Towards practical quantum metrology with photon counting

TLDR

Quantum metrology seeks to create sensors that reach ultimate precision limits, but optical loss, scheme complexity, and interferometric instability hinder practical quantum‑enhanced sensor realization. The study aims to develop interferometric schemes that tolerate realistic device loss and sample absorption to achieve quantum advantage. The authors employ photon‑counting of a generalized multi‑photon singlet state generated via spontaneous parametric down‑conversion to achieve loss‑tolerant quantum metrology. The approach yields rapidly oscillating interference fringes that survive realistic loss, as demonstrated by four‑photon coincidence detection dominated by the four‑photon singlet term, and indicates that state‑of‑the‑art photonics can deliver quantum advantage without post‑selection.

Abstract

Abstract Quantum metrology aims to realise new sensors operating at the ultimate limit of 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 generalised 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.