  We solve the nonequilibrium dynamics of qubits or quantum spin chains (s=1/2)
modeled by an anisotropic XY Hamiltonian, when the initial condition is
prepared as a spatially inhomogeneous state of the magnetization. Infinite
systems are studied analytically, yielding a universal relaxation driven by
quantum fluctuations. Particular long-lived excitations are found, for which
the relaxation time diverges as a consequence of constructive quantum
interference at degenerate stationary points. Those states are intrinsically
entangled and may be of interest for performing quantum computation. We also
numerically analyze finite samples to assess the extent of size effects.
