Theory of flux noise in the presence of an external magnetic field.*

Impurity spins randomly distributed at the surfaces and interfaces of superconducting wires are known to cause flux noise in Superconducting Quantum Interference Devices (SQUIDs), providing a dominant mechanism for decoherence in flux-tunable superconducting qubits. Recently, we developed a second principles method that is able to compute flux noise from the microscopic Hamiltonian of interacting spins [1]. The method provides explicit numerical predictions for the flux noise power law observed in experiments, including its temperature dependence.

Here we present calculations of flux noise in the presence of an external magnetic field. We show that the results are strongly dependent on the angle between the external field B and the flux vector defining the flux produced by an impurity spin at a certain location in the device. Spins at locations where the flux vector and the B field are parallel have their flux noise suppressed, while spins at locations where the flux vector is perpendicular to B have low frequency flux noise transferred to a resonance peak at the Zeeman frequency.

The peak’s amplitude and width are quite sensitive to spin-spin exchange interaction and spin-lattice energy relaxation. Thus, measurements of flux noise under an external B field provide additional information on the proper microscopic model required to explain the origin of flux noise in superconducting devices.

[1] J. A. Nava Aquino and R. de Sousa, Phys. Rev. B 106, 144506 (2022).