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arXiv:2603.11469

Impact of Oxygen Vacancies in Josephson Junction on Decoherence of Superconducting Qubits

Hanqin Bai, Shi-Yao Hou, Mu Lan

quant-phcond-mat.supr-conSubmitted Mar 12, 2026• Announced Mar 13, 2026• 0 citations
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Abstract

Superconducting quantum circuits are promising platforms for scalable quantum computing, where qubit coherence is critically determined by microscopic defects in the oxide tunneling barrier of Josephson junctions. Amorphous Al$_2$O$_3$ is widely used as a barrier material, but under irradiation, oxygen vacancy (V$_O$) defects are readily generated, introducing noise sources that accelerate qubit decoherence. We systematically investigate the structural characteristics and electronic impact of V$_O$ defects in amorphous Al$_2$O$_3$ using first-principles calculations and \textit{ab initio} molecular dynamics. Our results show that both the coordination environment and concentration of V$_O$s strongly influence electrical conductivity. In particular, two- and three-coordinated V$_O$s, unique to the amorphous structure, enhance conductivity more than conventional four-coordinated vacancies. Increasing V$_O$ concentration amplifies conductivity fluctuations, which we link to critical current noise in Josephson junctions. Using a noise model, we estimate that higher V$_O$ densities lead to shorter qubit coherence times. These findings provide insights for radiation-hard design of superconducting quantum devices.

Comments
17 pages, 13 figures

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Superconducting quantum circuits are promising platforms for scalable quantum computing, where qubit coherence is critically determined by microscopic defects in the oxide tunneling barrier of Josephson junctions. Amorphous Al2_22​O3_33​ is widely used as a barrier material, but under irradiation, oxygen vacancy (VO_OO​) defects are readily generated, introducing noise sources that accelerate qubit decoherence. We systematically investigate the structural characteristics and electronic impact of VO_OO​ defects in amorphous Al2_22​O3_33​ using first-principles calculations and \textit{ab initio} molecular dynamics. Our results show that both the coordination environment and concentration of VO_OO​s strongly influence electrical conductivity. In particular, two- and three-coordinated VO_OO​s, unique to the amorphous structure, enhance conductivity more than conventional four-coordinated vacancies. Increasing VO_OO​ concentration amplifies conductivity fluctuations, which we link to critical current noise in Josephson junctions. Using a noise model, we estimate that higher VO_OO​ densities lead to shorter qubit coherence times. These findings provide insights for radiation-hard design of superconducting quantum devices.
Al/AlOx/Al Josephson junctions are the building blocks of a wide range of superconducting quantum devices that are key elements for quantum computers, extremely sensitive magnetometers and radiation detectors. The properties of the junctions and the superconducting quantum devices are determined by the atomic structure of the tunnel barrier. The nanoscale dimension and disordered nature of the barrier oxide have been challenges for the direct experimental investigation of the atomic structure of the tunnel barrier. Here we show that the miniaturized dimension of the barrier and the interfacial interaction between crystalline Al and amorphous AlOx give rise to oxygen deficiency at the metal/oxide interfaces. In the interior of the barrier, the oxide resembles the atomic structure of bulk aluminium oxide. Atomic defects such as oxygen vacancies at the interfaces can be the origin of the two-level systems and contribute to decoherence and noise in superconducting quantum circuits.
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Constructing circuits which invoke the tunnelling effect of Josephson junctions in the superconducting regime manifest controllable, quantum properties. The development of a quantum computer employing these phenomena is just one of the many advantages to be gained from building such devices. However, the use of quantum bits for computation is dependent on the ability to operate them in an essentially isolated environment. The phenomenon of decoherence refers to the instability of a quantum state of a system when it interacts with the surrounding environment. Superconducting qubits are sensitive to decoherence mechanisms within the readout leads connecting to the device, and more importantly from the materials from which they are manufactured. Removal or control of these imperfections is required before quantum computers using superconducting circuit architecture can be realised. One identifiable noise source is the so called 'strongly coupled' two-level system (TLS). Comparable resonance frequencies to the qubit; strong coupling strengths and decoherence times long enough to allow coherent oscillations between the qubit and TLS have been experimentally measured. The premise of this thesis is that positional anharmonicity of oxygen atoms arises within the AlOx barrier of the Josephson junction solely due to its amorphous construction. This ansatz allows the existence of various spatial configurations throughout the layer, causing unique TLS properties based solely on atomic positions and rotation in relation to the external electric field. To validate this conjecture, Josepson junction models are constructed using a hybrid ab initio and molecular mechanics approach, with various stoichiometry and density properties to reflect experimental observations of the barrier. The resultant atomic positions provide input conditions through a Voronoi classification scheme to a framework describing an oxygen atom that has the capacity to become spatially delocalised as bonds perturb away from a crystalline structure. A direct digitalisation method is developed for low dimensional descriptions and a Wick-rotated time-dependent Schrödinger equation implementation is used for three dimensional investigations. Calculated properties are compared to many current experimental strongly coupled TLS measurements in phase qubits, which shows the models' capacity to explain how an oxygen atom can generate a large dipole and appropriate ground to first excited state splitting values expected of a TLS by merely migrating from its preferred lattice position.
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One of the key problems facing superconducting qubits and other Josephson junction devices is the decohering effects of bistable material defects. Although a variety of phenomenological models exist, the true microscopic origin of these defects remains elusive. For the first time we show that these defects may arise from delocalization of the atomic position of the oxygen in the oxide forming the Josephson junction barrier. Using a microscopic model, we compute experimentally observable parameters for phase qubits. Such defects are charge neutral but have nonzero response to both applied electric field and strain. This may explain the observed long coherence time of two-level defects in the presence of charge noise, while still coupling to the junction electric field and substrate phonons.
Improving qubit lifetime is essential for fault-tolerant quantum computing. It is widely acknowledged that intricate processing techniques introduce defects at the interfaces and surfaces of superconducting quantum circuits, which can create sources of decoherence. Therefore, a precise understanding of how a material's structure contributes to decoherence is critical for enhancing the performance of superconducting qubits. This talk highlights our recent advancements in understanding surface oxides in resonators and ultrathin aluminum oxide barriers in Josephson junctions (JJs) using transmission electron microscopy and spectroscopy techniques [1-4]. The Superconducting Quantum Materials & Systems Center (SQMS) demonstrated that encapsulating niobium surfaces with a ∼10nm tantalum layer results in 2-5x longer T1​ than bare niobium [2]. Our collaborative investigation shows that this improvement is attributed to tantalum's less lossy surface oxide, which contains fewer sub-oxides than niobium oxide. Moreover, the amorphous Ta2O5 shows a bonding nature closer to crystalline form than amorphous Nb2O5, potentially reducing hydrogen diffusion toward the oxide/metal interface [1, 2]. Moreover, we will briefly discuss our preliminary study of the thin film structure under cryogenic conditions down to 20K, conducted using a liquid helium TEM holder in a Thermo Cs-corrected Titan. We also explored the effect of aluminum deposition rates on aluminum oxide microstructure and qubit lifetime. Although increasing the deposition rate from 0.5 Å/s to 5 Å/s significantly improved oxide layer roughness and thickness variation, the qubits’ coherence remained unchanged. This is likely due to Cooper pairs only tunneling through the thinnest parts of the barrier, making them less affected by the grain boundaries in the thin aluminum films. Additionally, we uncovered a stress-induced grain boundary sliding mechanism, which contributes to short-circuit failures in JJs [4]. Our discovery provides vital insight into oxide microstructure and superconducting qubits’ functionality [5].