Towards practical quantum metrology with photon counting

Quantum metrology aims to realise new sensors operating with the ultimate in precision measurement. However, optical loss, the complexity of proposed metrology schemes, and interferometric instability each prevent the realisation of practical quantum-enhanced sensors. To obtain a quantum-advantage in interferometry using these capabilities, new schemes are required that tolerate realistic device loss and sample absorption. We show that loss-tolerant quantum metrology is achievable with photon-counting measurements of the generalized multi-photon singlet state, which is readily generated from spontaneous parametric downconversion without any further state engineering. The power of this scheme comes from coherent superpositions, which give rise to rapidly-oscillating interference fringes that persist in realistic levels of loss. We have demonstrated the key enabling principles through the four-photon coincidence detection of outcomes that are dominated by the four-photon singlet term of the four-mode downconversion state. Combining state-of-the-art quantum photonics will enable a quantum advantage to be achieved without using post selection and without any further changes to the approach studied here.