Quantum phase transitions occur when the ground state of a quantum system undergoes a qualitative change when an external control parameter reaches a critical value. Here, we demonstrate a technique for studying quantum systems undergoing a phase transition by coupling the system to a probe qubit. It uses directly the increased sensibility of the quantum system to perturbations when it is close to a critical point. Using an NMR quantum simulator, we demonstrate this measurement technique for two different types of quantum phase transitions in an Ising spin chain.
@article{arxiv.0709.3273,
title = {Detection of quantum critical points by a probe qubit},
author = {Jingfu Zhang and Xinhua Peng and Nageswaran Rajendran and Dieter Suter},
journal= {arXiv preprint arXiv:0709.3273},
year = {2008}
}