  There are a number of different strategies to measure the phase shift between
two pathways of light more efficiently than suggested by the standard quantum
limit. One way is to use highly entangled photons. Another way is to expose
photons to a non-linear or interacting Hamiltonian. This paper emphasises that
the conditional dynamics of open quantum systems provides an interesting
additional tool for quantum-enhanced metrology. As a concrete example, we
review a recent scheme which exploits the conditional dynamics of a
laser-driven optical cavity with spontaneous photon emission inside a quantum
feedback loop. Deducing information from second-order photon correlation
measurements requires neither optical non-linearities nor entangled photons and
should therefore be of immediate practical interest.
