A 2026 scanning-gate-microscopy study of live superconducting microwave resonators identified a previously unrecognized decoherence mechanism: 'Sisyphus defects', microscopic metallic grains in which Coulomb blockade and microwave-driven single-charge tunnelling dissipate device energy with a photon-number-independent signature that conventional characterization would misattribute to quasiparticles, vortex, or radiation loss. The study, performed on NbN-on-sapphire and Nb-on-Si devices, left the decisive questions open: the exact chemical composition and formation mechanism of the grains are not uniquely determined (candidates include Nb grains in an amorphous oxide matrix, hydride precipitates, dopant clusters, and processing contaminants); the prevalence of the defects across other materials and fabrication processes is unknown; their impact on qubit architectures such as transmons and fluxoniums, whose charge-noise engineering does not protect against the microwave-driven mechanism, is unexplored; and the proposed mitigations (deposition optimization, epitaxial growth, encapsulation) are unvalidated against this loss channel. The problem is to close this loop: identify the microscopic origin of Sisyphus defects at the sites where they cause loss, quantify their prevalence and their impact on qubit architectures, and demonstrate a fabrication route that eliminates or suppresses them. No independent work has yet addressed these open questions, so the problem remains open.

Background

Superconducting quantum circuits — qubits such as the transmon (a superconducting island shunted by a large capacitor and a Josephson junction, i.e. a tunnel junction between two superconductors) and coplanar-waveguide microwave resonators — lose coherence through coupling to microscopic defects in the thin films, surfaces, and interfaces from which they are made. The prevailing paradigm attributes most of this loss to two-level systems (TLS), defects with two nearly degenerate quantum states that can resonate with the device's electric field; remaining budgets are assigned to nonequilibrium quasiparticles (unpaired electrons above the superconducting gap, e.g. from stray infrared radiation), moving magnetic vortices, and radiation loss. Because a TLS saturates at high microwave drive power while the other mechanisms do not, power-dependence measurements are the standard tool for separating them — but they cannot distinguish mechanisms that share the same power dependence.

Scanning gate microscopy (SGM) provides a way to localize individual defects in operando: a conducting atomic-force-microscope tip in a dilution refrigerator is used as a movable electrostatic gate above a live superconducting device while its microwave transmission S21S_{21} is monitored, so a defect's response appears as contours in S21S_{21} as the tip position and voltage are varied. This platform was previously used to image individual TLS defects in live superconducting circuits (Hegedüs et al. 2025).

Banerjee et al. (arXiv:2607.15252, 2026) used SGM on three superconducting hanger resonators (microwave resonators coupled in transmission to a feedline) — one NbN film on sapphire and two Nb films on silicon, fabricated in two different cleanrooms — and found defects whose signature is a periodic sequence of peaks in the resonator loss rate κ\kappa as the tip voltage is swept, with single-electron (1e1e) periodicity. This is the hallmark of Coulomb blockade in a microscopic metallic island ('electron box'): a grain so small that its charging energy EC=e2/2CΣE_C = e^2/2C_\Sigma exceeds both the thermal energy and the relevant microwave photon-scale excitation, so its charge state is quantized and tunnelling across the barrier between grain and film occurs only near charge-degeneracy points. Under the device's own microwave field, the driven island repeatedly climbs its charge-parabola energy landscape and relaxes by tunnelling a single charge, dissipating energy on every cycle — the same mechanism as the 'Sisyphus resistance' measured in engineered, microwave-driven single-electron boxes (Persson et al. 2010). The authors call these objects Sisyphus defects and model them with an Anderson–Holstein-type impurity Hamiltonian (a quantum-impurity model of a localized electronic level coupled to the conduction electrons of the surrounding film) treated in the Orthodox theory of single-electron tunnelling (the standard master-equation treatment of incoherent sequential charge tunnelling on a Coulomb-blockaded island). They report per-defect loss rates up to about 5 kHz — enough that a handful of defects can limit coherence to the millisecond scale — and argue the defects are as common and as debilitating as TLS in the devices studied. The idea that stray metallic grains trouble charge-sensitive quantum devices is older: Kafanov et al. (2008) proposed them as a source of charge noise in single-electron transistors and charge qubits. Crucially, the Sisyphus loss is photon-number independent, so conventional power-dependent characterization would misattribute it to quasiparticle loss, vortex movement, or radiation loss — meaning its share of the loss budget in existing devices is presently unknown.

