Continuous superradiant laser on a millihertz atomic clock transition
In plain words
A laser whose light is stored in atoms rather than mirrors could be far more stable than today's best lasers, because mirror vibrations would barely matter. Short bursts have been shown on a strontium clock line, but a laser that runs continuously has not.
Precise statement
Bad-cavity laser with Sr-87 atoms on the 1S0-3P0 clock transition (natural linewidth about 1 mHz), continuously loaded and repumped, as proposed by Meiser, Ye, Carlson and Holland (PRL 2009). Achieve steady-state superradiant output with linewidth below 1 Hz and identify the process that limits it (atom-number fluctuations, inhomogeneous broadening, heating, loading noise). Answer: measured linewidth and fractional frequency stability of the continuous output.
What would settle it
Heterodyne measurement of a continuously operating superradiant clock-transition laser with sub-Hz linewidth against an independent reference.
Status in the literature
Pulsed superradiance on the Sr clock line was shown in 2016 (Norcia et al., Science Advances, doi:10.1126/sciadv.1601231); continuous lasing has been reported only on broader lines, for example the Sr-88 intercombination line (Kristensen et al., PRL 2023).