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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.
This finding matters because the implicated defects — microscopic metallic grains that can form near oxidized patterned edges in standard thin-film processing — appear to be common and harmful. The paper reports fitted device examples with resonator frequencies around 5–7 GHz and island tunnel resistances and capacitances consistent with tiny grains (for example, a fit using Rt ≈ 600 kΩ and Cj ≈ 0.1 fF for one defect). Crucially, conventional loss-characterization methods could misidentify this microwave-driven Coulomb-blockade loss as other, power-independent processes or as TLS loss. That means some efforts to improve coherence that focus only on TLS may miss this separate and widespread mechanism.
There are important caveats. The observations come from scanning-gate measurements on several resonators and are strongest when the tiny island sits inside the microwave mode volume; not all devices will have such defects in the right place. The defects show stochastic offset-charge jumps on minute time scales and survive thermal cycling to room temperature in the experiments reported, which suggests they are stable but also unpredictable in time. The authors note that modern materials-science tools can identify and help eliminate metallic grains, and that doing so provides a practical route to reduce this source of noise and improve coherence, but the work implies that fabrication control will be required to remove the problem.