The team of Academician Guo Guangcan of our university has made important progress in the research of silicon-based semiconductor quantum chips. In collaboration with Jianjun Zhang of the Institute of Physics of the Chinese Academy of Sciences, Xuedong Hu of the State University of New York at Buffalo, and Benyuan Quantum Computing Co., LTD., the team, Prof. Guopping Guo and Haiou Li, achieved the efficiency regulation of spin-orbit coupling strength in silicon-based germanium hole quantum dots. It provides important guidance for the system to realize spin orbit switch and improve the quality of spin qubits. The research results are titled "Gate-Tunable Spin-Orbit Coupling in a Germanium Hole Double Quantum Dot." It was published online on April 27 in the internationally renowned journal of Applied physics, Physical Review Applied.

Silicon-based spin qubits have attracted a lot of attention because of their long quantum decoherence time and high control fidelity, and are a strong contender for realizing quantum computers in the future. In addition, its compatibility with modern semiconductor processes makes it possible to expand it on a large scale. High control fidelity requires the bit to have a sufficiently fast control rate while having a long quantum decoherence time. The traditional bit manipulation mode, electron spin resonance, has a slow flip rate due to the limitation of heating effect. When there is strong spin-orbit coupling in the system, both theoretical and experimental studies show that spin bit flipping can be achieved by using electric dipole spin resonance, and the flipping rate is proportional to the spin-orbit coupling strength, which can greatly improve the bit manipulation rate. Therefore, the study of the spin-orbit coupling effect in the system can provide an important physical basis for realizing the high-fidelity manipulation of spin qubits.

In recent years, a series of systematic experiments have been carried out by Li Haiou and Guo Guoping, aiming at the strong spin-orbit coupling interaction characteristic of one-dimensional germanium nanowires. By measuring the anisotropy of the leakage current in the spin blocking interval in the double quantum dots, the Landau G-factor tensor and the direction of the spin-orbit coupling field in the hole quantum dots of silicon-based germanium nanowires have been measured and regulated for the first time [Nano Letters 21, 3835-3842 (2021)]. On this basis, in 2022, the research group used electric dipole spin resonance to achieve the fastest spin qubit manipulation in the world, with a flipping rate up to 540MHz[Nature Communications 13, 206 (2022)].

In order to further study the spin-orbit coupling mechanism in silicon-based germanium nanowire hole system and realize the adjustable height, the research group systematically measured the relationship between the leakage current in the spin-blocking interval with the magnitude of the external magnetic field and the detuning of the quantum dot energy level. Through theoretical modeling and numerical analysis, the spin-orbit strength in the system was obtained. By adjusting the grid voltage and changing the coupling strength between the two quantum dots, the coupling strength of spin orbit in the system can be controlled on a large scale. At the same time, the researchers pointed out that in the recently realized one-dimensional germanium nanowire system with a new pattern controlled growth, due to the Dresselhaus spin-orbit coupling caused by interface asymmetry and the direct Rashba spin-orbit coupling which can be adjusted by efficiency, we can adjust the spin coupling strength in the system and change the growth direction of the nanowires. It is possible to find a position in the momentum space where the spin-orbit coupling is completely off, or it is possible to use the spin-orbit switch to find a sweet spot that allows the bit to maintain a long quantum decoherence time while achieving the super-fast control rate of the bit. This discovery provides an important research basis for realizing high-fidelity bit manipulation and improving the quality of spin qubits.

Figure 1. (a) the variation of spin-orbit coupling length (a representation of spin-orbit coupling strength) with gate voltage VC; (b) In momentum space, spin-orbit fields caused by different mechanisms are represented by arrows of different colors: Blue, Direct Rashba spin orbit Field (BR), green, Dresselhaus Spin Orbit Field (BD), red, Total Spin Orbit Field (Btotal). When the amplitudes of BR and BD are equal and opposite, the total spin-orbit coupling field at the red star is zero, and spin-orbit coupling is turned off.

Liu He, Zhang Ting and Wang Ke, PhD students at the Key Laboratory of Quantum Information of Chinese Academy of Sciences, are the co-first authors of the paper, and Li Haiou, special professor at the Key Laboratory of Quantum Information of Chinese Academy of Sciences, and Guo Guoping, professor, are the co-corresponding authors of the paper. The work was funded by the Ministry of Science and Technology, the National Foundation Commission, the Chinese Academy of Sciences and Anhui Province. Professor Li Haiou is supported by the Zhongying Young Scholars Program of the University of Science and Technology of China.

Paper link：https://doi.org/10.1103/PhysRevApplied.17.044052

(Key Laboratory of Quantum Information, Academy of Quantum Information and Quantum Technology Innovation, Scientific Research Department, Chinese Academy of Sciences)