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Thermalization dynamics close to a quantum phase transition

We investigate the dissipative dynamics of a quantum critical system in contact with a thermal bath. In analogy with the standard protocol employed to analyze aging, we study the response of a system to a sudden change of the bath temperature. The specific example of the XY model in a transverse magnetic field whose spins are locally coupled to a set of bosonic baths is considered. The peculiar nature of the dynamics is encoded in the correlations developing out of equilibrium. By means of a kinetic equation we analyze the spin-spin correlations and block correlations. We identify some universal features in the out-of-equilibrium dynamics. Two distinct regimes, characterized by different time and length scales, emerge. During the initial transient the dynamics is characterized by the same critical exponents as those of the equilibrium quantum phase transition and resembles the dynamics of thermal phase transitions. At long times equilibrium is reached through the propagation along the chain of a thermal front in a manner similar to the classical Glauber dynamics.