的频率线宽（相对于拉曼修饰激光测量）小于单粒子退相干线宽，并且比通常应用于“好腔”光学激光的量子线宽限制小一万倍，对于后者，腔线宽远小于原子线宽。这些结果证明了未来超辐射激光的几个关键预测，这些预测可能用于提高无源原子钟的稳定性，并可能导致对超出标准模型的物理的新搜索。
