Do micromagnets expose spin qubits to charge and Johnson noise?

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 T-2* for Si and GaAs. The effect of the micromagnet on the Si qubits is comparable in size to that of spin-orbit coupling at an applied field of B = 1 T, whilst dephasing in GaAs is expected to be dominated by spin-orbit coupling. Tailoring the magnetic field gradient can efficiently reduce T-2* in Si. In contrast, the Johnson noise generated by a micromagnet will only be important for highly coherent spin qubits. (C) 2015 AIP Publishing LLC.