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arXiv:1512.02553 · Superconductor Science and Technology

Analysis of high quality superconducting resonators: consequences for TLS properties in amorphous oxides

Jonathan Burnett, Lara Faoro, T Lindström

Published Mar 11, 2016• 69 citations
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Abstract

noise caused by microscopic two-level systems (TLS) is known to be very detrimental to the performance of superconducting quantum devices but the nature of these TLS is still poorly understood. Recent experiments with superconducting resonators indicates that interaction between TLS in the oxide at the film-substrate interface is not negligible. Here we present data on the loss and frequency noise from two different Nb resonators with and without Pt capping and discuss what conclusions can be drawn regarding the properties of TLS in amorphous oxides. We also estimate the concentration and dipole moment of the TLS.

DOI
10.1088/0953-2048/29/4/044008

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Analysis of high quality superconducting resonators: consequences for TLS properties in amorphous oxides

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Abstract Losses from two-level systems (TLS) associated with amorphous oxides remain one of the primary limitations to the performance of superconducting qubits and microwave cavities. Niobium resonators are widely used in quantum science experiments, yet niobium’s natural oxide layer contains various types of oxides whose relative contributions to TLS loss have not been clearly distinguished. Here, we use a superconducting 3D microwave cavity to measure commercial 99.9% trace metal pure, microcrystalline oxide powders Nb 2 O 5 and NbO 2 in bulk amounts. Using this approach, we directly compare the loss characteristics of Nb 2 O 5 and NbO 2 . Our measurements show that the nominal Nb 2 O 5 bulk oxide powder samples exhibit losses which have the power and temperature behavior expected for TLS. Moreover, the measurements agree with existing theoretical models. Analogous measurements performed on NbO 2 bulk powder samples do not show any detectable TLS loss signatures. Based on our results we propose that the TLS losses might be reduced if a high quality microcrystalline NbO 2 oxide dominates the Nb 2 O 5 oxide in practical Nb cavities. These results establish a materials based strategy for isolating oxide specific TLS losses and provide a reference measurement for niobium oxide phases relevant to superconducting quantum devices.

