  Advancements in physics are often motivated/accompanied by advancements in
our precision measurements abilities. The current generation of atomic and
optical interferometers is limited by shot noise, a fundamental limit when
estimating a phase shift with classical light or uncorrelated atoms. In the
last years, it has been clarified that the creation of special quantum
correlations among particles, which will be called here useful entanglement,
can strongly enhance the interferometric sensitivity. Pioneer experiments have
already demonstrated the basic principles. We are probably at the verge of a
second quantum revolution where quantum mechanics of many-body systems is
exploited to overcome the limitations of classical technologies. This review
illustrates the deep connection between entanglement and sub shot noise
sensitivity.
