Physicists Revive 1990s Laser Concept To Propose a Next-Generation Atomic Clock

Source: Slashdot

Background

Physicists have proposed a new kind of atomic clock based on a revived superradiant laser concept. The device could produce an extraordinarily stable signal with a linewidth around 100 µHz, potentially the narrowest ever for an optical laser.

“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.” – Phys.org

Superradiant Laser Concept

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.

Early theoretical ideas emerged in the 1990s, but the concept gained traction in 2008 when researchers at the University of Colorado suggested that superradiant lasers could serve as a new kind of atomic clock.

How Atomic Clocks Use Lasers

Atomic clocks probe a very precise transition in an atom with laser light, 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 such as vibrations or temperature fluctuations.

Challenges

• Heating: The concept was first demonstrated experimentally in 2012 in a pulsed regime. Heating has limited superradiant lasers from reaching their full potential.
• Continuous Operation: To keep the laser running continuously as an atomic clock, atoms must be constantly replenished with energy. Atom‑by‑atom pumping delivers random kicks that heat the atomic sample and disrupt the lasing process, confining it to brief pulses rather than a steady beam.

New Study

Reilly’s team investigated whether a modification to earlier theoretical concepts could enable a continuous superradiant laser suitable for an atomic clock.

• Three‑level scheme: Previous studies treated atoms as simple two‑level systems (ground ↔ excited). The new proposal adds an extra ground state.
• Separate pumping and decay channels: In a two‑level system, collective pumping and decay through the cavity impose constraints that prevent stable, continuous lasing. With three levels, pumping and decay can occur on distinct transitions, breaking the constraint and allowing continuous operation.

The findings have been published in Physical Review Letters.