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A steady-state superradiant laser with less than one intracavity photon


The spectral purity of an oscillator is central to many applications, such as detecting gravity waves1, defining the second2,3, ground-state cooling and quantum manipulation of nanomechanical objects4, and quantum computation5. Recent proposals6,7,8,9 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 lasers10, 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 clocks3 and which may lead to new searches for physics beyond the standard model11,12.

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Figure 1: A steady-state superradiant laser.
Figure 2: Repumping-induced quenching.
Figure 3: Phase coherence maintained with no intracavity photons.
Figure 4: Beyond good-cavity, optical laser stability.

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We thank J. Ye and A. M. Rey for discussions. J.G.B., Z.C., J.M.W. and J.K.T. acknowledge support from NSF PFC, NIST and ARO. M.J.H. and D.M. acknowledge support from the DARPA QuASaR programme through a grant from ARO and from NSF. J.G.B. acknowledges support from NSF GRF, and Z.C. acknowledges support from A*STAR Singapore.

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J.G.B., Z.C., J.M.W. and J.K.T. designed and built the experiment. J.G.B. and Z.C. performed the measurements. J.G.B., Z.C., J.M.W. and J.K.T. analysed the results. D.M., M.J.H. and J.K.T. provided the theoretical analysis. J.G.B. and J.K.T. wrote the manuscript. All authors discussed the results and text of the manuscript.

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Correspondence to James K. Thompson.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text that expands on details in the original paper, comprising: Experimental Details; Primary Experimental Configuration; Secondary Experimental Configuration; Atom Loss; Photon Number and Phase Uncertainty; Phasor Correlation; and Lorentzian Fits. The file also includes Supplementary Figures 1-2, which are detailed diagrams of the physical experimental setup and atomic energy levels. (PDF 489 kb)

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Bohnet, J., Chen, Z., Weiner, J. et al. A steady-state superradiant laser with less than one intracavity photon. Nature 484, 78–81 (2012).

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