Sun Kai, Hao Ze-Yan, Wang Yan, Li Jia-Kun, Xu Xiao-Ye, Xu Jin-Shi, Han Yong-Jian, Li Chuan-Feng, Guo Guang-Can
CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026, China.
CAS Center for Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China.
Light Sci Appl. 2022 Jul 5;11(1):203. doi: 10.1038/s41377-022-00891-9.
A major challenge in practical quantum computation is the ineludible errors caused by the interaction of quantum systems with their environment. Fault-tolerant schemes, in which logical qubits are encoded by several physical qubits, enable to the output of a higher probability of correct logical qubits under the presence of errors. However, strict requirements to encode qubits and operators render the implementation of a full fault-tolerant computation challenging even for the achievable noisy intermediate-scale quantum technology. Especially the threshold for fault-tolerant computation still lacks experimental verification. Here, based on an all-optical setup, we experimentally demonstrate the existence of the threshold for the fault-tolerant protocol. Four physical qubits are represented as the spatial modes of two entangled photons, which are used to encode two logical qubits. The experimental results clearly show that when the error rate is below the threshold, the probability of correct output in the circuit, formed with fault-tolerant gates, is higher than that in the corresponding non-encoded circuit. In contrast, when the error rate is above the threshold, no advantage is observed in the fault-tolerant implementation. The developed high-accuracy optical system may provide a reliable platform to investigate error propagation in more complex circuits with fault-tolerant gates.
实际量子计算中的一个主要挑战是量子系统与其环境相互作用所导致的不可避免的误差。容错方案通过多个物理量子比特对逻辑量子比特进行编码，在存在误差的情况下能够输出更高概率的正确逻辑量子比特。然而，对量子比特和算子编码的严格要求使得即使对于可实现的有噪声的中等规模量子技术，实现完全容错计算也具有挑战性。特别是容错计算的阈值仍缺乏实验验证。在此，基于全光装置，我们通过实验证明了容错协议阈值的存在。四个物理量子比特由两个纠缠光子的空间模式表示，用于编码两个逻辑量子比特。实验结果清楚地表明，当错误率低于阈值时，由容错门构成的电路中正确输出的概率高于相应的未编码电路。相反，当错误率高于阈值时，在容错实现中未观察到优势。所开发的高精度光学系统可为研究具有容错门的更复杂电路中的误差传播提供可靠平台。
