Procedure for verifying the quantum device [Image: Guo Guangcan’s team]

The development of quantum technology is currently one of the most popular frontiers of advanced science, and is considered as a significant indicator of a country’s sci-tech level. On a structural basis, a quantum computer consists of multiple quantum gates. Fault-tolerant quantum computation requires high-fidelity operation of prepared quantum gates, and prioritizes developing a reliable and efficient way to examine their fidelity.

Recently, a research group led by Academician Guo Guangcan from the University of Science and Technology of China (USTC) made great progress in developing techniques of measuring and examining quantum gates. Their work was published in Physical Review Letters.

Due to the exponential growth of measurements and computation, the traditional quantum state tomography method no longer remains as practicable and the future of quantum technology lies in gates and routes of large scale. A new theoretical method known as “quantum gate testing” has lately been suggested, but lacks robustness in limiting the defects of quantum gates or experimental error.

The research group combined the idea of quantum gate testing with a multi-parameter quantum precision measurement platform developed by the group in recent years, which improved the algorithm applied in quantum gate testing and increased its robustness in error prevention while retaining high efficiency.

Through multiple local projection measurements of the output of the quantum gate, the improved testing method achieved optimal sample complexity (1/∈), as was proven by experimental results. More importantly, the sample complexity required by this method does not increase in the quantum gates’ scale.

Using the newly developed quantum gate testing method, the research group then tested the 2-bits CNOT gate (Controlled-not gate) and the 3-bits Toffoli gate (Controlled-controlled-not gate), and obtained an average of 1,600 and 2,600 measurement times to determine whether fidelity had surpassed 99 percent and 97 percent, using 20 and 32 measuring bases. In contrast, the traditional method needs 324 and 4,096 bases and millions of times of measurement to make that determination.

The work contributes greatly to the development of advanced quantum computers.

For more information, please contact:

Dr. & Professor Li Chuanfeng

E-mail: cfli@ustc.edu.cn

University of Science and Technology of China (USTC),

Chinese Academy of Sciences

Source: University of Science and Technology of China (USTC),

Chinese Academy of Sciences