Lasers by their nature emit light where each photon has nearly the same frequency. That "nearly" is good enough for most applications, but there are still cases where we'd like to do better: atomic clocks, gravitational wave detectors, and tests of variations in physical constants. All of these bump up against the limits of current lasers. A laser with a more stable frequency known as a superradiant laser has been studied theoretically, and now a prototype has been built shows what must be done to make it a practical reality.

Justin G. Bohnet et al. (of JILA/NIST) constructed a demonstration superradiant laser using ultracold rubidium atoms, in which the laser's photons act to synchronize the electronic transitions within the atoms. While a standard laser has many photons present in the laser cavity, this superradiant laser has a cavity that, at any given time, may be empty of photons. Where in a normal laser the light is coherent and the atoms are uncorrelated, in a superradiant laser, it's the atoms that are coherent, transitioning between energy states in concert. While the prototype is not a fully-working superradiant laser, it shows what steps are necessary to construct the real thing, and demonstrates how it should work.



The word "laser" is an acronym, standing for Light Amplification by the Stimulated Emission of Radiation. In a normal laser, atoms are "pumped" by an external power source until they all perform the same electronic transitions. They do not typically transition as a single synchronized unit, but since they all undergo the same quantum absorption and emission, the light they emit is close to being the same wavelength—nearly monochromatic.



In an ordinary laser, a standing wave formed of many photons reflects between mirrors at opposite ends of a cavity. Thermal fluctuations in the mirrors and other vibrations (say from passing footsteps) will slightly change the length of the cavity, which in turn creates small variations in the wavelength of the photons when they are sent out in a beam.

A superradiant laser, on the other hand, exploits atomic transitions with wavelengths that are much less than the length of the cavity, so that small fluctuations in the positions of the mirrors are mostly irrelevant. In this type of system, very few photons may be present in the cavity at any given time—instead of a standing wave of light, the coherence is due the atoms themselves. ("Superradiance" refers not to the number of photons, but to the synchronization of the atoms.)

In fact, the superradiant laser prototype built by Bohnet et al., generally doesn't have any photons in it. At any given moment, there is only a 20 percent chance of finding a single photon in the cavity.

The researchers trapped several million rubidium atoms in a cavity approximately 2 centimeters in length. Rubidium atoms are particularly easy to cool and trap (the original Bose-Einstein condensate was achieved using rubidium, for example). However, rubidium isn't ideal for lasers, since it doesn't normally have the correct types of electronic transitions.

However, the right behavior can be induced by driving the rubidium atoms with another regular laser. The externally supplied photons induce the atoms to allow the right kind of transitions as long as the laser is shining. This allowed the authors to demonstrate the predicted behavior of a superradiant laser: synchronization between the atoms drove the emission of photons in the cavity (although not many of them).

However, since the regular laser is subject to the same vibrational fluctuations in frequency, the rubidium superradiant laser Bohnet et al. achieved wasn't any better than a normal laser, even though the atoms were synchronized.

If a true self-sustaining superradiant laser can be built, it will have far lower variability due to the fluctuations in the length of the cavity. As a result, superradiant lasers can be used to calibrate even more precise atomic clocks than those currently in use. Additionally, gravitational wave detectors—which use interference between laser beams—need to be very sensitive to extremely tiny displacements, which means any fluctuations in the laser frequency need to be even smaller. Finally, detecting any fluctuations in physical constants (such as the fine-structure constant that sets the strength of the electromagnetic force) requires precision measurements, so superradiant lasers could be used in such experiments.

In short, if we can work out some of the limits in this demonstration, it opens the door to lots of interesting experiments.

Nature, 2012. DOI: 10.1038/nature10920 (About DOIs).