Handling fabrication defects in hex-grid surface codes

TL;DR

This paper introduces a method to handle fabrication defects in hex-grid surface codes, maintaining high error correction performance despite broken qubits and couplers, thus advancing scalable quantum hardware implementation.

Contribution

It extends the LUCI framework to effectively manage fabrication defects in hexagonal surface code architectures, enabling large-scale quantum error correction.

Findings

Broken qubits reduce circuit distance by one

Broken couplers reduce circuit distance in one or both bases

Dropout strategy restores fault tolerance in defective grids

Abstract

Recent work has shown that a hexagonal grid qubit layout, with only three couplers per qubit, is sufficient to implement the surface code with performance comparable to that of a traditional four-coupler layout [McEwen et al., 2023]. In this work we propose a method for handling broken qubits and couplers even in hex-grid surface code architectures, using an extension of the LUCI framework [Debroy et al., 2024]. We show that for isolated broken qubits, the circuit distance drops by one, while for isolated broken couplers, the distance drops by one in one or both bases. By providing a viable dropout strategy, we have removed a critical roadblock to the implementation of hexagonal qubit grids in hardware for large-scale quantum error correction.