  Quantum states of light, such as squeezed states or entangled states, can be
used to make measurements (metrology), produce images, and sense objects with a
precision that far exceeds what is possible classically, and also exceeds what
was once thought to be possible quantum mechanically. The primary idea is to
exploit quantum effects to beat the shot-noise limit in metrology and the
Rayleigh diffraction limit in imaging and sensing. Quantum optical metrology
has received a boost in recent years with an influx of ideas from the rapidly
evolving field of optical quantum information processing. Both areas of
research exploit the creation and manipulation of quantum-entangled states of
light. We will review some of the recent theoretical and experimental advances
in this exciting new field of quantum optical metrology, focusing on examples
that exploit a particular two-mode entangled photon state -- the High-N00N
state.
