  We evaluate exactly the non-Markovian effect on the decoherence dynamics of a
qubit interacting with a dissipative vacuum reservoir and find that the
coherence of the qubit can be partially trapped in the steady state when the
memory effect of the reservoir is considered. Our analysis shows that it is the
formation of a bound state between the qubit and its reservoir that results in
this residual coherence in the steady state under the non-Markovian dynamics. A
physical condition for the decoherence suppression is given explicitly. Our
results suggest a potential way to decoherence control by modifying the
system-reservoir interaction and the spectrum of the reservoir to the
non-Markovian regime in the scenario of reservoir engineering.
