  Entangled states can potentially be used to outperform the standard quantum
limit which every classical sensor is bounded by. However, entangled states are
very susceptible to decoherence, and so it is not clear whether one can really
create a superior sensor to classical technology via a quantum strategy which
is subject to the effect of realistic noise. This paper presents an
investigation of how a quantum sensor composed of many spins is affected by
independent dephasing. We adopt general noise models including non-Markovian
effects, and in these noise models the performance of the sensor depends
crucially on the exposure time of the sensor to the field. We have found that,
by choosing an appropriate exposure time within non-Markovian time region, an
entangled sensor does actually beat the standard quantum limit. Since
independent dephasing is one of the most typical sources of noise in many
systems, our results suggest a practical and scalable approach to beating the
standard quantum limit.
