Constant-Overhead Magic State Injection into qLDPC Codes with Error Independence Guarantees

TL;DR

This paper presents a scalable, low-overhead method for injecting magic states into qLDPC codes, ensuring error independence and supporting fault-tolerant universal quantum computation.

Contribution

It introduces a generic, parallelized code surgery technique for magic state injection with proven error independence and constant overhead, applicable to various qLDPC codes.

Findings

Supports injection into Θ(k) logical qubits with constant qubit overhead

Achieves injection with time complexity of ~O(d^2)

Numerical simulations validate error independence and method effectiveness

Abstract

Magic states are essential yet resource-intensive components for realizing universal fault-tolerant quantum computation. Preparing magic states within emerging quantum low-density parity-check (qLDPC) codes poses additional challenges, due to the complex encoding structures. Here, we introduce a generic and scalable method for magic state injection into arbitrarily selected logical qubits encoded using qLDPC codes. Our approach, based on parallelized code surgery, supports the injection from either physical qubits or low-distance logical qubits. For qLDPC code families with asymptotically constant encoding rates, the method achieves injection into $\Theta(k)$ logical qubits -- where $k$ denotes the logical qubit number of the code -- with only constant qubit overhead and a time complexity of $\tilde{O}(d^2)$, where $d$ is the code distance. A central contribution of this work is a rigorous proof that errors affecting the injected magic states remain independent throughout the procedure. This independence ensures the resilience of logical qubits against interactions with noisy ancillae and preserves the presumption of subsequent magic state distillation protocols. We further support our theoretical results with numerical validation through circuit-level simulations. These findings advance the feasibility of scalable, fault-tolerant universal quantum computing using qLDPC codes, offering a pathway to significantly reduced qubit resource requirements in magic state injection.