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.
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First superradiant lasers produce nearly no photons (and that’s expected)