  Stimulated radiation and gravitational waves (GWs) are two of the most
important predictions made by Albert Einstein. In this work, we demonstrate
that stimulated GW radiation can occur within gravitational atoms, which
consist of Kerr black holes and the surrounding boson clouds formed through
superradiance. The presence of GWs induces mixing between different states of
the gravitational atoms, leading to resonant transitions between two states
when the GW wavenumber closely matches the energy difference. Consequently, the
energy and angular momentum released from these transitions lead to the
amplification of GWs, resulting in an exponential increase in the transition
rate. Remarkably, the transitions complete within a much shorter time compared
to the lifetime of the cloud. These stimulated transitions give rise to a novel
GW signal that is strong and directed, distinguished from the previously
predicted continuous GWs originating from clouds of ultralight bosons.
