The realization of quantum error correction is an essential ingredient for reaching the full potential of fault-tolerant universal quantum computation. Using a range of different schemes, logical qubits that are resistant to errors can be redundantly encoded in a set of error-prone physical qubits. One such scalable approach is based on the surface code. Here we experimentally implement its smallest viable instance, capable of repeatedly detecting any single error using seven superconducting qubits—four data qubits and three ancilla qubits. Using high-fidelity ancilla-based stabilizer measurements, we initialize the cardinal states of the encoded logical qubit with an average logical fidelity of 96.1%. We then repeatedly check for errors using the stabilizer readout and observe that the logical quantum state is preserved with a lifetime and a coherence time longer than those of any of the constituent qubits when no errors are detected. Our demonstration of error detection with its resulting enhancement of the conditioned logical qubit coherence times is an important step, indicating a promising route towards the realization of quantum error correction in the surface code.

量子纠错的实现是实现容错通用量子计算的全部潜力的基本要素。使用一系列不同的方案，可以在一组容易出错的物理量子位中冗余编码抗错误的逻辑量子位。一种这样的可扩展方法是基于表面代码。在这里，我们实验性地实现了它的最小可行实例，该实例能够使用七个超导量子位（四个数据量子位和三个辅助量子位）重复检测任何单个错误。使用基于保真度的基于ancilla的稳定器测量，我们以96.1％的平均逻辑保真度初始化编码逻辑qubit的基态。然后，我们使用稳定器读数反复检查错误，并观察到逻辑量子态的寿命和相干时间比未检测到任何组成量子位的时间长，并且相干时间更长。我们对错误检测的演示及其对条件逻辑qubit相干时间的增强是一个重要的步骤，这表明在表面代码中实现量子错误校正的一种有希望的途径。