  Ultralight scalars could extract energy and angular momentum from a Kerr
black hole (BH) because of superradiant instability. Multiple modes labelled
with $nlm$ grow while rotating around the BH, emitting continuous gravitational
wave (GW). In this work, we carefully study the contribution of the subdominant
modes with $n\geq 1$ in the evolution of the BH-condensate system. We find the
BH still evolves along the Regge trajectory of the $n=0$ modes even with the
presence of the subdominant modes. The interference of the dominant and the
subdominant modes produces beats in the emitted GW, which could be used to
distinguish the BH-condensate systems from other monochromatic GW sources, such
as neutron stars.
