  We set up a real-time path integral to study the evolution of quantum systems
driven in real-time completely by the coupling of the system to the
environment. For specifically chosen interactions, this can be interpreted as
measurements being performed on the system. For a spin-1/2 system, in
particular, when the measurement results are averaged over, the resulting sign
problem completely disappears, and the system can be simulated with an
efficient cluster algorithm.
