Unconditional and robust quantum metrological advantage beyond NOON states

TL;DR

This paper introduces a new quantum metrology scheme that achieves unconditional, scalable, and robust measurement enhancement surpassing NOON states, demonstrating a 5.8-fold Fisher information improvement without loss discounting.

Contribution

The authors propose and experimentally realize a novel quantum metrology method combining nonlinear interferometers and stimulated emission, outperforming ideal NOON states in robustness and scalability.

Findings

Achieved 5.8-fold Fisher information enhancement above shot-noise limit.

Demonstrated robustness to photon loss and imperfections.

Scalable and practical for low photon flux quantum metrology.

Abstract

Quantum metrology employs quantum resources to enhance the measurement sensitivity beyond that can be achieved classically. While multi-photon entangled NOON states can in principle beat the shot-noise limit and reach the Heisenberg limit, high NOON states are difficult to prepare and fragile to photon loss which hinders it from reaching unconditional quantum metrological advantages. Here, we combine the idea of unconventional nonlinear interferometers and stimulated emission of squeezed light, previously developed for photonic quantum computer Jiuzhang, to propose and realize a new scheme that achieves a scalable, unconditional, and robust quantum metrological advantage. We observe a 5.8(1)-fold enhancement above the shot-noise limit in the Fisher information extracted per photon, without discounting for photon loss and imperfections, which outperforms ideal 5-NOON states. The Heisenberg-limited scaling, the robustness to external photon loss, and the ease-to-use of our method make it applicable in practical quantum metrology at low photon flux regime.