Realizing repeated quantum error correction in a distance-three surface code

Quantum computers hold the promise of solving computational problems that are intractable using conventional methods(1). For fault-tolerant operation, quantum computers must correct errors occurring owing to unavoidable decoherence and limited control accuracy(2). Here we demonstrate quantum error correction using the surface code, which is known for its exceptionally high tolerance to errors(3-6). Using 17 physical qubits in a superconducting circuit, we encode quantum information in a distance-three logical qubit, building on recent distance-two error-detection experiments(7-9). In an error-correction cycle taking only 1.1 mu s, we demonstrate the preservation of four cardinal states of the logical qubit. Repeatedly executing the cycle, we measure and decode both bit-flip and phase-flip error syndromes using a minimum-weight perfect-matching algorithm in an error-model-free approach and apply corrections in post-processing. We find a low logical error probability of 3% per cycle when rejecting experimental runs in which leakage is detected. The measured characteristics of our device agree well with a numerical model. Our demonstration of repeated, fast and high-performance quantum error-correction cycles, together with recent advances in ion traps(10), support our understanding that fault-tolerant quantum computation will be practically realizable.