  In this letter, we investigate the dynamics of a single superconducting
artificial atom capacitively coupled to a transmission line with a
characteristic impedance comparable or larger than the quantum resistance. In
this regime, microwaves are reflected from the atom also at frequencies far
from the atom's transition frequency. Adding a single mirror in the
transmission line then creates cavity modes between the atom and the mirror.
Investigating the spontaneous emission from the atom, we then find Rabi
oscillations, where the energy oscillates between the atom and one of the
cavity modes.