Superconductor Science and Technology•Aug 1, 2026•0 citations
noise caused by microscopic two-level systems (TLS) is known to be very detrimental to the performance of superconducting quantum devices but the nature of these TLS is still poorly understood. Recent experiments with superconducting resonators indicates that interaction between TLS in the oxide at the film-substrate interface is not negligible. Here we present data on the loss and frequency noise from two different Nb resonators with and without Pt capping and discuss what conclusions can be drawn regarding the properties of TLS in amorphous oxides. We also estimate the concentration and dipole moment of the TLS.
1/f noise caused by microscopic two-level systems (TLS) is known to be very detrimental to the performance of superconducting quantum devices but the nature of these TLS is still poorly understood. Recent experiments with superconducting resonators indicates that interaction between TLS in the oxide at the film-substrate interface is not negligible. Here we present data on the loss and 1/f frequency noise from two different Nb resonators with and without Pt capping and discuss what conclusions can be drawn regarding the properties of TLS in amorphous oxides. We also estimate the concentration and dipole moment of the TLS.
While several experiments claim that two-level system (TLS) defects in amorphous surfaces and interfaces are responsible for energy relaxation in superconducting resonators and qubits, none can provide quantitative explanation of their data in terms of the conventional noninteracting TLS model. Here a model that interpolates between the interacting and noninteracting TLS loss tangent is proposed to perform numerical analysis of experimental data and extract information about TLS parameters and their distribution. As a proof of principle, the model is applied to TESLA cavities that contain only a single lossy material in their interior, the niobium-niobium oxide interface. The best fits show interacting TLSs with a sharp modulus of electric dipole moment for both thin (5nm5\phantom{\rule{0.2em}{0ex}}\mathrm{nm}5nm) and thick (100nm100\phantom{\rule{0.2em}{0ex}}\mathrm{nm}100nm) oxides, indicating that the TLSs are ``atomic'' instead of ``glassy.'' The proposed method can be applied to other devices with multiple material interfaces and substrates, with the goal of elucidating the nature of TLSs causing energy loss in resonators and qubits.
Superconducting films are inherently limited by losses due to two-level system (TLS) defects within the amorphous oxide layers surrounding them and from quasiparticles in the film. In this thesis I will discuss novel theoretical and experimental methods toward understanding superconducting resonator loss from deleterious surface TLS defects as well as a loss transition from non-equilibrium quasiparticles in granular TiN. I will show using finite element solver software that a resonator with submicron linewidth and linespacing can be used to better characterize and simulate surface TLS as part of a circuit QED system. I have observed individual surface TLS and found coupling values in the range of g/2π =50 kHz -280 kHz with a maximum dipole moment pz-max = 4.5 Debye (.93 eÅ). I have found in in simulation of experiment that over 80% of the strongly coupled TLS reside within 50 nm of the corner between the Metal-Substrate (MS) and Substrate-Air (SA) interface. Additionally I have studied a loss transition from non-equilibrium quasiparticles in TiN films. These films exhibit an anomalous loss dependence on substrate treatment and film thickness. The films of interest are ones grown thin on oxidized substrates, which exhibit an order of magnitude decrease in internal quality factor (Qi) relative to either thicker ˝films or films grown without the oxidized substrate. These films additionally exhibit a grain size on average of 7.5 nm, a higher inhomogeneous gap, a transition to lower stress and a preference for the [111] crystal growth. The temperature dependence of the conductivity is fit and a factor of two difference in quasiparticle lifetime is found between the two films where the thinner film has a shorter lifetime. A two gap quasiparticle trapping model is fit to the temperature dependent loss data. The data is consistent with a model where non-equilibrium quasi-particles are trapped in low gapped grains on the inside of the films. From these works and others presented in my thesis the understanding of TLSs on surfaces and non-equilibrium quasiparticles in TiN has improved. This will help illuminate some of the most important absorption mechanisms plaguing superconducting qubits and resonators.
The analytical solution for the low-temperature 1/f1/f1/f noise in the microwave dielectric constant of amorphous films at frequency ${\ensuremath{\nu}}_{0}\ensuremath{\sim}5\phantom{\rule{0.28em}{0ex}}\mathrm{GHz}$ due to tunneling two-level systems (TLSs) is derived within the standard tunneling model including the weak dipolar or elastic TLS-TLS interactions. The 1/f1/f1/f frequency dependence is caused by TLS spectral diffusion characterized by the width growing logarithmically with time. Temperature and field dependencies are predicted for the noise spectral density in typical glasses with universal TLSs. The satisfactory interpretation of the recent experiment by J. Burnett et al. [Nat. Commun. 5, 4119 (2014)] in Pt-capped Nb superconducting resonators is attained by assuming a smaller density of TLSs compared to ordinary glasses, which is consistent with the very high internal quality factor in those samples.
Superconducting quantum computing is experiencing a tremendous growth. Although major milestones have already been achieved, useful quantum-computing applications are hindered by a variety of decoherence phenomena. Decoherence due to two-level systems (TLSs) hosted by amorphous dielectric materials is ubiquitous in planar superconducting devices. We use high-quality quasilumped-element resonators as quantum sensors to investigate TLS-induced loss and noise. We perform two-tone experiments with a probe and pump electric fields; the pump is applied at different power levels and detunings. We measure and analyze time series of the quality factor and resonance frequency for very long time periods, up to 1000 h. We additionally carry out simulations based on the TLS interacting model in the presence of a pump field. We find that loss and noise are reduced at medium and high power, matching the simulations, but not at low power.
Abstract Losses from two-level systems (TLS) associated with amorphous oxides remain one of the primary limitations to the performance of superconducting qubits and microwave cavities. Niobium resonators are widely used in quantum science experiments, yet niobium’s natural oxide layer contains various types of oxides whose relative contributions to TLS loss have not been clearly distinguished. Here, we use a superconducting 3D microwave cavity to measure commercial 99.9% trace metal pure, microcrystalline oxide powders Nb 2 O 5 and NbO 2 in bulk amounts. Using this approach, we directly compare the loss characteristics of Nb 2 O 5 and NbO 2 . Our measurements show that the nominal Nb 2 O 5 bulk oxide powder samples exhibit losses which have the power and temperature behavior expected for TLS. Moreover, the measurements agree with existing theoretical models. Analogous measurements performed on NbO 2 bulk powder samples do not show any detectable TLS loss signatures. Based on our results we propose that the TLS losses might be reduced if a high quality microcrystalline NbO 2 oxide dominates the Nb 2 O 5 oxide in practical Nb cavities. These results establish a materials based strategy for isolating oxide specific TLS losses and provide a reference measurement for niobium oxide phases relevant to superconducting quantum devices.