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Light produced by the superradiant laser could stay coherent along the entire journey from Earth to Uranus. Credit: Jarrod Reilly
Researchers in the US and Germany have unveiled a theoretical blueprint for an atomic clock driven by a highly synchronized laser, where atoms work in concert rather than independently. Publishing their results in Physical Review Letters, Jarrod Reilly at the University of Colorado, Simon Jäger at the University of Bonn, and their colleagues in the US and Germany revived an idea first proposed in the 1990s—possibly charting a course toward the narrowest-linewidth lasers ever achieved.
Superradiant lasers and atomic clocks
In a conventional laser, a mirrored cavity bounces light back and forth between atoms, building up a bright, coherent beam. A superradiant laser works differently: rather than relying on the cavity to maintain coherence, the atoms themselves act as single coordinated emitters, collectively synchronizing their light emission.
Following early theoretical ideas emerged in the 1990s, the concept didn't gain concrete traction until 2008, when researchers at the University of Colorado proposed that superradiant lasers could serve as a new kind of atomic clock.
Atomic clocks work by using laser light to probe a very precise transition in an atom, causing electrons to transition between energy levels at an extraordinarily stable frequency. Because a superradiant laser stores its coherence in the atoms rather than the cavity, its output frequency is far less vulnerable to environmental disturbances like vibrations or temperature fluctuations.
As Reilly explains, "Superradiant lasers are so promising to use as a new generation of atomic clocks because they have incredibly small linewidths (small uncertainty in frequency) and are very insensitive to timing errors from small shifts in the clock light's frequency from the environment."
Yet although this concept was first demonstrated experimentally in 2012 in a pulsed regime, the influence of heating has so far held superradiant lasers back from their full potential.
To keep the laser running continuously as an atomic clock requires, atoms must be constantly replenished with energy. Doing this atom-by-atom delivers random kicks that heat the atomic sample and disrupt the lasing process, confining it to brief pulses rather than a steady beam.
Adding an extra energy level
In their study, Reilly's team considered whether a modification to earlier theoretical concepts could make a continuous laser suitable for an atomic clock. In almost all previous studies, atoms were treated as simple two-level systems: an electron sitting in a ground state, occasionally jumping up to an excited state and back again. The team proposed that the heating problem could be solved by adding one extra ground state to the picture.
In a two-level system, if both the pumping (re-energizing) and decay processes happen collectively through the cavity, the mathematics constrains the system in a way that prevents stable, continuous lasing. But with three levels available, pumping and decay can operate on entirely separate transitions, breaking that constraint and allowing the collective approach to work.
"With this extra level, the collective decay and collective pump can occur between different states," Reilly explains. "This allows us to overcome the problems from the model from the 1990s, while still using a collective pump which causes much less heating than what happens in single-particle pump schemes."
The narrowest laser ever?
Running theoretical calculations using parameters relevant to the element barium, the team found that their scheme could produce a laser with a linewidth (a measure of how precisely defined its frequency is) of around 100 microhertz. That would be the narrowest linewidth ever achieved for an optical laser, corresponding to a coherence length (how far the laser light reaches before becoming out of phase) stretching from the sun to the orbit of Uranus.
The team also investigated how sensitive the laser's frequency is to external disturbances. This property, known as "cavity pulling," describes how strongly the output frequency gets dragged toward the cavity's own resonant frequency. Conventional lasers are highly susceptible to this effect: earlier superradiant schemes reduced it significantly, but Reilly's three-level approach does better still.
"In our scheme, because of its multi-level nature, we found that we could tune the cavity pulling from positive to negative in a near-linear fashion," he describes. "This means that we can tune the cavity pulling to be orders of magnitude smaller than what was possible previously, including a point where the cavity pulling theoretically hits zero."
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Beyond atomic clocks
The implications of this result could stretch well beyond timekeeping. A laser immune to environmental frequency shifts would be a powerful tool in optical interferometry—using interference patterns in light to make ultra-precise measurements.
"Our superradiant laser could be used in an optical interferometry setup," says Reilly. "This could potentially be used to make gravitational wave detectors that are insensitive to the environment so that frequency shifts are solely caused by the curved spacetime of the gravitational wave."
Because their scheme relies entirely on collective atomic behavior, the team also notes that it may open the door to an active nuclear clock, which uses transitions within an atomic nucleus rather than its electrons. If achieved, their approach could pave the way for the most accurate timekeepers ever produced.
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