Robust surface code topology against sparse fabrication defects in a superconducting-qubit array

Abstract

Superconducting qubits arrays have become one of the most promising architectures for the practical implementation of universal quantum computing with quantum error correction codes. Here we propose a defect-tolerant surface code topology which is resistant to sparse fabrication defects. The topology is equivalent to a disk folded into $N$ layers. A physically defective qubit can be replaced with a working physical qubit within the same unit cell from a different layer. Thereby sparse fabrication errors can be collected into one sacrificial layer and isolated from the working layers by turning off their controllable couplers. We propose two schemes to realize this robust topology. One is to use flux qubits, and the other is based on Xmon qubits. A modified quantum circuit for the stabilization cycle of the Xmon qubits scheme is developed. We calculate and compare the per-operation error thresholds for these two schemes and find that the error threshold for the modified quantum circuit is close to that for the original stabilization cycle. A possible way to implement the intersecting connections between different Xmon qubits on the circuit level is also provided.