  Quantum metrology research promises approaches to build new sensors that
achieve the ultimate level of precision measurement and perform fundamentally
better than modern sensors. Practical schemes that tolerate realistic
fabrication imperfections and environmental noise are required in order to
realise quantum-enhanced sensors and to enable their real-world application. We
have demonstrated the key enabling principles of a practical, loss-tolerant
approach to photonic quantum metrology designed to harness all multi-photon
components in spontaneous parametric downconversion---a method for generating
multiple photons that we show requires no further fundamental state engineering
for use in practical quantum metrology. We observe a quantum advantage of 28%
in precision measurement of optical phase using the four-photon detection
component of this scheme, despite 83% system loss. This opens the way to new
quantum sensors based on current quantum-optical capabilities.
