Sub-Natural Linewidth Superradiant Lasing with Cold $^{88}$Sr Atoms

Abstract: Superradiant lasers operate in the bad-cavity regime, where the phase coherence is stored in the spin state of an atomic medium rather than in the intracavity electric field. Such lasers use collective effects to sustain lasing and could potentially reach considerably lower linewidths than a conventional laser. Here, we investigate the properties of superradiant lasing in an ensemble of ultracold $^{88}$Sr atoms inside an optical cavity. We extend the superradiant emission on the 7.5 kHz wide $^3P_1$ $\rightarrow$ $^1S_0$ intercombination line to several milliseconds, and observe steady parameters suitable for emulating the performance of a continuous superradiant laser by fine tuning the repumping rates. We reach a lasing linewidth of 820 Hz for 1.1 ms of lasing, nearly an order of magnitude lower than the natural linewidth