Bounds on universal quantum computation with perturbed two-dimensional cluster states

Motivated by the possibility of universal quantum computation under noise perturbations, we compute the phase diagram of the two-dimensional (2D) cluster state Hamiltonian in the presence of Ising terms and magnetic fields. Unlike in previous analysis of perturbed 2D cluster states, we find strong evidence of a very well-defined cluster phase, separated from a polarized phase by a line of first- and second-order transitions compatible with the 3D Ising universality class and a tricritical end point. The phase boundary sets an upper bound for the amount of perturbation in the system so that its ground state is still useful for measurement-based quantum computation purposes. Moreover, we also compute the local fidelity with the unperturbed 2D cluster state. Besides a classical approximation, we determine the phase diagram by combining series expansion and variational infinite projected entangled-pair states methods. Our work constitutes an analysis of the nontrivial effect of few-body perturbations in the 2D cluster state, which is of relevance for experimental proposals.