# Demonstrating quantum coherence and metrology that is resilient to transversal noise

## TL;DR

This paper demonstrates that quantum coherence can be resilient to certain noise types and can be exploited to achieve metrological advantages in noisy environments, using photonic GHZ states under Markovian noise.

## Contribution

It provides experimental evidence that quantum coherence resilience can be harnessed for enhanced quantum metrology despite Markovian noise, highlighting passive control strategies.

## Key findings

- Quantum coherence shows resilience to Markovian noise in photonic GHZ states.
- Metrological advantage surpassing the standard quantum limit is achieved under noise.
- Passive control can complement quantum error correction in noisy quantum hardware.

## Abstract

Quantum systems can be exploited for disruptive technologies but in practice quantum features are fragile due to noisy environments. Quantum coherence, a fundamental such feature, is a basis-dependent property that is known to exhibit a resilience to certain types of Markovian noise. Yet, it is still unclear whether this resilience can be relevant in practical tasks. Here, we experimentally investigate the resilient effect of quantum coherence in a photonic Greenberger-Horne-Zeilinger state under Markovian bit-flip noise, and explore its applications in a noisy metrology scenario. In particular, using up to six-qubit probes, we demonstrate that the standard quantum limit can be outperformed under a transversal noise strength of approximately equal magnitude to the signal, providing experimental evidence of metrological advantage even in the presence of uncorrelated Markovian noise. This work highlights the important role of passive control in noisy quantum hardware, which can act as a low-overhead complement to more traditional approaches such as quantum error correction, thus impacting on the deployment of quantum technologies in real-world settings.