Nonlinear Quantum Transport and Current Noise

We study nonequilibrium transport and noise in a generic dissipative tight-binding model. Within a real-time path integral approach, we derive formally exact series expressions in the number of tunneling events for the noise valid for arbitrary bias, frequency and temperature. At zero temperature, the low-frequency noise can be summed in analytic form. The resulting Shiba-like $|\omega|$-singularity is a consequence of the $1/t^2$ decay law for the current correlation function. At finite temperature, this singularity is smoothed out.