Local Predecoder to Reduce the Bandwidth and Latency of Quantum Error Correction

A fault-tolerant quantum computer will be supported by a classical decoding system interfacing withquantum hardware to perform quantum error correction. It is important that the decoder can keep pacewith the quantum clock speed, within the limitations on communication that are imposed by the physicalarchitecture. To this end, we propose a local "predecoder," which makes greedy corrections to reducethe amount of syndrome data sent to a standard matching decoder. We study these classical overheadsfor the surface code under a phenomenological phase-flip noise model with imperfect measurements. Wefind substantial improvements in the run time of the global decoder and the communication bandwidth byusing the predecoder. For instance, to achieve a logical-failure probability off=10-15using qubits withphysical error ratep=10-3and a distanced=22 code, we find that the bandwidth cost is reduced by afactor of 1000 and that the time taken by a matching decoder is sped up by a factor of 200. To achieve thistarget failure probability, the predecoding approach requires a 50% increase in the qubit count compared with the optimal decoder