Quantum quench from a thermal tensor state: boundary effects and generalized Gibbs ensemble

We consider a quantum quench in a non-interacting fermionic one-dimensional field-theory. The system of size $L$ is initially prepared into two halves $\mathcal{L}$ ($[-L/2,0]$) and $\mathcal{R}$ ($[0,L/2]$), each of them thermalized at two different temperatures, ${T_{L}}$ and ${T_{R}}$ respectively. At a given time the two halves are joined together by a local coupling and the whole system is left to evolve unitarily. For an infinitely extended system ($L\rightarrow \infty$), we show that the time evolution of the particle and energy densities is well described via a hydrodynamic approach which allows us to evaluate the correspondent stationary currents. We show, in such a case, that the two-point correlation functions are deduced, at large times, from a simple non-equilibrium steady state. Otherwise, whenever the boundary conditions are retained (in a properly defined thermodynamic limit), any current is suppressed at large times, and the stationary state is described by a generalized Gibbs ensemble, which is diagonal and depends only on the post-quench mode occupation.