  Coupling a quantum system to a bosonic environment always give rise to
inelastic processes, which reduce the coherency of the system. We measure
energy dependent rates for inelastic tunneling processes in a fully
controllable two-level system of a double quantum dot. The emission and
absorption rates are well repro-duced by Einstein's coefficients, which relate
to the spontaneous emission rate. The inelastic tunneling rate can be
comparable to the elastic tunneling rate if the boson occupation number becomes
large. In the specific semiconductor double dot, the energy dependence of the
inelastic rate suggests that acoustic phonons are coupled to the double dot
piezoelectrically.
