Decoherence is a fundamental phenomenon through which classical behavior emerges from the quantum laws of nature. It is the main obstacle when scientists harness these laws for information processing. In practical terms for quantum computing, decoherence is caused by external factors such as stray radiation that can cause a quantum computer’s qubits to change their quantum states and lose their stored information. Quantum error correction (QEC) repairs the effects of decoherence on quantum information. However, in essentially all past experimental attempts to implement QEC, the speed of the error correction was slower than the effect of decoherence. This meant a quantum system lost information faster than QEC could keep up. Breakeven is the point where the added complexity of the correction circuit just barely makes up for the induced decoherence.
In this experiment, researchers enhanced the lifetime of quantum information past breakeven by more than twofold. This establishes that there is no fundamental obstacle to significantly extending the lifetime of quantum information through active intervention. This confirms scientists’ expectations based on theory. It also opens the pathway towards quantum information processing in the presence of noise from radiation, cosmic rays, and other sources. Noise is an unavoidable nuisance for all real-world quantum systems. Looking forward, one of the next challenges for this platform is to realize high-fidelity logical operations between two error-corrected qubits.
This experiment realized the grid code in an electromagnetic mode residing in a superconducting cavity. The quantum state of this mode is controlled by an auxiliary superconducting circuit called the transmon. Scientists cooled this experimental system inside a dilution refrigerator to a temperature 100 times colder than the cosmic background of outer space. An external controller orchestrated the quantum error correction process with a latency of only a few hundred nanoseconds. A reinforcement learning agent optimized the process to counteract imperfections of the experimental setup and the controller.
This work was conducted at Yale University and funded in part by the Co-design Center for Quantum Advantage (C2QA), a national quantum information science research center led by Brookhaven National Laboratory.
Research Scientist, Google Quantum AI
(The research reported here was conducted when Sivak was at Yale University)
Frederick W. Beinecke Professor of Applied Physics, Yale University
This research was supported by the U.S. Army Research Office and by the Department of Energy Office of Science, National Quantum Information Science Research Center, Co-design Center for Quantum Advantage (C2QA). The use of fabrication facilities was supported by the Yale Institute for Nanoscience and Quantum Engineering and the Yale School of Engineering & Applied Science Cleanroom.
Sivak, V.V., et al., Real-time quantum error correction beyond break-even. Nature 616, 50–55 (2023). [DOI: 10.1038/s41586-023-05782-6]