Q3DE: A fault-tolerant quantum computer architecture for multi-bit burst errors by cosmic rays

TL;DR

The paper tackles the fragility of fault-tolerant quantum computing to multi-bit burst errors caused by cosmic rays. It introduces Q3DE, an architecture that combines in-situ anomaly detection from syndrome statistics, temporal code deformation to dynamically enlarge code distance, and decoder re-execution with rollback and non-uniform weighted matching. Numerical evaluations show Q3DE can reduce MBBE exposure by about $10^3$ in time and shrink burst regions, while enabling up to roughly a tenfold reduction in required qubits and potential throughput gains relative to naive baselines, at moderate hardware overhead (e.g., ~40% LUT usage for decoding). This approach is versatile across device families and highlights a scalable path to FTQC by tolerating temporal and spatial error variations without demanding drastic changes to qubit designs.

Abstract

Demonstrating small error rates by integrating quantum error correction (QEC) into an architecture of quantum computing is the next milestone towards scalable fault-tolerant quantum computing (FTQC). Encoding logical qubits with superconducting qubits and surface codes is considered a promising candidate for FTQC architectures. In this paper, we propose an FTQC architecture, which we call Q3DE, that enhances the tolerance to multi-bit burst errors (MBBEs) by cosmic rays with moderate changes and overhead. There are three core components in Q3DE: in-situ anomaly DEtection, dynamic code DEformation, and optimized error DEcoding. In this architecture, MBBEs are detected only from syndrome values for error correction. The effect of MBBEs is immediately mitigated by dynamically increasing the encoding level of logical qubits and re-estimating probable recovery operation with the rollback of the decoding process. We investigate the performance and overhead of the Q3DE architecture with quantum-error simulators and demonstrate that Q3DE effectively reduces the period of MBBEs by 1000 times and halves the size of their region. Therefore, Q3DE significantly relaxes the requirement of qubit density and qubit chip size to realize FTQC. Our scheme is versatile for mitigating MBBEs, i.e., temporal variations of error properties, on a wide range of physical devices and FTQC architectures since it relies only on the standard features of topological stabilizer codes.

Figures (10)

• Figure 1: Design of our Q3DE architecture. Components in the blue region are classical computing units and those in the pink region are quantum computing units. The yellow region is their interface. Our proposal is to add components and interactions inside the red dotted square. We also modified the procedures of the units colored in orange.
• Figure 2: Schematic diagram of $d=4$ surface codes and the processing flow of syndrome values. Surface codes are allocated on the qubit plane. Z/X-syndrome values are extracted and stored in each buffer as 3D lattices in each surface code. Once a cosmic ray strikes the qubit plane, physical error rates of qubits around the hit position become high.
• Figure 3: Logical error rates with and without an MBBE as a function of physical error rates. Dotted and solid lines indicate those with and without an MBBE, respectively.
• Figure 4: Overview of Q3DE. The timeline of interactions between the qubit plane and the syndrome lattice is shown.
• Figure 5: Three steps for op_expand instruction. These figures show a procedure to expand the code distance of a logical qubit from $d$ to $d_{\rm exp}$ and shrink it from $d_{\rm exp}$ to $d$.