Figure 3. (a) Schedule of shuttle and CNOT/CZ operations that implement the surface code X and Z checks in the required order. Each ancilla qubit (colored circle) is responsible for interacting with four data qubits in a rectangle, and it must interact in a specific order to avoid hook errors [31], as shown in the lower right. (b) Serialized measurement of a group of ancilla qubits for readout density $\rho=1$. Ancilla qubits in group $i$ are measured first, followed by qubits in group $i+1$.