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arXiv:2610.12556

TLS loss in superconducting resonators on epitaxial hBN thin films

J. Christiansen-Salameh, D. J. Temples, R. Linehan, S. Sussman, Aloyse Maille, F. Abouzahr, L. Alexis, K. Anyang, D. Bowring, G. Bratrud, E. Celi, A. H. Colón Cesaní, R. Khatiwada, D. Molenaar, O. Peek, A. Rodriguez, O. Seidel, D. Schlom, D. Sotir, S. Uemura, Y. Wu, E. Figueroa-Feliciano, D. Baxter, Z. Tian

quant-phhep-exSubmitted Oct 8, 2026• Announced Oct 12, 2026• 0 citations
arXivHTMLPDF

Abstract

The integration of low-loss dielectrics into superconducting circuits is of importance to the functionality of quantum computers and sensors. In this work, we fabricate and measure niobium resonators atop few-layer, epitaxial hexagonal boron nitride (hBN) grown by Molecular Beam Epitaxy on a sapphire substrate and compare against a control counterpart without hBN. We conduct radiofrequency measurements which probe the responses to power, temperature, and temporal fluctuations of two-level systems. The resonators integrating hBN maintain low dielectric loss with a low-power internal loss tangent on the order of $10^{-6}$, and demonstrate hBN can be added to the conventional superconducting qubit stack with minimal degradation of the quality factor. This research represents a first step towards a materials-driven strategy to eliminate phonon-mediated correlated errors in solid-state quantum systems, which leverages the thermal properties of 2D materials to prevent athermal phonons excited in the device substrate from transmitting across the superconductor-substrate interface. The same technique applied to quantum sensors would maximize phonon collection by blocking phonons from entering passive metal films.

