[论文解读] Interface induced spin-orbit interaction in silicon quantum dots and prospects for scalability

Rifat Ferdous, K. W. Chan+7 more|arXiv (Cornell University)|Mar 10, 2017

Quantum and electron transport phenomena参考文献 29被引用 58

一句话总结

本文 identifies interface monoatomic steps in Si/SiO2 and Si/SiGe heterostructures as the dominant source of variability in spin qubit dephasing times (T₂*) in silicon quantum dots. Using atomistic tight-binding simulations and experiments, it demonstrates that these steps induce spatially varying spin-orbit coupling, leading to inconsistent g-factors and Stark shifts. The key contribution is a method to suppress this variability and enhance T₂* by at least an order of magnitude through optimal alignment of the external magnetic field along specific crystallographic directions (e.g., [110]), leveraging the anisotropic nature of spin-orbit interaction in Si.

ABSTRACT

We identify the presence of monoatomic steps at the Si/SiGe or Si/SiO$_2$ interface as a dominant source of variations in the dephasing time of Si quantum dot (QD) spin qubits. First, using atomistc tight-binding calculations we show that the g-factors and their Stark shifts undergo variations due to these steps. We compare our theoretical predictions with experiments on QDs at a Si/SiO$_2$ interface, in which we observe significant differences in Stark shifts between QDs in two different samples. We also experimentally observe variations in the $g$-factors of one-electron and three-electron spin qubits realized in three neighboring QDs on the same sample, at a level consistent with our calculations. The dephasing times of these qubits also vary, most likely due to their varying sensitivity to charge noise, resulting from different interface conditions. More importantly, from our calculations we show that by employing the anisotropic nature of the spin-orbit interaction (SOI) in a Si QD, we can minimize and control these variations. Ultimately, we predict that the dephasing times of the Si QD spin qubits will be anisotropic and can be improved by at least an order of magnitude, by aligning the external DC magnetic field towards specific crystal directions.

研究动机与目标

• To identify and understand the microscopic origin of dephasing time (T₂*) variability in Si quantum dot spin qubits.
• To investigate how atomic-scale interface roughness—specifically monoatomic steps—affects spin-orbit coupling and g-factor variations.
• To establish a link between experimental observations of g-factor and T₂* variability and atomistic interface structure.
• To propose a scalable solution to minimize qubit variability by exploiting the anisotropic spin-orbit interaction in silicon.
• To demonstrate that optimal magnetic field orientation can enhance T₂* and reduce sensitivity to interface disorder.

提出的方法

• Employing atomistic sp3d5s* tight-binding simulations to model electron wavefunctions and spin-orbit coupling in Si quantum dots with interface steps.
• Using an effective mass model to derive analytical expressions for g-factor shifts due to Rashba and Dresselhaus spin-orbit coupling, particularly δg± ≈ 2|e|⟨z⟩/μBℏ(−α± + β± sin 2φ).
• Comparing theoretical predictions of g-factor and Stark shift variations with experimental data from gate-defined Si/SiO2 quantum dots.
• Measuring Ramsey oscillations to extract T₂* times for one- and three-electron spin qubits across multiple quantum dots on the same sample.
• Analyzing the dependence of g-factors and their electric field tunability on the relative position (x₀) of the quantum dot with respect to interface steps.
• Systematically varying the external magnetic field direction (e.g., [100] vs. [110]) to probe the anisotropy of spin-orbit coupling and its impact on dephasing.

实验结果

研究问题

• RQ1 What is the dominant microscopic origin of variability in dephasing times (T₂*) among Si quantum dot spin qubits?
• RQ2 How do monoatomic steps at Si/SiO2 or Si/SiGe interfaces affect the effective spin-orbit coupling and g-factor of electron states in quantum dots?
• RQ3 To what extent does the anisotropic nature of spin-orbit interaction in silicon allow for control over qubit dephasing through magnetic field orientation?
• RQ4 Can experimental observations of g-factor and T₂* variations across neighboring quantum dots be explained by interface step-induced variations in the Dresselhaus coefficient?
• RQ5 What magnetic field direction maximizes T₂* and minimizes sensitivity to interface disorder in Si quantum dots?

主要发现

• Monoatomic steps at Si/SiO2 and Si/SiGe interfaces cause sign reversal of the effective Dresselhaus spin-orbit coefficient (β), leading to spatially varying g-factors and Stark shifts.
• Experimental measurements show significant variations in g-factors and T₂* across one-electron and three-electron spin qubits in three neighboring quantum dots on the same sample, consistent with theoretical predictions.
• The Stark shift of the g-factor varies in both magnitude and sign depending on the quantum dot's position relative to an interface step, with the effect being strongest when the external magnetic field is aligned along the [110] crystal direction.
• The dephasing time T₂* is anisotropic and can be improved by at least an order of magnitude by aligning the external magnetic field along the [110] direction, due to the anisotropic spin-orbit interaction.
• When the magnetic field is oriented along [100], the g-factor variations due to interface steps are negligible, but this orientation does not suppress dephasing as effectively as [110].
• Theoretical modeling confirms that the dominant contribution to g-factor renormalization comes from the Dresselhaus term (β), which changes sign across interface steps, while the Rashba term (α) remains relatively unchanged.