  We study the dynamics of a two-level quantum system interacting with an
external environment that takes the form of an XY spin chain in the presence of
an external magnetic field. While the presence of the bath itself can enhance
the transition probability from the lower level to the upper level of the
system, we show that this noise-assisted phenomenon is sensitive to a change of
the quantum phase of the environment. The derivative of the transition
probability displays a maximum in correspondence with the critical value of the
applied field both in the case of isotropic and anisotropic chains.
