Physicists have proposed a new kind of atomic clock based on a revived superradiant laser concept that could produce an extraordinarily stable signal with a linewidth around 100 microhertz, potentially the narrowest ever for an optical laser. "The implications of this result could stretch well beyond timekeeping," reports Phys.org. "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." From the report: 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. 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.
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. The findings have been published in the journal Physical Review Letters.
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