The University of Science and Technology of China realizes the efficient regulation of spin-orbit coupling strength in silicon-based quantum chips

The team of academician Guo Guangcan of our university has made important progress in the research of silicon-based semiconductor quantum chips. The team, professor Guo Guoping, Professor Li Haiou and others, together with Zhang Jianjun, researcher of the Institute of Physics of the Chinese Academy of Sciences, Professor Hu Xuedong of the State University of New York at Buffalo, and Yuanyuan Quantum Computing Co., Ltd., have realized the efficient regulation of spin orbit coupling strength in silicon-based germanium hole quantum dots, which provides important guiding significance for the system to achieve spin orbital switching and improve the quality of spin qubits. The research results, titled “Gate-Tunable Spin-Orbit Coupling in a Germanium Hole Double Quantum Dot”, were published online on April 27 in the leading international journal of applied physics, Physical Review Applied.

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

Li Haiou and Guo Guoping have carried out a series of systematic experimental studies in recent years in view of the strong spin-orbit coupling interaction of one-dimensional germanium nanowires. By measuring the anisotropy of the leakage current in the spin blocking interval in the double quantum dots, the measurement and regulation of the Rondau g-factor tensor and the direction of the coupled field direction of the spin orbit were realized for the first time in the hole quantum dots of the silicon-based germanium nanowires [Nano Letters 21, 3835-3842 (2021)]。 On this basis, in 2022, the research group used electric dipole spin resonance to achieve the fastest rate of spin qubit manipulation in the world, and the flip rate can reach 540MHz[Nature Communications 13, 206 (2022)]。

In order to further study the spin orbit coupling mechanism in the silicon-based germanium nanowire hole system and achieve a high degree of tunableness, 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 amount of detuning of the energy level of the quantum dots, and obtained the spin orbital strength in the system through theoretical modeling and numerical analysis. By adjusting the gate voltage and changing the coupling strength between the two quantum dots, a large range of spin orbit coupling strength in the system is realized. At the same time, the researchers pointed out that in the recently realized new graphically controlled growth of one-dimensional germanium nanowire system, because of its Overlaphaus spin orbit coupling caused by interface asymmetry and the direct Rashba spin orbit coupling that can be efficiently adjusted, we can adjust the spin coupling strength in the system and change the growth direction of the nanowires, which can find a position completely closed in the momentum space. The spin orbit switch can also be used to find the best control point (sweet spot) to maintain a long quantum decoherence time for the bit while achieving an ultra-fast control rate of the bit. This discovery provides an important research basis for achieving high-fidelity manipulation of bits and improving the quality of spin qubits.

Figure 1. (a) The spin orbital coupling length (a representation of the strength of the spin orbit coupling) varies with the gate voltage VC, (b) in the momentum space, the spin orbital field caused by different mechanisms is represented by different colored arrows: blue, direct Rashba spin orbital field (BR), green, Dresselhaus spin orbital field (BD), red, total spin orbital field (Btotal). When the amplitudes of BR and BD are equal and in opposite directions, the total spin orbit coupling field at the red star is zero, and the spin orbit coupling is turned off.

Liu He, Zhang Ting, and Wang Ke, postdoctoral fellows of the Key Laboratory of Quantum Information of the Chinese Academy of Sciences, are the co-first authors of the paper, and Professor Li Haiou and Professor Guo Guoping of the Key Laboratory of Quantum Information of the Chinese Academy of Sciences are the co-corresponding authors of the paper. The work was funded by the Ministry of Science and Technology, the State Foundation Of China, the Chinese Academy of Sciences and Anhui Province. Professor Hai’ou Li was funded by the Zhongying Young Scholars Program of the University of Science and Technology of China.

Thesis Link:https://doi.org/10.1103/PhysRevApplied.17.044052

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