  Quantum metrology promises high-precision measurements beyond the capability
of any classical techniques, and has the potential to be integral to
investigative techniques. However, all sensors must tolerate imperfections if
they are to be practical. Here we show that photons with perfectly overlapped
modes, which are therefore fully indistinguishable, are not required for
quantum-enhanced measurement, and that partially-distinguishable photons do not
have to be engineered to mitigate the adverse effects of distinguishability. We
quantify the effect of distinguishability on quantum metrology experiments, and
report results of an experiment to verify that two- and four-photon states
containing partially-distinguishable photons can achieve quantum-enhanced
sensitivity with low-visibility quantum interference. This demonstrates that
sources producing photons with mixed spectral states can be readily utilized
for quantum metrology.
