Figure 9
From: Methodology for bus layout for topological quantum error correcting codes
Pictorial representation of the surface code. Data qubits reside on the vertices of the lattice and are here depicted by black dots. Products of data qubits around dark (light) squares correspond to plaquette (star) operators. The green squares are ancillary qubits necessary to measure plaquette and star operators in a non-demolition fashion. We choose the unit cell 0 (dashed square) to generate the whole lattice by translations. The two data qubits and the two ancillary qubits of a unit cell are labeled as shown in the figure. This choice of boundary conditions leads to a twofold degeneracy of the surface code \(\mathcal{S}\) in Eq. (18).