  We investigate that no-knowledge measurement-based feedback control is
utilized to obtain the estimation precision of the detection efficiency. For
the feedback operators that concern us, no-knowledge measurement is the optimal
way to estimate the detection efficiency. We show that the higher precision can
be achieved for the lower or larger detection efficiency. It is found that
no-knowledge feedback can be used to cancel decoherence. No-knowledge feedback
with a high detection efficiency can perform well in estimating frequency and
detection efficiency parameters simultaneously. And simultaneous estimation is
better than independent estimation given by the same probes.
