  Most studies in multiparameter estimation assume the dynamics is fixed and
focus on identifying the optimal probe state and the optimal measurements. In
practice, however, controls are usually available to alter the dynamics, which
provides another degree of freedom. In this paper we employ optimal control
methods, particularly the gradient ascent pulse engineering (GRAPE), to design
optimal controls for the improvement of the precision limit in multiparameter
estimation. We show that the controlled schemes not only capable to provide a
higher precision limit, but also have a higher stability to the inaccuracy of
the time point performing the measurements. This high time stability will
benefit the practical metrology where it is hard to perform the measurement at
a very accurate time point due to the response time of the measurement
apparatus.

Quantum Physics