June 24, 2026 -- Scientists at NPL have demonstrated the best reported laser frequency stability achieved with an optical reference cavity operating at room temperature, marking a major advance in ultrastable laser technology. The team’s results have beenpublished today (24 June) in Optica.

Ultrastable lasers produce light of exceptional spectral purity and are a critical enabling technology for optical atomic clocks. These are the next generation of atomic clocks based on atomic transitions in the optical domain. These clocks underpin the most precise timekeeping ever achieved and are central to future technologies ranging from advanced navigation to fundamental physics.

The NPL team measured a fractional frequency instability of 4 × 10⁻¹⁷, achieved for the first time using a room temperature optical reference cavity. Until now, comparable performance had only been realised internationally using complex cryogenic systems.

This breakthrough was made possible using an optical cavity measuring 68cm in length - the longest ever reported. By increasing the cavity length, the researchers were able to reduce the effect of the Brownian thermal noise introduced by the cavity mirrors, enabling state‑of‑the‑art laser stability without the need for cryogenic cooling.

Operating at room temperature significantly reduces system complexity, maintenance requirements and costs, making ultrapure laser light accessible to a wider range of users in research, industry and national laboratories.

The achieved laser stability is derived from the exceptionally low fractional length variation of the optical reference cavity. To put this result into perspective, a fractional frequency instability of 4 × 10⁻¹⁷ corresponds to a spurious change in cavity length equivalent to altering the distance between the Earth and the Sun by the size of a bacterium.

Improvements in laser stability have a cascading positive effect on optical clock performance. More stable lasers allow clocks to reach their target statistical uncertainty more quickly, significantly reducing measurement time and ultimately improving both precision and accuracy of the clocks.

This advancement will enable improved timekeeping and contributes directly to international efforts to redefine the SI second, moving from microwave based atomic clocks to those operating in the optical domain.