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arXiv:2310.06621 · Nature Communications

The effects of disorder in superconducting materials on qubit coherence

Ran Gao, Feng Wu, Hantao Sun, Jianjun Chen, Hao Deng, Xizheng Ma, Xiaohe Miao, Zhijun Song, Xin Wan, Fei Wang, Tian Xia, Make Ying, Chao Zhang, Yaoyun Shi, Hui‐Hai Zhao, Chunqing Deng

Published Apr 16, 2025• 11 citations
arXivHTMLPDF

Abstract

Introducing disorder in the superconducting materials has been considered promising to enhance the electromagnetic impedance and realize noise-resilient superconducting qubits. Despite a number of pioneering implementations, the understanding of the correlation between the material disorder and the qubit coherence is still developing. Here, we demonstrate a systematic characterization of fluxonium qubits with the superinductors made by spinodal titanium-aluminum-nitride with varied disorder. From qubit noise spectroscopy, the flux noise and the dielectric loss are extracted as a measure of the coherence properties. Our results reveal that the 1/f α flux noise dominates the qubit decoherence around the flux-frustration point, strongly correlated with the material disorder; while the dielectric loss are largely similar under a wide range of material properties. From the flux-noise amplitudes, the areal density (σ) of the phenomenological spin defects and material disorder are found to be approximately correlated by $$\sigma \propto {\rho }_{xx}^{3}$$ , or effectively $${({k}_{F}l)}^{-3}$$ . This work has provided new insights on the origin of decoherence channels beyond surface defects and within the superconductors, and could serve as a useful guideline for material design and optimization. Disorder has emerged as a promising tool to manipulate properties of superconducting circuits. Here the authors demonstrate the use of disordered spinodal superconductor for fluxonium qubit fabrication and reveal an interesting correlation between the material disorder and the 1/f-type flux noise.

