Coherent Photon Statistics in a Superradiant Laser

It has been widely accepted that the conventional lasing threshold, occurring around unity intracavity photon number, can be eliminated by making the so-called $\beta$ parameter approach unity. Recently, it has been shown that the photon statistics become incoherent when the intracavity mean photon number is decreased below unity even for $\beta \approx 1$. Here, we report systematic measurement of the photon statistics in a superradiant laser as a function of the intracavity photon number. Our superradiant laser was based on the single-atom superradiance driven by twolevel atoms in a quantum superposition state traversing a high-Q cavity one by one. The lasing did not exhibit the conventional threshold and the photon statistics remained near coherent even well below unity photon number. Moreover, the coherence got better as the excited-state population in the initial superposition state was made smaller.