Comments
14 pages, 11 figures

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The integration of low-loss dielectrics into superconducting circuits is of importance to the functionality of quantum computers and sensors. In this work, we fabricate and measure niobium resonators atop few-layer, epitaxial hexagonal boron nitride (hBN) grown by Molecular Beam Epitaxy on a sapphire substrate and compare against a control counterpart without hBN. We conduct radiofrequency measurements which probe the responses to power, temperature, and temporal fluctuations of two-level systems. The resonators integrating hBN maintain low dielectric loss with a low-power internal loss tangent on the order of 10−610^{-6}10−6, and demonstrate hBN can be added to the conventional superconducting qubit stack with minimal degradation of the quality factor. This research represents a first step towards a materials-driven strategy to eliminate phonon-mediated correlated errors in solid-state quantum systems, which leverages the thermal properties of 2D materials to prevent athermal phonons excited in the device substrate from transmitting across the superconductor-substrate interface. The same technique applied to quantum sensors would maximize phonon collection by blocking phonons from entering passive metal films.
The performance of superconducting microwave circuits is strongly influenced by the material properties of the superconducting film and substrate. While progress has been made in understanding the importance of surface preparation and the effect of surface oxides, the complex effect of superconductor film structure on microwave losses is not yet fully understood. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over one million at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasiparticle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Our results stress the connection between the crystal structure of superconducting films and the loss mechanisms suffered by the resonators and demonstrate that even a moderate change in temperature during thin film deposition can significantly affect the resulting quality factors.
The performance of superconducting microwave circuits is strongly influenced by the material properties of the superconducting film and substrate. While progress has been made in understanding the importance of surface preparation and the effect of surface oxides, the complex effect of superconductor film structure on microwave losses is not yet fully understood. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975K. The lowest-loss resonators that we measure have quality factors of over 106 at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasiparticle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Our results stress the connection between the crystal structure of superconducting films and the loss mechanisms suffered by the resonators and indicate that even a moderate change in temperature during thin film deposition can significantly affect the resulting quality factors.
This is a study of the growth of titanium-nitride (TiN) by plasma assisted molecular beam epitaxy (MBE) for applications in low loss quantum circuits. Titanium nitride is a material known to have low loss at microwave frequencies which sees practical use in many emerging quantum device architectures. The goal of this research is to investigate improvements in TiN thin films utilizing MBE to produce highly pure, highly crystalline films. A review of the theory of superconducting qubits is given and recent developments in superconducting qubit research are summarized to elucidate how loss has been reduced historically and the importance of materials improvements to the future of scalable quantum computation. The dynamics of epitaxial film growth are discussed, focusing on the formation of a minimum energy surface, the thermally driven kinetics of atoms deposited on the surface, and how these lead to the formation of large scale crystalline domains in the film. An overview of the MBE growth chamber is given and the growth procedure for TiN is discussed. Transport and morphology measurement results are discussed for MBE TiN where, for optimized growth conditions, films are highly crystalline with superconducting transition temperatures(Tc) as high as Tc = 6.1K, matching the value for bulk crystals and exceeding all reported thin film values. Microwave loss measurements of transmission line resonators fabricated from these films are shown to have low-power quality factors(Q) as high as 200,000 and an increase to Q = 400,000 at high power, consistent with the saturation of two-level-systems (TLS) at high power. These results confirm a connection between the high quality crystalline materials produced in this work and low loss in quantum circuits.
Abstract The performance of superconducting microwave circuits is strongly influenced by the material properties of the superconducting film and substrate. While progress has been made in understanding the importance of surface preparation and the effect of surface oxides, the complex effect of superconductor film structure on microwave losses is not yet fully understood. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over 10 6 at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasiparticle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Our results stress the connection between the crystal structure of superconducting films and the loss mechanisms suffered by the resonators and indicate that even a moderate change in temperature during thin film deposition can significantly affect the resulting quality factors.
Epitaxially grown superconductor/dielectric/superconductor trilayers have the potential to form high-performance superconducting quantum devices and may even allow scalable superconducting quantum computing with low-surface-area qubits such as the merged-element transmon. In this work, we measure the power-independent loss and two-level-state (TLS) loss of epitaxial, wafer-bonded, and substrate-removed Al/GaAs/Al trilayers by measuring lumped element superconducting microwave resonators at millikelvin temperatures and down to single-photon powers. The power-independent loss of the device is (4.8±0.1)×10−5, and the resonator-induced intrinsic TLS loss is (6.4±0.2)×10−5. Dielectric loss extraction is used to determine a lower bound of the intrinsic TLS loss of the trilayer of 7.2×10−5. The unusually high power-independent loss is attributed to GaAs’s intrinsic piezoelectricity.
Superconducting coplanar waveguide resonators are essential components in quantum processors, where their internal quality factor (Qi) constrains qubit coherence and readout fidelity. In niobium devices, microwave losses at millikelvin temperatures are strongly influenced by two-level systems (TLS) associated with the complex NbOx surface oxide. To mitigate these losses, we investigate a surface-engineering approach in which Nb films are capped in situ with a thin tantalum layer to suppress Nb2O5 formation and replace the native NbOx interface with a Ta-based oxide. We fabricate Nb/Ta bilayer and reference Nb resonators on high-resistivity silicon using identical DC sputtering and wet etching conditions, and characterize their performance at millikelvin temperatures. Fresh Ta-encapsulated devices exhibit internal quality factors up to 2.4 x 10^6 in the near-single-photon regime, with power dependence consistent with reduced TLS-related loss at the metal-air interface. A control Nb device fabricated under the same process shows comparatively lower Q_TLS, consistent with the beneficial effect of the Ta capping layer. Furthermore, ageing tests performed on Nb/Ta resonators after six months reveal a moderate reduction in Q_TLS relative to their initial values, yet the performance remains superior to newly fabricated Nb-only devices. These results suggest that thin Ta encapsulation enhances interface quality and contributes to improved temporal stability while remaining compatible with Nb-based fabrication workflows.