  We present a measurement scheme capable of achieving the quantum limit of
parameter estimation using an adaptive strategy that minimizes the parameter's
variance at each step. The adaptive rule we propose makes the scheme robust
against errors, in particular imperfect readouts, a critical requirement to
extend adaptive schemes from quantum optics to solid-state sensors. Thanks to
recent advances in single-shot readout capabilities for electronic spins in the
solid state (such as Nitrogen Vacancy centers in diamond), this scheme can be
as well applied to estimate the polarization of a spin bath coupled to the
sensor spin. In turns, the measurement process decreases the entropy of the
spin bath resulting in longer coherence times of the sensor spin.
