Spin-orbit coupling and operation of multi-valley spin qubits

TL;DR

This paper demonstrates high-fidelity control of one- and three-electron spin qubits in silicon quantum dots, revealing a linear, opposite dependence of g-factors on electric fields due to spin-valley coupling, with implications for qubit tuning.

Contribution

It introduces a theory explaining the opposite electric field dependence of g-factors in multi-valley silicon spin qubits based on spin-valley coupling and inter-valley spin-flip tunneling.

Findings

High-fidelity qubit operation achieved via pulsed ESR.

Opposite linear g-factor dependence on electric field for one- and three-electron qubits.

Inter-valley spin-flip tunneling influences spin-orbit coupling and g-factors.

Abstract

Spin qubits composed of either one or three electrons are realized in a quantum dot formed at a Si/SiO_2-interface in isotopically enriched silicon. Using pulsed electron spin resonance, we perform coherent control of both types of qubits, addressing them via an electric field dependent g-factor. We perform randomized benchmarking and find that both qubits can be operated with high fidelity. Surprisingly, we find that the g-factors of the one-electron and three-electron qubits have an approximately linear but opposite dependence as a function of the applied dc electric field. We develop a theory to explain this g-factor behavior based on the spin-valley coupling that results from the sharp interface. The outer "shell" electron in the three-electron qubit exists in the higher of the two available conduction-band valley states, in contrast with the one-electron case, where the electron is in the lower valley. We formulate a modified effective mass theory and propose that inter-valley spin-flip tunneling dominates over intra-valley spin-flips in this system, leading to a direct correlation between the spin-orbit coupling parameters and the g-factors in the two valleys. In addition to offering all-electrical tuning for single-qubit gates, the g-factor physics revealed here for one-electron and three-electron qubits offers potential opportunities for new qubit control approaches.