Magnetic noise from Kondo charge traps (Presentation Recording)

Abstract

Magnetic noise impacts a wide variety of solid-state devices, from quantum bits in superconductor and semiconductor-based quantum computer architectures to spintronic devices made of metals and semiconductors. Developing a theory of magnetic noise will have great impact in minimizing fluctuations in these devices. Magnetic noise is commonly detected as flux noise in superconducting quantum interference devices (SQUIDs). At low frequencies, SQUID flux noise spectral density decreases with frequency as $1/f^{\alpha}$ with $\alpha=0.5-0.8$ in a wide variety of devices. However, at higher frequencies (above ~1~GHZ) flux noise was found to be Ohmic, i.e. increasing linearly with frequency. This puzzling behavior is not explained by any model of magnetic fluctuations. Here we present a theory for the magnetic noise produced by local charge traps, elucidating the kind of noise that the majority of defects produce in a typical solid-state device. Our numerical renormalization group calculations reveal a deviation from 1/f behavior in the magnetic noise of charge traps in the Kondo regime over a wide range of frequencies. Remarkably, such behavior is not present in the charge noise, which is dominated by single-particle processes, consistent with a mean-field picture. The results show that, when Kondo correlations are present, magnetic noise originating from charge traps has a many-particle character, while charge noise does not. Since Kondo temperatures can be relatively high in charge traps, these findings indicate that electron-electron interactions can lead to a strong contribution to the magnetic noise that has not been captured by current models.