  Classical measurement strategies in many areas are approaching their maximum
resolution and sensitivity levels, but these levels often still fall far short
of the ultimate limits allowed by the laws of physics. To go further,
strategies must be adopted that take into account the quantum nature of the
probe particles and that optimize their quantum states for the desired
application. Here, we review some of these approaches, in which quantum
entanglement, the orbital angular momentum of single photons, and quantum
interferometry are used to produce optical measurements beyond the classical
limit.