DOI
10.1038/s41467-025-58745-y

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Introducing disorder in the superconducting materials has been considered promising to enhance the electromagnetic impedance and realize noise-resilient superconducting qubits. Despite a number of pioneering implementations, the understanding of the correlation between the material disorder and the qubit coherence is still developing. Here, we demonstrate a systematic characterization of fluxonium qubits with the superinductors made by spinodal titanium-aluminum-nitride with varied disorder. From qubit noise spectroscopy, the flux noise and the dielectric loss are extracted as a measure of the coherence properties. Our results reveal that the 1/f α flux noise dominates the qubit decoherence around the flux-frustration point, strongly correlated with the material disorder; while the dielectric loss are largely similar under a wide range of material properties. From the flux-noise amplitudes, the areal density (σ) of the phenomenological spin defects and material disorder are found to be approximately correlated by σ∝ρxx3\sigma \propto {\rho }_{xx}^{3}σ∝ρxx3​ , or effectively (kFl)−3{({k}_{F}l)}^{-3}(kF​l)−3 . This work has provided new insights on the origin of decoherence channels beyond surface defects and within the superconductors, and could serve as a useful guideline for material design and optimization. Disorder has emerged as a promising tool to manipulate properties of superconducting circuits. Here the authors demonstrate the use of disordered spinodal superconductor for fluxonium qubit fabrication and reveal an interesting correlation between the material disorder and the 1/f-type flux noise.
Using a fluxonium qubit with in situ tunability of its Josephson energy, we characterize its energy relaxation at different flux biases as well as different Josephson energy values. The relaxation rate at qubit energy values, ranging more than 1 order of magnitude around the thermal energy kBT{k}_{B}TkB​T, can be quantitatively explained by a combination of dielectric loss and 1/f1/f1/f flux noise with a crossover point. The amplitude of the 1/f1/f1/f flux noise is consistent with that extracted from the qubit dephasing measurements at the flux sensitive points. In the dielectric loss dominant regime, the loss is consistent with that arising from the electric dipole interaction with two-level-system (TLS) defects. In particular, by increasing the Josephson energy, thus decreasing the qubit frequency at the flux insensitive spot, we find that the qubit exhibits increasingly weaker coupling to TLS defects, which is desirable for high-fidelity quantum operations. Our work establishes a generic noise model for the design of high coherence fluxonium and future generations of noise resilient superconducting qubits.
The dominant source of decoherence in contemporary frequency-tunable superconducting qubits is 1/f1/f1/f flux noise. To understand its origin and find ways to minimize its impact, we systematically study flux noise amplitudes in more than 50 flux qubits with varied superconducting quantum interference device (SQUID) geometry parameters and compare our results to a microscopic model of magnetic spin defects located at the interfaces surrounding the SQUID loops. Our data are in agreement with an extension of the previously proposed model, based on numerical simulations of the current distribution in the investigated SQUIDs. Our results and detailed model provide a guide for minimizing the flux noise susceptibility in future circuits.
Superconducting quantum devices, from qubits and magnetometers to dark matter detectors, are influenced by magnetic flux noise originating from paramagnetic surface defects and impurities. These spin systems can feature complex dynamics, including a Berezinskii-Kosterlitz-Thouless transition, that depend on the lattice, interactions, external fields, and disorder. However, the disorder included in typical models is not materials-specific, diminishing the ability to accurately capture measured flux noise phenomena. We present a first principles-based simulation of a spin lattice consisting of paramagnetic O2_22​ molecules on an Al2_22​O3_33​ surface, a likely flux noise source in superconducting qubits, to elucidate opportunities to mitigate flux noise. We simulate an ensemble of surface adsorbates with disordered orientations and calculate the orientation-dependent exchange couplings using density functional theory. Thus, our spin simulation has no free parameters or assumed functional form of the disorder, and captures correlation in the defect landscape that would appear in real systems. We calculate a range of exchange interactions between electron pairs, with the smallest values, 0.016 meV and -0.023 meV, being in the range required to act as a two-level system and couple to GHz resonators. We calculate the flux noise frequency, temperature, and applied external magnetic field dependence, as well as the susceptibility-flux noise cross-correlation. Calculated trends agree with experiment, demonstrating that a surface harboring paramagnetic adsorbates arranged with materials-specific disorder and interactions captures the various properties of magnetic flux noise observed in superconducting circuits. In addition, we find that an external electric field can tune the spin-spin interaction strength and reduce magnetic flux noise.
Noise from material defects at device interfaces is known to limit the coherence of superconducting circuits, yet our understanding of the defect origins and noise mechanisms remains incomplete. Here we investigate the temperature and in-plane magnetic-field dependence of energy relaxation in a low-frequency fluxonium qubit, where the sensitivity to flux noise and charge noise arising from dielectric loss can be tuned by applied flux. We observe an approximately linear scaling of flux noise with temperature TTT and a power-law dependence of dielectric loss T3T^3T3 up to 100 mK. Additionally, we find that the dielectric-loss-limited T1T_1T1​ decreases with weak in-plane magnetic fields, suggesting a potential magnetic-field response of the underlying charge-coupled defects. We implement a multi-level decoherence model in our analysis, motivated by the widely tunable matrix elements and transition energies approaching the thermal energy scale in our system. These findings offer insight for fluxonium coherence modeling and should inform microscopic theories of intrinsic noise in superconducting circuits.
Fluxonium superconducting qubits have demonstrated long coherence times and high single- and two-qubit gate fidelities, making them a favorable building block for superconducting quantum processors. We investigate the dominant limitations to fluxonium qubit energy relaxation time T1T_1T1​ using a set of eight planar, aluminum-on-silicon qubits. We find that a circuit-based model for capacitive dielectric loss best captures the frequency dependence of T1T_1T1​, which we analyze within both a two-level and a six-level energy relaxation model. We convert the measured T1T_1T1​ into an effective capacitive quality factor QCeffQ_\mathrm{C}^{\mathrm{eff}}QCeff​ to compare qubits on equal footing, accounting for independently estimated contributions from 1/f1/f1/f flux noise and radiative loss to the control and readout circuitry. We apply this methodology to compare qubits from two fabrication processes: a baseline process and one that applies a fluorine-based wet treatment prior to Josephson junction deposition. We resolve a small improvement of (13.8 ±\pm± 8.4)%)\%)% in the process mean QCeffQ_\mathrm{C}^{\mathrm{eff}}QCeff​, indicating that the fluorine treatment may have reduced loss from the metal-substrate interface, but did not address the primary source of loss in these fluxonium qubits.
The performance of superconducting qubits is orders of magnitude below what is expected from theoretical estimates based on the loss tangents of the constituent bulk materials. This has been attributed to the presence of uncontrolled surface oxides formed during fabrication which can introduce defects and impurities that create decoherence channels. Here, we develop an ab initio Shiba theory to investigate the microscopic origin of magnetic-induced decoherence in niobium thin film superconductors and the formation of native oxides. Our ab initio calculations encompass the roles of structural disorder, stoichiometry, and strain on the formation of decoherence-inducing local spin moments. With parameters derived from these first-principles calculations we develop an effective quasi-classical model of magnetic-induced losses in the superconductor. We identify d-channel losses (associated with oxygen vacancies) as especially parasitic, resulting in a residual zero temperature surface impedance. This work provides a route to connecting atomic scale properties of superconducting materials and macroscopic decoherence channels affecting quantum systems.