  The quantum coherence control of a solid-state charge qubit is studied by
using a suboptimal continuous feedback algorithm within the Bayesian feedback
scheme. For the coherent Rabi oscillation, the present algorithm suggests a
simple bang-bang control protocol, in which the control parameter is modulated
between two values. For the coherence protection of idle state, the present
approach is applicable to arbitrary states, including those lying on the
equator of the Bloch sphere which are out of control in the previous Markovian
feedback scheme.
