  We study the evolution of reduced density matrix of an impurity coupled to a
Fermi sea after the coupling is switched on at time $t=0$. We find the
non-diagonal elements of the reduced density matrix decay exponentially, and
the decay constant is the impurity level width $\Gamma$. And we study the
information transfer rate between the impurity and the Fermi sea, which also
decays exponentially. And the decay constant is $k\Gamma$ with $k=2\sim 4$. Our
results reveal the relation between information transfer rate and decoherence
rate.
