Magnetic flux noise in superconducting qubits and the gap states continuum

Abstract

In the present study we investigate the selected local aspects of the metal-induced gap states (MIGSs) at the disordered metal–insulator interface, that were previously proposed to produce magnetic moments responsible for the magnetic flux noise in some of the superconducting qubit modalities. Our analysis attempts to supplement the available studies and provide new theoretical contribution toward their validation. In particular, we explicitly discuss the behavior of the MIGSs in the momentum space as a function of the onsite energy deviation, that mimics random potential disorder at the interface in the local approximation. It is found, that when the difference between the characteristic electronic potentials in the insulator increases, the corresponding MIGSs become more localized. This effect is associated with the increasing degree of the potential disorder that was earlier observed to produce highly localized MIGSs in the superconducting qubits. At the same time, the presented findings show that the disorder-induced localization of the MIGSs can be related directly to the decay characteristics of these states as well as to the bulk electronic properties of the insulator. As a result, our study reinforces plausibility of the previous corresponding investigations on the origin of the flux noise, but also allows to draw future directions toward their better verification.

Measuring magnetic 1/f noise in superconducting microstructures and the fluctuation-dissipation theorem

“…In MMCs, 1/f noise scales with the amount of magnetic moments in the sensor material, suggesting that it originates from the paramagnetic, erbium-doped gold or silver sensor [14]. For SQUIDs, flux, and phase qubits, different models consider magnetic flux noise from the stochastic hopping of electrons [15], paramagnetic dangling bonds [16], strongly interacting surface spins [17,18], metal-induced gap states [19,20], or adsorbed O 2 molecules [21,22]. Both the experimental verification of theories and the reduction of noise in specific set-ups can be difficult, since devices usually suffer from a combination of different noise sources, which are hard to disentangle and therefore hard to eliminate.…”

Robust Strong-Coupling Architecture in Circuit Quantum Electrodynamics

“…Nevertheless, the architecture of a coupling junction at the constricted central line of a coplanar waveguide (CPW) is a practical choice to achieve ultrastrong coupling, while isolating the qubit from magnetic flux noise. References [19,20] report on the dependence of magnetic flux noise magnitude over the superconducting loop area. Furthermore, local effects in the superconducting material and substrate can produce magnetic flux noise [21][22][23][24].…”