Do micromagnets expose spin qubits to charge and Johnson noise?

TL;DR

This paper investigates how micromagnets used in quantum dot spin qubits introduce charge and Johnson noise, affecting qubit coherence times, with implications for material choice and device design.

Contribution

It derives an effective Hamiltonian to quantify micromagnet-induced noise effects on spin qubits and estimates their impact on dephasing times in Si and GaAs.

Findings

Micromagnet effects on Si qubits are comparable to spin-orbit coupling at 1T.

Dephasing in GaAs is mainly due to spin-orbit coupling.

Magnetic field gradient tailoring can reduce T2* in Si.

Abstract

An ideal quantum dot spin qubit architecture requires a local magnetic field for one-qubit rotations. Such an inhomogeneous magnetic field, which could be implemented via a micromagnet, couples the qubit subspace with background charge fluctuations causing dephasing of spin qubits. In addition, a micromagnet generates magnetic field evanescent-wave Johnson noise. We derive an effective Hamiltonian for the combined effect of a slanting magnetic field and charge noise on a single-spin qubit and estimate the free induction decay dephasing times T2* for Si and GaAs. The effect of the micromagnet on Si qubits is comparable in size to that of spin-orbit coupling at an applied field of B=1T, whilst dephasing in GaAs is expected to be dominated by spin-orbit coupling. Tailoring the magnetic field gradient can efficiently reduce T2* in Si. In contrast, the Johnson noise generated by a micromagnet will only be important for highly coherent spin qubits.