  Quantum-enhanced measurements use highly non-classical quantum states in
order to enhance the precision of the measurement of classical quantities, like
the length of an optical cavity. The major goal is to beat the standard quantum
limit (SQL), i.e. a precision of order $1/\sqrt{N}$, where $N$ is the number of
quantum resources (e.g. the number of photons or atoms used), and to achieve a
scaling $1/N$, known as the Heisenberg limit. Doing so would have tremendous
impact in many areas, but so far only three experiments have demonstrated a
slight improvement over the SQL. The required quantum states are generally
difficult to produce, and very prone to decoherence. Here we show that
decoherence itself may be used as an extremely sensitive probe of system
properties. This should allow for a new measurement principle with the
potential to achieve the Heisenberg limit without the need to produce highly
entangled states.
