Probing Quantum Phase Transition in Macroscopic Qubit Array via circuit   QED architecture
  We demonstrate a universal physical mechanism to probe the macroscopic quantum phase transition based on circuit QED architecture. We found that, with certain parameters, the Josephson junction qubit array behaves as an antiferromagnetic Ising model in transverse field and the coupled transmission line resonator serves as a bosonic quantum probe. Our investigation shows that, at the critical point, the drastic broadening of the spectrum of the probe indicates the quantum phase transition. 
Y. D. Wang, Fei Xue, C. P. Sun