  Quantum phenomena such as entanglement can improve fundamental limits on the
sensitivity of a measurement probe. In optical interferometry, a probe
consisting of $N$ entangled photons provides up to a $\sqrt{N}$ enhancement in
phase sensitivity compared to a classical probe of the same energy. Here, we
employ high-gain parametric down-conversion sources and photon-number-resolving
detectors to perform interferometry with heralded quantum probes of sizes up to
$N=8$ (i.e. measuring up to 16-photon coincidences). Our probes are created by
injecting heralded photon-number states into an interferometer, and in
principle provide quantum-enhanced phase sensitivity even in the presence of
significant optical loss. Our work paves the way towards quantum-enhanced
interferometry using large entangled photonic states.
