  Quantum black holes (BHs), thought of as an excited multilevel system, can be
effectively modelled by replacing an event horizon with a partially reflective
membrane. This emergent feature affects their interaction with hosting
environments, with the most pronounced effects happening for environments of
particles with mass~$m_{\rm p}\lesssim (10
M_{\odot}/M)\,10^{-11}\mathrm{\,eV/c^2}$, with~$M$ the BH mass. We show that
ultra-light bosons -- a viable dark matter candidate -- can be used to probe
the quantum nature of BHs. We derive analytical expressions for the accretion
rate and dynamical friction acting on exotic compact objects moving through an
ultra-light scalar field, which we then use to estimate the orbital de-phasing
in the inspiralling of different Laser Inteferometer Space Antenna (LISA)
binaries. Our results indicate that LISA may be able to discriminate quantum
from classical BHs.
