  We use a superconducting microresonator as a cavity to sense absorption of
microwaves by a superconducting quantum point contact defined by surface gates
over a proximitized two-dimensional electron gas. Renormalization of the cavity
frequency with phase difference across the point contact is consistent with
adiabatic coupling to Andreev bound states. Near $\pi$ phase difference, we
observe random fluctuations in absorption with gate voltage, related to quantum
interference-induced modulations in the electron transmission. We identify
features consistent with the presence of single Andreev bound states and
describe the Andreev-cavity interaction using a dispersive Jaynes-Cummings
model. By fitting the weak Andreev-cavity coupling, we extract ~GHz decoherence
consistent with charge noise and the transmission dispersion associated with a
localized state.
