  We study the dynamics of the biased sub-Ohmic spin-boson model by means of a
time-dependent variational matrix product state (TDVMPS) algorithm. The
evolution of both the system and the environment is obtained in the weak- and
the strong-coupling regimes, respectively characterized by damped spin
oscillations and by a nonequilibrium process where the spin freezes near its
initial state, which are explicitly shown to arise from a variety of reactive
environmental quantum dynamics. We also explore the rich phenomenology of the
intermediate-coupling case, a nonperturbative regime where the system shows a
complex dynamical behavior, combining features of both the weakly and the
strongly coupled case in a sequential, time-retarded fashion. Our work
demonstrates the potential of TDVMPS methods for exploring otherwise elusive,
nonperturbative regimes of complex open quantum systems, and points to the
possibilities of exploiting the qualitative, real-time modification of quantum
properties induced by nonequilibrium bath dynamics in ultrafast transient
processes.
