  Over the past 20 years, bright sources of entangled photons have led to a
renaissance in quantum optical interferometry. Optical interferometry has been
used to test the foundations of quantum mechanics and implement some of the
novel ideas associated with quantum entanglement such as quantum teleportation,
quantum cryptography, quantum lithography, quantum computing logic gates, and
quantum metrology. In this paper, we focus on the new ways that have been
developed to exploit quantum optical entanglement in quantum metrology to beat
the shot-noise limit, which can be used, e.g., in fiber optical gyroscopes and
in sensors for biological or chemical targets. We also discuss how this
entanglement can be used to beat the Rayleigh diffraction limit in imaging
systems such as in LIDAR and optical lithography.
