Fault-tolerant quantum computation using large spin cat-codes

TL;DR

This paper introduces a fault-tolerant quantum error correction protocol using large spin qudits with spin-cat codes, demonstrating improved thresholds and resource efficiency for quantum computing, especially in neutral-atom systems.

Contribution

The authors develop a novel spin-cat code-based fault-tolerant protocol tailored to dominant errors in large spin systems, including a universal gate set and measurement-free error correction schemes.

Findings

Fault-tolerance threshold exceeds standard qubit encodings.

Universal gate set includes a rank-preserving CNOT gate.

Implementation feasible with neutral-atom quantum computing.

Abstract

We construct a fault-tolerant quantum error-correcting protocol based on a qubit encoded in a large spin qudit using a spin-cat code, analogous to the continuous variable cat encoding. With this, we can correct the dominant error sources, namely processes that can be expressed as error operators that are linear or quadratic in the components of angular momentum. Such codes tailored to dominant error sources {can} exhibit superior thresholds and lower resource overheads when compared to those designed for unstructured noise models. To preserve the dominant errors during gate operations, we identify a suitable universal gate set. A key component is the CNOT gate that preserves the rank of spherical tensor operators. Categorizing the dominant errors as phase and amplitude errors, we demonstrate how phase errors, analogous to phase-flip errors for qubits, can be effectively corrected. Furthermore, we propose a measurement-free error correction scheme to address amplitude errors without relying on syndrome measurements. Through an in-depth analysis of logical CNOT gate errors, we establish that the fault-tolerant threshold for error correction in the spin-cat encoding surpasses that of standard qubit-based encodings. We consider a specific implementation based on neutral-atom quantum computing, with qudits encoded in the nuclear spin of $^{87}$Sr, and show how to generate the universal gate set, including the rank-preserving CNOT gate, using quantum control and the Rydberg blockade. These findings pave the way for encoding a qubit in a large spin with the potential to achieve fault tolerance, high threshold, and reduced resource overhead in quantum information processing.