  Quantum sensors allow the estimation of parameters with precision higher than
that obtained with classical strategies. Devices based on quantum physics have
allowed the precise estimation of the gravitational field, the detailed imaging
of the brain, the detection of gravitational-wave sources more than 400 million
light years away, and an ever-increasing precision in the measurement of time.
Quantum metrology, which is the conceptual framework that encompasses all these
devices, is reviewed here, emphasizing recent results regarding noisy systems.
