Real-time adaptive quantum error correction by model-free multi-agent learning

Abstract: Can we build efficient Quantum Error Correction (QEC) that adapts on the fly to time-varying noise? In this work we say yes, and show how. We present a two level framework based on Reinforcement Learning (RL) that learns to correct even non-stationary errors from scratch. At the first level we take advantage of model-free Multi-Agent RL (MARL) to automatically discover full QEC cycle -- logical state encoding, stabilizer measurements, and recovery -- without any prior system knowledge, relying only on orthogonality conditions. Leveraging the stabilizer formalism, we demonstrate that our MARL framework can discover novel QEC codes tailored for multi-level quantum architectures. At the second level we introduce BRAVE (Bandit Retraining for Adaptive Variational Error correction), an efficient algorithm that tunes the variational layer on the fly to change the physical basis of the errors, adapting the QEC code to time-varying noise while minimizing computational overhead and reducing the number of retraining steps. By combining our MARL and BRAVE approaches and testing them on multi-level systems subjected to competing bit- and phase-flip errors over time across diverse scenarios, we observed an improvement in logical fidelity by more than an order of magnitude -- under time-dependent noise channels -- compared to conventional QEC schemes.