Measuring quantum systems with tunnel junctions

Full text: We present a formalism that allows to describe a quantum system modulating the transmission of a tunnel junction. The tunnel junction acts as an environment for the quantum system. Contrary to the conventional approach to open quantum systems we retain a degree of freedom of the environment, the charge passed through the junction, after averaging over the bath degrees of freedom, employing a projection operator technique. The resulting object characterizing the joint dynamics of the system and the charge is the charge specific density matrix. We derive a master equation describing the time evolution of the charge specific density matrix. We consider two examples of quantum systems coupled to the junction: a spin and a harmonic oscillator. In the spin case we are able to analyze a quantum measurement process in detail. For the oscillator we investigate the noise in the tunnel junction induced by the coupling. (author)