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Applied Physics Letters

Probing the density of states of two-level tunneling systems in silicon oxide films using superconducting lumped element resonators

Sebastian T. Skacel, Ch. Kaiser, S. Wünsch, Hannes Rotzinger, Alexander V. Lukashenko, Markus Jerger, G. Weiß, M. Siegel, Alexey Vladimirovich Ustinov

Published Jan 12, 2015• 25 citations
Publisher

Abstract

We have investigated dielectric losses in amorphous silicon oxide (a-SiO) thin films under operating conditions of superconducting qubits (mK temperatures and low microwave powers). For this purpose, we have developed a broadband measurement setup employing multiplexed lumped element resonators using a broadband power combiner and a low-noise amplifier. The measured temperature and power dependences of the dielectric losses are in good agreement with those predicted for atomic two-level tunneling systems (TLS). By measuring the losses at different frequencies, we found that the TLS density of states is energy dependent. This had not been seen previously in loss measurements. These results contribute to a better understanding of decoherence effects in superconducting qubits and suggest a possibility to minimize TLS-related decoherence by reducing the qubit operation frequency.

DOI
10.1063/1.4905149

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We have investigated dielectric losses in amorphous silicon oxide (a-SiO) thin films under operating conditions of superconducting qubits (mK temperatures and low microwave powers). For this purpose, we have developed a broadband measurement setup employing multiplexed lumped element resonators using a broadband power combiner and a low-noise amplifier. The measured temperature and power dependences of the dielectric losses are in good agreement with those predicted for atomic two-level tunneling systems (TLS). By measuring the losses at different frequencies, we found that the TLS density of states is energy dependent. This had not been seen previously in loss measurements. These results contribute to a better understanding of decoherence effects in superconducting qubits and suggest a possibility to minimize TLS-related decoherence by reducing the qubit operation frequency.
We have investigated dielectric losses in amorphous SiO thin films under operating conditions of superconducting qubits (mK temperatures and low microwave powers). For this purpose, we have developed a broadband measurement setup employing multiplexed lumped element resonators using a broadband power combiner and a low-noise amplifier. The measured temperature and power dependences of the dielectric losses are in good agreement with those predicted for atomic two-level tunneling systems (TLS). By measuring the losses at different frequencies, we found that the TLS density of states is energy dependent. This had not been seen previously in loss measurements. These results contribute to a better understanding of decoherence effects in superconducting qubits and suggest a possibility to minimize TLS-related decoherence by reducing the qubit operation frequency.
The improvement of the coherence times of superconducting qubits depends on the reduction of the dielectric losses in the insulating materials implemented for the device fabrication. These losses depend on the density of spurious dipoles of different nature (two-level systems, TLSs) which couple to phase qubits and, hence, limit their coherence times. Hydrogenated amorphous silicon (a-Si:H), because of its lower loss tangent (tan δ) among conventional dielectrics (a-SiO, a-SiO2, a-SiNx:H), is considered as the best amorphous dielectric for superconducting qubit application. We have developed a reliable method for the direct measurement of dielectric losses in amorphous dielectric thin films using a novel design based on four lumped superconducting LC resonators connected in series without coupling capacitors. The losses are obtained as tan δ = 1/Q0, where Q0 is the intrinsic quality factor of the resonator, measured at 3 dB above the resonance frequency without any fitting procedure. The series type LC resonators with a-Si:H as dielectric were fabricated by the Nb technology. The measurements were done at the conditions of a qubit application (0.5 -10 GHz frequency range and low temperatures). The low values of the loss tangent of a-Si:H (up to 2.5 x 10−5 at 4.2 K) have required the development of superconducting housing for the resonators in order to eliminate a spurious dependence of tan δ on the microwave power, by reducing losses which were not originated in the dielectric itself. The results of the simulations agree well with the experiments.
We have developed a reliable method for the direct measurement of dielectric losses in thin films using a special design of lumped element superconducting resonators. The method presented allows obtaining quantitative values for the losses in the film volume as well as the metal/dielectric interfaces. Numerous different resonator geometries were designed and measured in order to study the losses in dielectric thin films usually used for Josephson junction fabrication, such as Nb 2 O 5 , SiO and SiN x , at low temperatures and low gigahertz frequencies. The results show that for such amorphous materials, the bulk losses clearly exceed the interface losses. Furthermore, the frequency dependence of the losses in this working regime was studied for the first time. Our results are in good agreement with the universal law describing the frequency dependence of dielectric losses and suggest that the losses are due to many-body interacting dipoles, which is an important fact for the theoretical modelling of two-level fluctuators. We also investigated the losses in multi-layer films and find that the results are in good agreement with the theoretical expectations, which allows optimization of the dielectric multi-layers used in Josephson junction and qubit fabrication.
We report on the characterization of microwave loss of thin aluminum oxide films at low temperatures using superconducting lumped resonators. The oxide films are fabricated using plasma oxidation of aluminum and have a thickness of 5 nm. We measure the dielectric loss versus microwave power for resonators with frequencies in the GHz range at temperatures from 54 to 303 mK. The power and temperature dependence of the loss are consistent with the tunneling two-level system theory. These results are relevant to understanding decoherence in superconducting quantum devices. The obtained oxide films are thin and robust, making them suitable for capacitors in compact microwave resonators.
Two-level systems (TLS) are an important, if not dominant, source of loss and noise for superconducting resonators such as those used in kinetic inductance detectors and some quantum information science platforms. They are similarly important for loss in photolithographically fabricated superconducting mm-wave/THz transmission lines. For both lumped-element and transmission-line structures, native amorphous surface oxide films are typically the sites of such TLS in nonmicrostripline geometries, while loss in the (usually amorphous) dielectric film itself usually dominates in microstriplines. We report here on the demonstration of low TLS loss at GHz frequencies in hydrogenated amorphous silicon (a-Si:H) films deposited by plasma-enhanced chemical vapor deposition in superconducting lumped-element resonators using parallel-plate capacitors (PPCs). The values we obtain from two recipes in different deposition machines, $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ and $12\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, improve on the best achieved in the literature by a factor of 2--4 for a-Si:H and are comparable to recent measurements of amorphous germanium. Moreover, we have taken care to extract the true zero-temperature, low-field loss tangent of these films, accounting for temperature and field saturation effects that can yield misleading results. Such robustly fabricated and characterized films render the use of PPCs with deposited amorphous films a viable architecture for superconducting resonators and they also promise extremely low loss and high quality factor for photolithographically fabricated superconducting mm-wave/THz transmission lines used in planar antennas and resonant filters.
The investigation of two-level-state (TLS) loss in dielectric materials and interfaces remains at the forefront of materials research in superconducting quantum circuits. We demonstrate a method of TLS loss extraction of a thin film dielectric by using a lumped element resonator fabricated from a superconductor-dielectric-superconductor trilayer. We extract the dielectric loss by formulating a circuit model for a lumped element resonator with TLS loss and then fitting to this model using measurements from a set of three resonator designs: a coplanar waveguide resonator, a lumped element resonator with an interdigitated capacitor, and a lumped element resonator with a parallel plate capacitor that includes the dielectric thin film of interest. Unlike the commonly used single measurement technique, this method allows the accurate measurement of materials with TLS loss lower than 10−6. We demonstrate this method by extracting a TLS loss of 1.00×10−3 for sputtered Al2O3 using a set of samples fabricated from an Al/Al2O3/Al trilayer. We compare this method with the single measurement technique and observe a difference of 11% in extracted loss of the trilayer.