Two-photon phase-sensing with single-photon detection

TL;DR

This paper introduces a novel quantum phase-sensing method using two-photon states transferred to single-photon measurements, enabling quantum advantage without complex multiphoton detection, thus advancing practical quantum optical metrology.

Contribution

A new approach that transfers two-photon quantum information to single-photon states, allowing phase measurement with standard intensity detection and maintaining quantum advantage.

Findings

Achieved phase sensing with two-photon states using single-photon detection.

Observed twice as many interference fringes for the same phase shift.

Demonstrated resource-efficient quantum optical metrology.

Abstract

Path-entangled multi-photon states allow optical phase-sensing beyond the shot-noise limit, provided that an efficient parity measurement can be implemented. Realising this experimentally is technologically demanding, as it requires coincident single-photon detection proportional to the number of photons involved, which represents a severe challenge for achieving a practical quantum advantage over classical methods. Here, we exploit advanced quantum state engineering based on superposing two photon-pair creation events to realise a new approach that bypasses this issue. In particular, optical phase shifts are probed with a two-photon quantum state whose information is subsequently effectively transferred to a single-photon state. Notably, without any multiphoton detection, we infer phase shifts by measuring the average intensity of the single-photon beam on a photodiode, in analogy to standard classical measurements. Importantly, our approach maintains the quantum advantage: twice as many interference fringes are observed for the same phase shift, corresponding to N=2 path-entangled photons. Our results demonstrate that the advantages of quantum-enhanced phase-sensing can be fully exploited in standard intensity measurements, paving the way towards resource-efficient and practical quantum optical metrology.