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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.
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