One of the two silicon resonators (photo: PTB)

Two independent lasers, each with their own silicon Fabry-Perot cavities, and with laser linewidths of just 0.01 Hz have been created by researchers at Physikalisch-Technische Bundesanstalt (PTB; Braunschweig, Germany) and JILA, National Institute of Standards and Technology and University of Colorado (Boulder, CO).¹ This is a world record, and far smaller than the more-usual kilohertz- or megahertz-scale linewidths.

Cooled to 124 K, the lasers have a beat note between them of as small as 5 mHz at their 194 THz operating frequency (1545 nm wavelength). The mutual phase coherence times for these two lasers range up to 55 s. The thermal Brownian noise of the cavity mirrors is what limits the linewidth of these lasers from being even narrower.