Researchers used a scanning gate microscope to find a new, microscopic source of decoherence in superconducting quantum circuits. They show that tiny metallic grains in thin-film devices can act like isolated islands where electrons tunnel in and out in a way that is driven by the device’s microwave field. This Coulomb-blockade behavior produces dissipation and slow charge noise that can reduce device performance as much as the better-known two-level system (TLS) defects, but it works by a different physical mechanism.

The team put a conducting probe tip above live superconducting resonators at millikelvin temperatures and continuously measured the microwave transmission while moving the tip and changing its voltage. As the tip scanned, the device response showed clear concentric rings and a series of equally spaced peaks in tip voltage. The authors observed more than 20 peaks when tip voltages allowed, random jumps of the island’s offset charge on minute time scales, and a response that would not “saturate” when they increased the resonator drive power. These signatures match the behaviour expected for a small, tunnel-coupled metallic island subject to Coulomb blockade.

At a high level, Coulomb blockade means that adding one electron to a very small metal island costs a noticeable amount of energy. The probe tip changes the island’s electric potential, letting the researchers cross the energy thresholds for adding charge. When the island is also coupled to the resonator’s microwave field, microwave-driven tunnelling between discrete charge states produces dissipation and shifts in the resonator frequency. The authors model the system as an island with discrete charge states tunnel-coupled to a continuum, and they relate the effect to known impurity models and to a form of “Sisyphus” dissipation where the drive repeatedly pushes and relaxes the system.