A quantum phase transition from $Z_2 \times Z_2$ to $Z_2$ topological   order
  Although the topological order is known as a quantum order in quantum many-body systems, it seems that there is not a one-to-one correspondence between topological phases and quantum phases. As a well-known example, it has been shown that all one-dimensional (1D) quantum phases are topologically trivial\cite{spt}. By such a fact, it seems a challenging task to understand when a quantum phase transition between different topological models necessarily reveals different topological classes of them. In this paper, we make an attempt to consider this problem by studying a phase transition between two different quantum phases which belong to a universal topological phase. We define a Hamiltonian which describes an interpolation between the toric code model with $Z_2$ topological order and the color code model with $Z_2 \times Z_2$ topological order on a hexagonal lattice. We show such a model is exactly mapped to many copies of 1D quantum Ising model in transverse field by rewriting the Hamiltonian in a new complete basis. Consequently, we show that the universal topological phase of the color code model and the toric code model reflects in the 1D nature of the phase transition. We also consider the expectation value of Wilson loops by a perturbative calculation and show that behavior of the Wilson loop captures the non-topological nature of the quantum phase transition. The result on the point of phase transition also show that the color code model is strongly robust against the toric code model. 
Mohammad Hossein Zarei
quant-ph cond-mat.str-el