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arXiv:2408.16660 · Science Advances

In situ scanning gate imaging of individual quantum two-level system defects in live superconducting circuits

M. Hegedüs, Riju Banerjee, A. Hutcheson, Tomas Barker, Sumedh Mahashabde, Andrey V. Danilov, Sergey Kubatkin, Vladimir N Antonov, Sebastian E. de Graaf

Published Apr 30, 2025• 8 citations
arXivHTMLPDF

Abstract

The low-temperature physics of structurally amorphous materials is governed by low-energy two-level system (TLS) defects. Being impervious to most traditional condensed matter probes, the exact origin and nature of TLS remain elusive. Recent advances toward realizing stable high-coherence quantum computing platforms have increased the importance of studying TLS in solid-state quantum circuits, as they are a persistent source of decoherence and instability. Here, performing scanning gate microscopy on a live superconducting NbN resonator at millikelvin temperatures, we locate individual TLS, directly revealing their microscopic nature. Mapping and visualizing the most detrimental TLS in the bath pinpoints the dominant sources of ubiquitous 1/ f dielectric noise and energy relaxation. We also deduce the three-dimensional orientation of individual TLS electric dipole moments. Combining these insights with structural information of the underlying materials can help unravel the detailed microscopic nature and chemical origin of TLS, directing targeted strategies for their eventual mitigation.

