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

A superradiant laser with less than one intracavity photon is shown to synchronize its lasing medium spontaneously and simultaneously isolate it from the environment, producing emitted light with a linewidth ten thousand times smaller than the quantum limit for non-superradiant optical lasers. The 'no-photon' laser Recent proposals suggest that 'superradiant' laser oscillators based on very narrow optical transitions in atoms could be orders of magnitude more spectrally pure than conventional optical lasers. This paper demonstrates one such superradiant laser source, based on spontaneous synchronization of more than one million rubidium-87 atomic dipoles in an optical cavity. The atomic correlations are sustained by fewer than 0.2 photons on average inside the cavity. The low intracavity photon number isolates the oscillator (the collective atomic dipole) from the environment, which is crucial for reducing the sensitivity of such lasers to thermal and technical noise. The results demonstrate several key predictions for superradiant lasers, which could be used to improve the stability of atomic clocks and for new tests of fundamental physics. The spectral purity of an oscillator is central to many applications, such as detecting gravity waves 1 , defining the second 2 , 3 , ground-state cooling and quantum manipulation of nanomechanical objects 4 , and quantum computation 5 . Recent proposals 6 , 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 lasers 10