Fig. 4: Singular behavior in the susceptibility.

a The susceptibility χ diverges at the phase transition point J_x = J_y = 0.7ϵ. The subgraph shows the abrupt changes in the order parameters in the first-order quantum phase transition with qubit number N = 1000. b The susceptibility χ near the phase transition point increases linearly with the qubit number N. Here, the perturbation magnetic field is set as B_x = 10⁻⁵ϵ.