Physical systems close to a quantum phase transition exhibit a divergent susceptibility, suggesting that an arbitrarily-high precision may be achieved by exploiting quantum critical systems as probes to estimate a physical parameter. However, such an improvement in sensitivity is counterbalanced by the closing of the energy gap, which implies a critical slowing down and an inevitable growth of the protocol duration. Here, we design different metrological protocols that make use of the superradiant phase transition of the quantum Rabi model, a finite-component system composed of a single two-level atom interacting with a single bosonic mode. We show that, in spite of the critical slowing down, critical quantum optical systems can lead to a quantum-enhanced time-scaling of the quantum Fisher information, and so of the measurement sensitivity.
@article{arxiv.1910.00604,
title = {Critical Quantum metrology with a finite-component quantum phase transition},
author = {Louis Garbe and Matteo Bina and Arne Keller and Matteo G. A. Paris and Simone Felicetti},
journal= {arXiv preprint arXiv:1910.00604},
year = {2020}
}