  Entangled photons can be used to make measurements with an accuracy beyond
that possible with classical light. While most implementations of quantum
metrology have used states made up of a single colour of photons, we show that
entangled states of two colours can show supersensitivity to optical phase and
path-length by using a photonic crystal fibre source of photon pairs inside an
interferometer. This setup is relatively simple and robust to experimental
imperfections. We demonstrate sensitivity beyond the standard quantum limit and
show super-resolved interference fringes using entangled states of two, four,
and six photons.
