  Laser-cooled gases of atoms interacting with the field of an optical cavity
are a powerful tool for quantum sensing and the simulation of open and closed
quantum systems. They can display spontaneous self-organisation phase
transitions, time crystals, new lasing mechanisms, squeezed states for quantum
sensing, protection of quantum coherence, and dynamical phase transitions.
However, all of these phenomena are explored in a discontinuous manner due to
the need to stop and reload a new ensemble of atoms. Here we report the
observation of hours-long continuous lasing from laser-cooled $^{88}$Sr atoms
continuously loaded into a ring cavity. The required inversion to produce
lasing arises from inversion in the atomic momentum degree of freedom, a
mechanism related directly to self-organization phase transitions and
collective atomic recoil lasing, both of which were previously only observed in
a cyclic fashion compared to the truly continuous behavior here. Further, the
sensitivity of the lasing frequency to cavity frequency changes is 120 fold
suppressed due to an atomic loss mechanism, opening an interesting new path to
compensate cavity frequency noise for realizing narrow frequency references.
This work opens the way for continuous cavity QED quantum simulation
experiments as well as continuous superradiant lasers.
