  Quantum-enhanced measurements exploit quantum mechanical effects for
increasing the sensitivity of measurements of certain physical parameters and
have great potential for both fundamental science and concrete applications.
Most of the research has so far focused on using highly entangled states, which
are, however, difficult to produce and to stabilize for a large number of
constituents. In the following we review alternative mechanisms, notably the
use of more general quantum correlations such as quantum discord, identical
particles, or non-trivial hamiltonians; the estimation of thermodynamical
parameters or parameters characterizing non-equilibrium states; and the use of
quantum phase transitions. We describe both theoretically achievable
enhancements and enhanced sensitivities, not primarily based on entanglement,
that have already been demonstrated experimentally, and indicate some possible
future research directions.