DOI
10.1126/sciadv.adt8586

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The low-temperature physics of structurally amorphous materials is governed by low-energy two-level system (TLS) defects. Being impervious to most traditional condensed matter probes, the exact origin and nature of TLS remain elusive. Recent advances toward realizing stable high-coherence quantum computing platforms have increased the importance of studying TLS in solid-state quantum circuits, as they are a persistent source of decoherence and instability. Here, performing scanning gate microscopy on a live superconducting NbN resonator at millikelvin temperatures, we locate individual TLS, directly revealing their microscopic nature. Mapping and visualizing the most detrimental TLS in the bath pinpoints the dominant sources of ubiquitous 1/ f dielectric noise and energy relaxation. We also deduce the three-dimensional orientation of individual TLS electric dipole moments. Combining these insights with structural information of the underlying materials can help unravel the detailed microscopic nature and chemical origin of TLS, directing targeted strategies for their eventual mitigation.
A critical limitation of solid-state quantum devices arises from the materials from which they are fabricated: uncontrolled surfaces, interfaces, and structural imperfections introduce numerous sources of loss and decoherence. Despite extensive efforts, linking these decoherence mechanisms to their microscopic material origins -- essential for developing effective mitigation strategies -- remains an outstanding challenge that has slowed coherence improvements. Here, using scanning gate microscopy on live superconducting circuits we identify a previously unrecognised decoherence mechanism originating from Coulomb blockade and microwave-driven charge tunnelling in metallic grains. Such grains are ubiquitous in thin-film devices fabricated by standard lithography processes. By characterising multiple defects across different devices, we find such defects to be as common and as debilitating to device performance as two-level system (TLS) defects, while originating from a fundamentally different physical mechanism. Importantly, conventional characterisation techniques would misattribute this loss to other, microwave power-independent processes. Our observations thus reveal a widespread source of decoherence in superconducting circuits, challenging the prevailing paradigm that coherence lifetimes are primarily limited by TLS defects. Eliminating metallic grains during fabrication provides a clear and practical route to suppress this mechanism, offering a pathway towards improved coherence and reduced noise in microwave-based solid-state quantum devices.
Understanding the nature and dynamics of material defects in superconducting circuits is of paramount importance for improving qubit coherence and parameter stability and much needed for implementing large-scale quantum computing. Here we present measurements on individual highly coherent environmental two-level systems (TLS). We trace the spectral diffusion of specific TLS and demonstrate that it originates from the TLS coupling to a small number of low energy incoherent fluctuators. From the analysis of these fluctuations, we access the relevant parameters of low energy fluctuators: dipole moments, switching energies, and, more importantly, interaction energies. Our approach opens up the possibility of deducing the macroscopic observables in amorphous glassy media from direct measurements of local fluctuator dynamics at the microscopic level---a route towards substantiating commonly accepted, but so far phenomenological, models for the decohering environment.
Two-level systems (TLSs) are the dominant source of decoherence in superconducting qubits, a leading platform for quantum computing. While TLSs are believed to arise from atomic-scale tunneling defects, their precise structural origins, particularly at complex and disordered interfaces in real devices, have remained elusive. Conventional structural characterization is extremely challenging because these interfaces are non-periodic, containing steps and roughness, and device-relevant TLS defects involve only atomic motions with small energy differences. We have determined the atomic character of candidate TLSs at the Al-Si interface of a superconducting qubit using aberration-corrected scanning transmission electron microscopy (STEM) combined with experiment-constrained 3D structure modeling and computational defect searching. High-angle annular dark-field (HAADF) STEM images were obtained from a disordered interface containing a prominent step defect and localized strain. Structure modeling started with 3D models created in the Ingrained framework to match the experimental projections, accounting for mistilt and uniform strain. These models were then refined using FANTASTX (Fully Automated Nanoscale To Atomistic Structure from Theory and eXperiments), a multi-objective optimizer that simultaneously minimizes disagreement with the experimental image via structural similarity and the ab initio (DFT) total system energy. Transition-state searches with the activation relaxation technique nouveau (ARTn) on the refined models identified many atomic configurations occupying double-well potentials corresponding to TLS. Statistical analysis of over 1500 TLSs shows that candidate TLS with tunneling energies and asymmetries in the range relevant to superconducting qubits (∼5 GHz) predominantly involve Al atoms located near the interface. These atoms exhibit smaller median tunneling displacements (∼0.44 Å) than the non-candidate TLS and reside in locally disordered environments with marginally longer and less dense first-neighbor shells than those of non-candidate TLSs. The most probable candidate TLSs feature cooperative motion of one or two Al atoms, and sometimes with Si, in directions largely parallel to the interface. This work provides the first direct atomic-scale insight into the structural defects that may be responsible for decoherence at a realistic, device-relevant interface. It also demonstrates a scalable, generalizable workflow for solving the atomic structure of complex, disordered interfaces by integrating aberration-corrected STEM with advanced computational refinement and defect searching. The methodology is directly applicable to other materials systems and heterointerfaces critical to quantum devices [5].
Noise within solid-state systems at low temperatures can typically be traced back to material defects. In amorphous materials, these defects are broadly described by the tunneling two-level systems (TLSs) model. TLS have recently taken on further relevance in quantum computing because they dominate the coherence limit of superconducting quantum circuits. Efforts to mitigate TLS impacts have thus far focused on circuit design, material selection, and surface treatments. Our work takes an approach that directly modifies TLS properties. This is achieved by creating an acoustic bandgap that suppresses all microwave-frequency phonons around the operating frequency of a transmon qubit. For embedded TLS strongly coupled to the transmon qubit, we measure a pronounced increase in relaxation time by two orders of magnitude, with the longestT1time exceeding 5 milliseconds. Our work opens avenues for studying the physics of highly coherent TLS and methods for mitigating noise within solid-state quantum devices.
Recent progress with microfabricated quantum devices has revealed that an ubiquitous source of noise originates in tunneling material defects that give rise to a sparse bath of parasitic two-level systems (TLSs). For superconducting qubits, TLSs residing on electrode surfaces and in tunnel junctions account for a major part of decoherence and thus pose a serious roadblock to the realization of solid-state quantum processors. Here, we utilize a superconducting qubit to explore the quantum state evolution of coherently operated TLSs in order to shed new light on their individual properties and environmental interactions. We identify a frequency-dependence of TLS energy relaxation rates that can be explained by a coupling to phononic modes rather than by anticipated mutual TLS interactions. Most investigated TLSs are found to be free of pure dephasing at their energy degeneracy points, around which their Ramsey and spin-echo dephasing rates scale linearly and quadratically with asymmetry energy, respectively. We provide an explanation based on the standard tunneling model, and identify interaction with incoherent low-frequency (thermal) TLSs as the major mechanism of the pure dephasing in coherent high-frequency TLS.
Abstract The anomalous low-temperature properties of glasses arise from intrinsic excitable entities, so-called tunneling Two-Level-Systems (TLS), whose microscopic nature has been baffling solid-state physicists for decades. TLS have become particularly important for micro-fabricated quantum devices such as superconducting qubits, where they are a major source of decoherence. Here, we present a method to characterize individual TLS in virtually arbitrary materials deposited as thin films. The material is used as the dielectric in a capacitor that shunts the Josephson junction of a superconducting qubit. In such a hybrid quantum system the qubit serves as an interface to detect and control individual TLS. We demonstrate spectroscopic measurements of TLS resonances, evaluate their coupling to applied strain and DC-electric fields, and find evidence of strong interaction between coherent TLS in the sample material. Our approach opens avenues for quantum material spectroscopy to investigate the structure of tunneling defects and to develop low-loss dielectrics that are urgently required for the advancement of superconducting quantum computers.