  Feedback control is expected to considerably protect quantum states against
decoherence caused by interaction between the system and environment.
Especially, Markovian feedback scheme developed by Wiseman can modify the
properties of decoherence and eventually recover the purity of the steadystate
of the corresponding master equation. This paper provides a condition for which
the modified master equation has a pure steady state. By applying this
condition to a two-qubit system, we obtain a complete parametrization of the
feedback Hamiltonian such that the steady state becomes a maximally entangled
state.