What the 2026 work establishes about the defects' identity is indirect: capacitances extracted from the Coulomb-blockade fits correspond to island sizes of 10–30 nm; X-ray reflectometry and grazing-incidence diffraction on the Nb-on-Si films give an average crystallite size of about 21 nm and a roughly 7 nm amorphous surface oxide; scanning electron microscopy with energy-dispersive spectroscopy shows Nb and Nb oxide with granularity near lithographic edges. The defects survive thermal cycling to room temperature. But the composition of the specific grains that act as Sisyphus defects is not determined: candidates include Nb grains held in an insulating amorphous oxide matrix, niobium hydride precipitates, dopant clusters, and processing-induced contaminants. Two further limitations are stated explicitly: the tip-positioning hysteresis (about 1 μm) restricts localization precision, and the Fourier-based detection technique resolves only defects that couple strongly enough and fall within the Nyquist limit, so the population of weakly coupled defects — plausibly the bulk of the coherence budget — is uncharacterized.

The stakes for qubits are specific. The transmon and the fluxonium (a related qubit in which the junction is shunted by a large superconducting inductance instead of a capacitor; Koch et al. 2007; Manucharyan et al. 2009) were deliberately engineered to be insensitive to static offset-charge noise, which is what made them robust against the classical charge noise associated with metallic grains. The Sisyphus mechanism, however, is driven by the device's own microwave field, and the 2026 paper argues that such charge-noise engineering does not protect against it. The paper proposes elimination of the grains during fabrication — via, e.g., optimization of thin-film deposition conditions, epitaxial (single-crystal) growth, or encapsulation — as the mitigation route, citing related but distinct results on Nb film structure and resonator loss (Drimmer et al. 2025) and on coherence improvements from niobium surface encapsulation (Bal et al. 2024). None of these strategies has been tested against the Sisyphus loss channel itself.

Problem Statement

Establish the materials origin of Sisyphus defects — the Coulomb-blockaded metallic grains identified as a microwave-driven, photon-number-independent decoherence source in superconducting circuits — and demonstrate their elimination or suppression in superconducting quantum devices. The components of this objective are: (1) determine the chemical composition and structure of the grains at defect sites located in operando, distinguishing among the candidate origins (Nb metallic grains in an insulating amorphous oxide matrix, niobium hydride precipitates, dopant clusters, and processing-induced contaminants), and identify the formation mechanism in the fabrication process; (2) establish the prevalence of these defects across superconducting thin-film materials and fabrication processes beyond the NbN-on-sapphire and Nb-on-Si devices studied, with stated detection limits that bound the weakly coupled defect population invisible to current Fourier-based detection; (3) quantify the impact of Sisyphus defects on superconducting qubit architectures such as transmons and fluxoniums, whose offset-charge-noise engineering does not protect against the microwave-driven mechanism, and charge qubits; and (4) validate experimentally that a fabrication strategy (e.g., thin-film deposition optimization, epitaxial growth, or encapsulation) eliminates or suppresses the Sisyphus defects and the associated loss. The scope is superconducting quantum circuits — qubits and the microwave resonators from which they are built; the source paper notes only generically that the same mechanism has implications for other cryogenic solid-state devices fabricated from metallic thin films and operated by microwaves, and probing that wider device class is a natural extension of this problem rather than a completion requirement. These deliverables share one scientific objective — closing the loop from the observed decoherence mechanism to its microscopic materials origin and its engineered suppression — and one experimental platform, operando defect localization tied to ex-situ microstructural characterization. The problem is answered when the identity and formation mechanism of the defect are demonstrated at the sites where it causes loss, its prevalence (with stated detection limits that bound the weakly coupled defect population) and qubit-level impact are quantified, and a fabrication route is shown to suppress the Sisyphus loss channel.

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