Shulou(Shulou.com)11/24 Report--

Thanks to CTOnews.com netizen xiaocluoyuzi for the clue delivery! CTOnews.com, January 30, according to the release of the University of Science and Technology of China, du Jiangfeng, Shi Development and others of the key Laboratory of Micro Magnetic Resonance of the Chinese Academy of Science and Technology have made important progress in the field of quantum manipulation, realizing a quantum CNOT gate with a fidelity of 99.92% based on diamond nitrogen-vacancy (Nitrogen-Vacancy,NV) color center qubits. The research results are published under the title "99.92%-Fidelity CNOT Gates in Solids by Noise Filtering" in Physical Review Letters [Phys. Rev. Lett.130, 030601 (2023)].

Figure: the nitrogen-vacancy color center of diamond and the nuclear spin diagram around it. The shape pulse is used to resist the noise caused by nuclear spin. CTOnews.com learned that high-fidelity two-bit quantum gates play an important role in quantum information processing, especially in fault-tolerant quantum computing. However, qubits will inevitably interact with the environment, which greatly reduces the fidelity of logic gates, especially for solid-state quantum systems. After decades of efforts, quantum systems such as superconductivity, ion traps, solid-state defects and quantum dots have achieved two-bit gates whose fidelity exceeds the fault-tolerant threshold (about 99%). However, practical large-scale quantum computing requires a gate fidelity of at least 99.9%. Previously, only the ion trap system has achieved a two-bit gate with a fidelity of about 99.9%. Solid-state systems are disturbed by noisier solid-state environments, making it a daunting challenge to achieve a fidelity of more than 99.9%.

Theoretically, through the quantum error correction (QEC) process, as long as we implement the quantum gate whose error is lower than the fault-tolerant threshold in the physical bit, we can obtain the quantum gate with smaller error in the logical bit. Similarly, by dynamic error correction (DEC), gate errors on physical bits can cancel each other out between multiple control pulses if the non-Markov performance of ambient noise is fully utilized. Therefore, by combining the above two levels of error correction, the initial door fidelity requirements can be greatly reduced, which provides a feasible path for general quantum computing based on imperfect devices in reality. However, due to the extra complexity of the quantum error correction process itself, it is often difficult to obtain higher fidelity quantum gates in experiments. When the dynamic error correction method is applied to the two-bit sub-gate, it will encounter similar problems, often because it does not accurately grasp the noise characteristics, the use of more complex pulses and longer control time will cause bigger errors.

In this paper, through the detailed measurement of noise, an accurate and complete noise model is established, including static noise, time-dependent noise and quantum noise. Based on the idea of dynamic error correction, the researchers designed the shape pulse to resist all kinds of magnetic noise in the noise model, and finally reduced the influence of magnetic noise on the CNOT gate by two orders of magnitude to less than 10-4. Experimentally, the fidelity of the CNOT gate realized by the shape pulse is 99.920 (7)% measured by the RandomizedBenchmarking method, and the remaining errors are mainly due to the distortion of the shape pulse and the longitudinal relaxation of the electron spin. Both of these can be further eliminated technically, so it is expected to further increase the fidelity of CNOT doors to more than 99.99% in the future. The method of this work is universal and can be further extended to other solid-state systems, such as silicon quantum dots, diamond and other defects in silicon carbide, rare earth doping systems and so on.

Figure: (a) the noise resistance effect of CNOT gates realized by different pulses; (b) the measurement results of CNOT gate fidelity. Xie Tianyu, a postdoctoral student, and Zhao Zhiyuan, a doctoral student in the key Laboratory of Micro Magnetic Resonance, Chinese Academy of Sciences, are the first authors of this paper, and Academician du Jiangfeng and Professor Shi are the co-authors of the paper. The research was supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences and Anhui Province.

Links to papers:

Https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.030601

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