Coulomb blockade in microscopic material defects as a source of decoherence and noise in solid-state quantum circuits

Solid-state quantum devices, particularly superconducting circuits, are limited by material-related decoherence. While two-level system (TLS) defects have long been considered the primary culprit, many decoherence sources remain poorly understood. This paper investigates the microscopic origins of these losses by using scanning gate microscopy (SGM) to probe live superconducting resonators at millikelvin temperatures.

The researchers used an SGM tip to locally tune the electrostatic environment of superconducting resonators. By sweeping the tip voltage while monitoring the microwave transmission, they identified periodic, concentric ring patterns in the device's dissipation and frequency response. They modeled these observations using an Anderson-Holstein-type impurity model, which describes charge tunneling in microscopic metallic islands (grains) driven by the device's microwave field. They further validated this model by comparing experimental data with theoretical simulations of Sisyphus dissipation.

The study reveals that microscopic metallic grains, which are ubiquitous in thin-film devices, act as "Sisyphus defects." These grains undergo microwave-driven charge tunneling, leading to energy dissipation and frequency shifts in the resonator. Unlike TLS defects, which can often be saturated by increasing microwave power, these Sisyphus defects remain active and continue to cause decoherence even at higher power levels. The researchers demonstrate that these defects are as common and as detrimental to device performance as TLS defects, and they suggest that eliminating these metallic grains during fabrication is a viable path toward improving coherence in quantum circuits.

This work challenges the prevailing paradigm that coherence lifetimes are primarily limited by TLS defects. By identifying a specific, physically distinct mechanism for decoherence, the authors provide a clear target for materials science improvements. Because these defects are linked to structural imperfections like metallic grains, optimizing thin-film deposition and surface treatment processes could lead to significant gains in the performance of superconducting quantum devices.