Non-Markovian Quantum State Diffusion

We present a nonlinear stochastic Schroedinger equation for pure states describing non-Markovian diffusion of quantum trajectories. It provides an unravelling of the evolution of a quantum system coupled to a finite or infinite number of harmonic oscillators, without any approximation. Its power is illustrated by several examples, including measurement-like situations, dissipation, and quantum Brownian motion. In some examples, we treat the environment phenomenologically as an infinite reservoir with fluctuations of arbitrary correlation. In other examples the environment consists of a finite number of oscillators. In these quasi-periodic cases we see the reversible decay of a `Schroedinger cat' state. Finally, our description of open systems is compatible with different positions of the `Heisenberg cut' between system and environment.