Emergence of irreversibility in embedded quantum systems

We investigate irreversibility in a quantum system subject to unitary dynamics as it results from perturbations of the respective Hamiltonian and from the partitioning of the system into subsystems. We use the quantum fidelity as a distance measure of two states. For the whole system, a perturbation induces instability, but no preferred direction in time. We show that an arrow of time, which is an essential feature of irreversibility, shows up only when focusing on a small enough subsystem. This approach is exemplified by a numerical simulation of a finite spin network, for which we see how irreversibility emerges locally from an appropriate partitioning