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硅基半导体量子点自旋量子比特因具有相干时间长, 可控性好, 以及与现代先进集成电路制造工艺相兼容等特点, 成为有望实现容错量子计算的潜在候选体系之一, 受到科学界的广泛关注. 近年来, 由于在硅基材料性质, 量子点制造工艺和结构以及量子比特操控技术等方面取得的显著进步, 硅基半导体量子计算在自旋量子比特的高保真度态初始化和读取、单比特逻辑门和两比特逻辑门保真度等方面取得了重要研究进展, 实现了单比特以及两比特逻辑门保真度超过99%的重要突破. 本文将简要介绍硅基半导体量子点的基本概念, 着重讨论在提高单比特以及两比特门操控保真度过程中采用的最新技术手段, 最后简要讨论了需要重点关注的研究方向.
Spin qubits in silicon-based semiconductor quantum dots have become one of the prominent candidates for realizing fault-tolerant quantum computing due to their long coherence time, good controllability, and compatibility with modern advanced integrated circuit manufacturing processes. In recent years, due to the remarkable advances in silicon-based materials, the structure of quantum dot and its fabrication process, and qubit manipulation technology, the great progress of high-fidelity state preparation and readout, single- and two-qubit gates of spin qubits for silicon based semiconductor quantum computation has been achieved. The control fidelities for single- and two-qubit gates all exceed 99%—fault tolerance threshold required by the surface code known for its exceptionally high tolerance to errors. In this paper, we briefly introduce the basic concepts of silicon-based semiconductor quantum dots, discuss the state-of-art technologies used to improve the fidelities of single- and two-qubit gates, and finally highlight the research directions that should be focused on. This paper is organized as follows. Firstly, we introduce three major types of quantum dot (QD) devices fabricated on different silicon-based substrates, including Si/SiGe heterojunction and Si/SiO 2. The spin degree of electron or nucleus hosted in QD can be encoded into spin qubits. Electron spin qubits can be thermally initialized to ground state by using an electron reservoirs, and can be read out by spin-charge conversion mechanism: energy-selective readout (Elzerman readout) with reservoirs or Pauli spin blockade (PSB) without reservoirs. Additionally, high fidelity single-shot readout has been demonstrated by using radio-frequency gate reflectometry combined with the PSB, which has unique advantages in large-scale qubit array. To coherently control the spin qubits, electron dipole resonance (ESR) or electron dipole spin resonance (EDSR) for electron and nuclear magnetic resonance (NMR) for nucleus are introduced. With the help of isotope purification greatly improving the dephasing time of qubit and fast single-qubit manipulation based on EDSR, fidelity above 99.9% can be reached. For the two-qubit gates based on exchange interaction between electron spins, the strength of interaction $ J $ combined with Zeeman energy difference$ \Delta {E}_{z} $ determines the energy levels of system, which lead to the different two-qubit gates, such as controlled-Z (CZ), controlled-Rotation (CROT), and the square root of the SWAP gate ($ \sqrt{\rm{S}\rm{W}\rm{A}\rm{P}} $ ) gates. In order to improve the fidelity of two-qubit gates, a series of key technologies is used experimentally, they being isotope purification, symmetry operation, careful Hamiltonian engineering, and gate set tomography. Fidelity of two-qubit gates exceeding 99% has been demonstrated for electron spin qubits in Si/SiGe quantum dots and nuclear spin qubits in donors. These advances have pushed the silicon-based spin qubit platform to become a major stepping stone towards fault-tolerant quantum computation. Finally, we discuss the future study of spin qubits, that is, how to effectively expand the number of qubits, and many other problems to be explored and solved.-
Keywords:
- quantum computing/
- silicon-based semiconductor quantum dot/
- spin qubit/
- logic gate/
- fidelity
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