  A double quantum dot device is a tunable two-level system for electronic
energy states. A dc electron current directly measures the rates for elastic
and inelastic transitions between the two levels. For inelastic transitions
energy is exchanged with bosonic degrees of freedom in the environment. The
inelastic transition rates are well described by the Einstein coefficients,
relating absorption with stimulated and spontaneous emission. The most
effectively coupled bosons in the specific environment of our semiconductor
device are acoustic phonons. The experiments demonstrate the importance of
vacuum fluctuations in the environment for little circuits of coherent quantum
devices.
