近日，美国IBM量子公司的Dmitri Maslov及其研究团队取得一项新进展。经过不懈努力，他们成功开发出高阈值和低开销容错量子存储器。相关研究成果已于2024年3月27日在国际权威学术期刊《自然》上发表。

该研究团队提出了一种端到端量子纠错协议，该协议基于一组低密度奇偶校验码实现了容错存储器。该研究方法为标准电路噪声模型实现了0.7%的误差阈值，与20年来在误差阈值方面领先的表面码相当。在该协议中，长度为n的代码的校验子测量周期需要n个辅助量子比特以及一个深度为8的电路，涉及CNOT门、量子比特初始化和测量等步骤。此外，所需的量子比特连通性由两个边不相交的平面子图组成的6级图来确保。

特别值得一提的是，研究人员发现，在假设物理误差率为0.1%的情况下，仅使用288个物理量子比特，就能实现12个逻辑量子比特保存近100万个校验子周期，这一性能与表面码相比具有显著优势，后者需要近3000个物理量子比特才能达到相同的性能。这项研究的发现为近期量子处理器提供了低开销容错量子存储器的演示。

据悉，物理误差的积累阻碍了当前量子计算机大规模算法的执行。量子纠错为解决量子计算中的误差问题提供了有效方案。通过将k个逻辑量子比特编码到n个物理量子比特上，量子纠错能显著抑制物理误差，使计算得以在可接受的保真度下运行。只要物理误差速率低于某一特定阈值，量子纠错即可实现。这一阈值的大小则取决于量子码的类型、校验子测量电路的设计和译码算法的选择。

附：英文原文

Title: High-threshold and low-overhead fault-tolerant quantum memory

Author: Bravyi, Sergey, Cross, Andrew W., Gambetta, Jay M., Maslov, Dmitri, Rall, Patrick, Yoder, Theodore J.

Issue&Volume: 2024-03-27

Abstract: The accumulation of physical errors prevents the execution of large-scale algorithms in current quantum computers. Quantum error correction promises a solution by encoding k logical qubits onto a larger number n of physical qubits, such that the physical errors are suppressed enough to allow running a desired computation with tolerable fidelity. Quantum error correction becomes practically realizable once the physical error rate is below a threshold value that depends on the choice of quantum code, syndrome measurement circuit and decoding algorithm. We present an end-to-end quantum error correction protocol that implements fault-tolerant memory on the basis of a family of low-density parity-check codes. Our approach achieves an error threshold of 0.7% for the standard circuit-based noise model, on par with the surface code that for 20years was the leading code in terms of error threshold. The syndrome measurement cycle for a length-n code in our family requires n ancillary qubits and a depth-8 circuit with CNOT gates, qubit initializations and measurements. The required qubit connectivity is a degree-6 graph composed of two edge-disjoint planar subgraphs. In particular, we show that 12 logical qubits can be preserved for nearly 1 million syndrome cycles using 288 physical qubits in total, assuming the physical error rate of 0.1%, whereas the surface code would require nearly 3,000 physical qubits to achieve said performance. Our findings bring demonstrations of a low-overhead fault-tolerant quantum memory within the reach of near-term quantum processors.

DOI: 10.1038/s41586-024-07107-7

Source: https://www.nature.com/articles/s41586-024-07107-7