  The temporal evolution of the entanglement between two qubits evolving by
random interactions is studied analytically and numerically. Two different
types of randomness are investigated. Firstly we analyze an ensemble of systems
with randomly chosen but time-independent interaction Hamiltonians. Secondly we
consider the case of a temporally fluctuating Hamiltonian, where the unitary
evolution can be understood as a random walk on the SU (4) group manifold. As a
by-product we compute the metric tensor and its inverse as well as the
Laplace-Beltrami for SU (4).
