Nature 484, 78–81 (2012)

One of the defining features of a laser is its spectrally narrow emission. Justin Bohnet and colleagues have now demonstrated that devices operating in the superradiant regime can dramatically reduce this linewidth further still. This could lead to substantial improvements in the resolution of optics-based sensors and detectors.

Like lasing, superradiance is coherent emission from an ensemble of light sources. Conventional lasers achieve this by using a high-quality cavity, formed by two mirrors, to feed back many of the generated photons and stimulate further emission. But thermal effects on these mirrors broaden the optical mode. Superradiance, on the other hand, relies on the ensemble of sources acting collectively. This synchronization can be achieved with far fewer photons in the cavity mode — superradiance, it is said, works in a 'bad-cavity' regime. The emission is less sensitive to thermal effects; rather it is related to the narrower linewidth of the sources themselves.

With their superradiant laser, Bohnet et al. have demonstrated spontaneous synchronization of rubidium-87 atoms in a cavity containing, on average, fewer than 0.2 photons. This light source has a linewidth more than 10,000 times narrower than an equivalent 'good-cavity' optical laser.