  We use non-equilibrium dynamical mean-field theory in combination with a
recently developed Quantum Monte Carlo impurity solver to study the real-time
dynamics of a Hubbard model which is driven out of equilibrium by a sudden
increase in the on-site repulsion U. We discuss the implementation of the
self-consistency procedure and some important technical improvements of the QMC
method. The exact numerical solution is compared to iterated perturbation
theory, which is found to produce accurate results only for weak interaction or
short times. Furthermore we calculate the spectral functions and the optical
conductivity from a Fourier transform on the finite Keldysh contour, for which
the numerically accessible timescales allow to resolve the formation of Hubbard
bands and a gap in the strongly interacting regime. The spectral function, and
all one-particle quantities that can be calculated from it, thermalize rapidly
at the transition between qualitatively different weak- and strong-coupling
relaxation regimes.
