Effects of local decoherence on quantum critical metrology

TL;DR

This paper investigates how local decoherence impacts the enhanced sensitivity of quantum critical metrology, revealing that local noise can revert the system to classical limits and emphasizing the need to protect quantum coherence.

Contribution

It demonstrates that local decoherence destroys the sub-Heisenberg scaling in quantum critical metrology, highlighting the universality of noise effects and the importance of coherence protection.

Findings

Local decoherence recovers the standard quantum limit.

Decoherence destroys many-body entanglement at critical points.

Noise effects are universal across quantum critical systems.

Abstract

The diverging responses to parameter variations of systems at quantum critical points motivate schemes of quantum metrology that feature sub-Heisenberg scaling of the sensitivity with the system size (e.g., the number of particles). This sensitivity enhancement is fundamentally rooted in the formation of Schr\"odinger cat states, or macroscopic superposition states at the quantum critical points. The cat states, however, are fragile to decoherence caused by local noises on individual particles or coupling to local environments, since the local decoherence of any particle would cause the collapse of the whole cat state. Therefore, it is unclear whether the sub-Heisenberg scaling of quantum critical metrology is robust against the local decoherence. Here we study the effects of local decoherence on the quantum critical metrology, using a one-dimensional transverse-field Ising model as a representative example. Based on a previous work [Phys. Rev. Lett. 94, 047201 (2005)] on the critical behaviors of the noisy Ising model, which shows that the universality class of the quantum criticality is modified by the decoherence, we find that the standard quantum limit is recovered by the single-particle decoherence, which is equivalent to local quantum measurement conducted by the environment and destroys the many-body entanglement in the ground state at the quantum critical point. Following the renormalization group analysis [Phys. Rev. B 69, 054426 (2004)], we argue that the noise effects on quantum critical metrology should be universal. This works demonstrates the importance of protecting macroscopic quantum coherence for quantum sensing based on critical behaviors.