Tunability Of Spin-qubit Systems In Silicon-based Quantum Dots

In recent years,the development of quantum computing has been in full swing,and the corresponding theoretical and technological progress has been rapid.After a long period of exploration,several quantum systems stand out,such as superconducting system,semiconductor quantum dot system,and Ion trap system are viewed as reliable platforms for quantum computing.Take semiconductor quantum dot system as an example,demonstrative experiments based on this system have shown promising results,which greatly inspires confidence in quantum computing.Despite the rapid progress,the realization of universal quantum computing is still not a short-term goal.There are still some obstacles between the reality and the goal,and there are still many topics remained to be studied,e.g.,tunability of a qubit system.The tunability of qubits reflects the ability of the external environment to control the quantum system.Some important qubit gates rely on the tunability of the system.In addition,the tunability of the system is also important in quantum simulation.As a kind of "artificial atom",semiconductor quantum dots have excellent tunability.In practice,we can effectively tune the energy level of the quantum dot system by adjusting the external field such as gate voltage and magnetic field,offering an excellent test-bed for varieties of phenomena and quantum simulation schemes.This thesis will introduce my research work on the tunability of semiconductor quantum dot systems during my Ph.D.study,and discuss the influence of these results in corresponding community.The main contents of the full text include the following points:1.Introduction of the background of semiconductor quantum dots,briefly reviewing the development process of semiconductor quantum computation,elaborating the advantages of silicon-based spin qubit system and the significance of studying its tunability;2.The processing process of semiconductor quantum dots,the measurement and control facilities,and the basic characterization process of quantum dots are briefly introduced.On this basis,the commonly used control and readout scheme of the spin qubit is introduced;Brief introduction of the theoretical models and approximation methods,and brief introduction of the spin-orbit coupling in the crystal lattice and the valley splitting in the silicon lattice;3.Studies on the influence of external magnetic field on silicon-based spin qubits,including the influence of magnetic orientation on relaxation time,resonant frequency,and manipulation speed.Quantitative analysis of experimental data and discussion on the underlying physical picture is presented.4.Effective Hamiltonian of the driven multilevel system and its applications.Introduction of the driving-induced tunability and the consequent characteristics of the driven system’s spectrum.Numerical demonstration of dark state and proposal of coherent population trapping based on longitudinal driving.5.Studies on the dark state in a double-quantum-dot system,studies on the oddeven effect in its spectrum,and revealing the driving-induced tunability experimentally.The originality of this thesis can be summarized as follows1.Studying the influence of external magnetic field to spin qubits,and demonstrating the existence of the phase angle of spin-orbit coupling;2.Exploring a procedure to obtain the effective Hamiltonian of a periodically driven multilevel system.Compared to the previous works,our approach is more accurate and intuitive;3.Studying the tunability of a driven multilevel system induced by longitudinal driving.Several counterparts of multilevel physics phenomena in quantum optics are investigated,which could be the indicator in the future experimental demonstration.In the context of quantum computing,we discuss the potential application of the effective model,and propose a procedure of quantum state manipulation based on adiabatic passage.4.A preliminary examination of our theoretical results.We study the driven double-quantum-dot system based on nanowire,and effective couplings tuned by microwave field are observed as expected,which is the cornerstone for further relevant study such as microwave manipulation of qubits,interference in multilevel systems,coherent state transfer,and et al.