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Superradiant Lasers: A Quantum Leap for Precision Lasers

Precision laser science at JILA has taken a quantum leap, thanks to some seminal theory work by the Murray Holland group and promising basic research and technical development by the James Thompson and Jun Ye groups. The story began in 2004 just after Ye had finished a talk on the advantages of focusing on strontium (Sr) in basic physics research. Daniel Kleppner, Lester Wolfe Professor of Physics, Emeritus at MIT, asked Ye if he’d ever considered making a laser with Sr. To that point, Ye hadn’t, but the more he thought about it, the more he was intrigued with the idea.

Ye approached theorist Holland and postdoc Dominic Meiser about whether to pursue the idea of using Sr to make a laser. A few years later in 2009, the Holland group proposed an innovative photonic quantum device for producing long-lasting phase-coherent light—like a laser. However, the device would be based on entirely different principles than a conventional laser.

The heart of the device would be a linear chain of neutral Sr atoms in an optical lattice. The atoms would oscillate exactly in sync when connected by a single “lasing” photon running around inside an optical cavity. As the lasing photon traveled through the chain of Sr atoms, it would “glue” the atoms together and lock them in phase. In the process, all the atoms would automatically evolve into a coherent collective superposition of their ground and excited electronic states.

This collective superposition would cause each atom to oscillate like a spring, but because all the atoms would be glued together, the entire chain of atoms would extend and contract in unison. This collective “superspring” would lose energy by emitting coherent lasing photons. The collective spring action would result in the device emitting far more intense coherent radiation that would occur with independent atoms. This collective emission is known as super-radiance. Because the device would be a superradiant system, its resulting power was predicted to be about 10-12 W, four orders of magnitude higher than if all the Sr atoms emitted their excess energy one at a time.

This amount of power piqued the interest of Thompson and Ye. Ye predicted the device would produce just enough power to make it feasible to use it with the Sr-lattice optical atomic clock under development in the Ye labs. When built, the Sr superradiant laser could have a frequency linewidth of about a thousandth of a hertz—on the order of, or possibly even narrower than, the Sr-lattice clock transition itself. Such a narrow linewidth laser could improve the stability of the Sr-lattice clock a hundredfold.

The results confirmed Ye’s decision to focus on basic research on Sr, which soon became an even more active research frontier in the Ye labs. The group’s fundamental studies of light-matter interactions involving Sr atoms included the optical atomic clock, studies of the quantum mechanics of Sr atomic interactions, the development (with Ana Maria Rey) of a new quantum simulator, ultracold molecules whose coupling resembles collective atomic Sr oscillations that give rise to superradiance, and superradiance itself.

In the meantime, the Thompson group decided it would be relatively straightforward to build a superradiant laser based on the quantum synchronization of a million rubidium atoms (87Rb). Unlike a superradiant Sr laser that would emit visible light, however, the superradiant Rb laser emits light at microwave frequencies. With theory help from the Holland group, the Thompson group’s Rb-based superradiant laser was up and running in 2012. The new laser has, on average, less than a one photon inside of it at any given moment and has been aptly described as a nearly lightless laser.

The Thompson group “looked inside” its new laser to better understand how it works. In one experiment, the researchers investigated the cause of observed fluctuations in the emitted light. First, they deduced that the fluctuations were due to some kind of oscillation occurring in the atoms in the heart of the laser. Second, they “tickled” the laser (with an ordinary laser undergoing controlled power fluctuations). Finally, they used an innovative quantum-measurement technique to precisely determine how the atoms responded.

The researchers found that under certain conditions the superradiant laser was not very ticklish. However when the tickling was done at just the right frequency, the Rb atoms wobbled like crazy. The superradiant laser also exhibited a kind of self-awareness in which it measured atomic wobblings and then adjusted itself to reduce the amount of wobble. The researchers were also able to figure out how to get the laser to increase atomic wobbling.

The results of the Thompson 87Rb superradiant laser experiment are now guiding the group's development of a superradiant Sr laser. Superradiant lasers will not only improve the precision measurement of time, but also the measurement of gravitational waves and fundamental constants. The development of superradiant lasers and the basic physics research behind this development promise to enrich our understanding of many aspects of quantum mechanics for many years to come.