IT House news on January 17, according to the University of Science and Technology of China released that the team of academician Guo Guangcan of our university has made important progress in the measurement and inspection of quantum gates. The team's research team of Li Chuanfeng and Xiang Guoyong cooperated with Zhu Huangjun of Fudan University and Shang Jiangwei of Beijing Institute of Technology to achieve the optimal quantum gate test based on local operation for the first time. The research results were published online on January 14, 2022 in the internationally renowned journal Physical Review Letters, and were selected as "Editor's Recommended" articles.
Quantum gates are the basic units for building quantum computers, and achieving high-fidelity quantum gate operations is a necessary condition for fault-tolerant quantum computing. How to test whether the fidelity of the actually prepared quantum gate meets the requirements is the first problem to be solved to achieve fault-tolerant quantum computing. Since the number of parameters in the quantum gate increases exponentially with the size of the quantum system, the number of measurements and the amount of computation of the traditional quantum process chromatography scheme increase exponentially with the increase of the scale of the quantum gate. For the mass characterization of large-scale quantum gates and quantum circuits in the future, the traditional quantum process chromatography method is no longer practically operable. The new theoretical method recently developed internationally, the quantum gate test, brings the dawn to solve the above problems, which can achieve optimal sample complexity for most quantum gates and only use local operations. However, this method is difficult to apply in practice, and it is not robust to experimental errors and quantum gate imperfections.
Xiang Guoyong's group and collaborators combined the ideas of quantum gate testing with the multi-parameter quantum precision measurement platform developed by the group in recent years, [Nat.Commun.9, 1 (2018); PRL124,060502 (2020); PRL125,020501 (2020);P RL 126,070503 (2021)], improves the data processing algorithm for quantum gate tests, making quantum gate tests more robust to errors while retaining high efficiency. The improved quantum gate test estimates the upper bound of the distortion of the quantum gate by making multiple local projection measurements of∈ the quantum gate output. The experimental results show that the estimation of the ∈ and the number of samples n are closely inversely proportional, that is, the estimation accuracy of 1 / n. This means that the work achieves an optimal quantum gate test with an optimal sample complexity of 1/∈, and more importantly, the sample complexity required by the method does not increase with the increase in the size of the quantum gate.
Figure 1 Schematic diagram of the optimal quantum gate test scheme
Using the optimal quantum gate test, the research team tested the two-bit CNOT gate and the three-bit Toffoli gate respectively. On average, it takes only 1600 and 2600 measurements to verify fidelity of 99% and 97% or more, using only 20 and 32 measurement bases. For comparison, chromatography of quantum processes to characterize their distortion requires 324 and 4096 measurement bases, as well as millions of measurements. Therefore, quantum gate testing is of great significance for the verification of large-scale quantum gates and quantum circuits.
Figure 2 Test results for the CNOT gate.
(a) Significance level (b) Upper bound of distortion
Zhang Ruiqi, a graduate student of the Key Laboratory of Quantum Information of the Chinese Academy of Sciences, and Hou Zhibo, an associate researcher, are the co-first authors of the work. The work was supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences, the Ministry of Education and Anhui Province.
Thesis Link:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.020502