  A pivotal task in quantum metrology, and quantum parameter estimation in
general, is to de- sign schemes that achieve the highest precision with given
resources. Standard models of quantum metrology usually assume the dynamics is
fixed, the highest precision is achieved by preparing the optimal probe states
and performing optimal measurements. However, in many practical experimental
settings, additional controls are usually available to alter the dynamics. Here
we propose to use optimal control methods for further improvement on the
precision limit of quantum parameter estimation. We show that by exploring the
additional degree of freedom offered by the controls higher precision limit can
be achieved. In particular we show that the precision limit under the
controlled schemes can go beyond the constraints put by the coherent time,
which is in contrast to the standard scheme where the precision limit is always
bounded by the coherent time.
