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Short bursts have been shown on a strontium clock line, but a laser that runs continuously has not.","posed_since":"2009","precise":"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.","problem_ref":null,"references":"","settled_by":"Heterodyne measurement of a continuously operating superradiant clock-transition laser with sub-Hz linewidth against an independent reference.","status_note":"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).","title":"Continuous superradiant laser on a millihertz atomic clock 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