  We consider the XY spin chain with arbitrary time-dependent magnetic field
and anisotropy. We argue that a certain subclass of Gaussian states, called
Coherent Ensemble (CE) following [1], provides a natural and unified framework
for out-of-equilibrium physics in this model. We show that $all$ correlation
functions in the CE can be computed using form factor expansion and expressed
in terms of Fredholm determinants. In particular, we present exact
out-of-equilibrium expressions in the thermodynamic limit for the previously
unknown order parameter one-point function, dynamical two-point function and
equal-time three-point function.
