Promatch: Extending the Reach of Real-Time Quantum Error Correction with Adaptive Predecoding

Fault-tolerant quantum computing relies on Quantum Error Correction (QEC), which encodes logical qubits into data and parity qubits. Error decoding is the process of translating the measured parity bits into types and locations of errors. To prevent a backlog of errors, error decoding must be performed in real-time (i.e., within 1𝜇𝑠 on superconducting machines). Minimum Weight Perfect Matching (MWPM) is an accurate decoding algorithm for surface code, and recent research has demonstrated real-time implementations of MWPM (RT-MWPM) for a distance of up to 9. Unfortunately, beyond d=9, the number of flipped parity bits in the syndrome, referred to as the Hamming weight of the syndrome, exceeds the capabilities of existing RT-MWPM decoders. In this work, our goal is to enable larger distance RT-MWPM decoders by using adaptive predecoding that converts high Hamming weight syndromes into low Hamming weight syndromes, which are accurately decoded by the RT-MWPM decoder.

An effective predecoder must balance both accuracy (as any erroneous decoding by the predecoder contributes to the overall Logical Error Rate, termed as LER) and coverage (as the predecoder must ensure that the hamming weight of the syndrome is within the capability of the final decoder). In this paper, we propose Promatch, a real-time adaptive predecoder that predecodes both simple and complex patterns using a locality-aware, greedy approach. Our approach ensures two crucial factors: 1) high accuracy in prematching flipped bits, ensuring that the decoding accuracy is not hampered by the predecoder, and 2) enough coverage adjusted based on the main decoder's capability given the time constraints. Promatch represents the first real-time decoding framework capable of decoding surface codes of distances 11 and 13, achieving an LER of 2.6 × 10 -14 for distance 13. Moreover, we demonstrate that running Promatch concurrently with the recently proposed Astrea-G achieves LER equivalent to MWPM LER, 3.4 × 10 -15 , for distance 13, representing the first real-time accurate decoder for up-to a distance of 13.