  The non-equilibrium evolution of the block entanglement entropy is
investigated in the XY chain in a transverse magnetic field after the
Hamiltonian parameters are suddenly changed from and to arbitrary values. Using
Toeplitz matrix representation and multidimensional phase methods, we provide
analytic results for large blocks and for all times, showing explicitly the
linear growth in time followed by saturation. The consequences of these
analytic results are discussed and the effects of a finite block length is
taken into account numerically.
