  The invention of the maser stimulated many revolutionary technologies such as
lasers and atomic clocks. Despite enormous progress, the realizations of masers
are still confined to a limited variety of systems, in particular, the physics
of masers remains unexplored in periodically driven (Floquet) systems, which
are generally defined by time-periodic Hamiltonians and enable to observe many
exotic phenomena such as time crystals. Here we investigate the Floquet system
of periodically driven 129Xe gas under damping feedback, and surprisingly
observe a multi-mode maser that oscillates at frequencies of transitions
between Floquet states. Our findings extend maser techniques to Floquet
systems, and open a new avenue to probe Floquet phenomena unaffected by
decoherence, enabling a new class of maser sensors. As a first application, our
maser offers a unique capability of measuring low-frequency (1-100 mHz)
magnetic fields with femtotesla-level sensitivity, which is significantly
better than state-of-the-art magnetometers, and can be immediately applied to,
for example, ultralight dark matter searches.
