  The model of Unruh-DeWitt detector coupled to the scalar field for finite
time is studied. A systematic way of computing finite time corrections in
various cases is suggested and nonperturbative effects like thermalization are
discussed. It is shown in particular that adiabatic switching off the coupling
between the detector and the thermal bath leaves non-vanishing corrections to
the detector's levels distribution. Considering the two level detector as an
information bearing degree of freedom encoding one bit of information, limits
on external work for the detector's (de)couling in finite time following from
the Landauer's bound are formulated.
