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arXiv:1912.09119 · New Journal of Physics

Phononic loss in superconducting resonators on piezoelectric substrates

Marco Scigliuzzo, Laure E Bruhat, Andreas Bengtsson, Jonathan Burnett, Anita Fadavi Roudsari, Per Delsing

Published Mar 16, 2020• 39 citations
arXivHTMLPDF

Abstract

Abstract We numerically and experimentally investigate the phononic loss for superconducting resonators fabricated on a piezoelectric substrate. With the help of finite element method simulations, we calculate the energy loss due to electromechanical conversion into bulk and surface acoustic waves. This sets an upper limit for the resonator internal quality factor Q i. To validate the simulation, we fabricate quarter wavelength coplanar waveguide resonators on GaAs and measure Q i as function of frequency, power and temperature. We observe a linear increase of Q i with frequency, as predicted by the simulations for a constant electromechanical coupling. Additionally, Q i shows a weak power dependence and a negligible temperature dependence around 10 mK, excluding two level systems and non-equilibrium quasiparticles as the main source of losses at that temperature.

DOI
10.1088/1367-2630/ab8044

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Abstract We numerically and experimentally investigate the phononic loss for superconducting resonators fabricated on a piezoelectric substrate. With the help of finite element method simulations, we calculate the energy loss due to electromechanical conversion into bulk and surface acoustic waves. This sets an upper limit for the resonator internal quality factor Q i. To validate the simulation, we fabricate quarter wavelength coplanar waveguide resonators on GaAs and measure Q i as function of frequency, power and temperature. We observe a linear increase of Q i with frequency, as predicted by the simulations for a constant electromechanical coupling. Additionally, Q i shows a weak power dependence and a negligible temperature dependence around 10 mK, excluding two level systems and non-equilibrium quasiparticles as the main source of losses at that temperature.
Abstract The development of superconducting quantum processors relies on understanding and mitigating decoherence in superconducting qubits. Piezoelectric coupling contributes to decoherence by mediating energy exchange between microwave photons and acoustic phonons. Although bulk centrosymmetric materials like silicon and sapphire are non-piezoelectric and commonly used as qubit substrates, the lack of centrosymmetry at interfaces may induce piezoelectric losses. This effect was predicted decades ago but never experimentally observed in superconducting devices. Here, we report interface piezoelectricity at aluminum-silicon junctions and demonstrate it as a significant loss channel in superconducting devices. Using aluminum interdigital transducers on silicon, we observe piezoelectric transduction from room to millikelvin temperatures, with an effective electromechanical coupling factor K 2 ≈ (3 ± 0.4) × 10 −5 %, comparable to weakly piezoelectric substrates. Modeling shows this mechanism limits qubit quality factors to Q ~ 10 4 − 10 8 , depending on surface participation and mode matching. These findings reveal interface piezoelectricity as a major dissipation channel and highlight the need for heterostructure and phononic engineering in next-generation superconducting qubits.
We investigated the loss mechanisms of superconducting circuits with epitaxially grown NbN(100) on MgO(100) substrates by measuring quarter-wave coplanar waveguide resonators with various strip and slot widths. The power dependence of the internal quality factor and the temperature dependence of the resonance frequency shift were evaluated in a dilution refrigerator. The measurement and analysis of the loss in these resonators indicate that the loss was dominated by a two-level system (TLS) loss and the TLS host materials were primarily distributed at the surface of the substrate.
Measuring the internal quality factor of coplanar waveguide superconducting resonators is an established method of determining small losses in superconducting devices. Often, resonator losses are attributed to two-level system (TLS) defects using a power dependent model for the quality factor. However, excess non-equilibrium quasiparticles can also limit the quality factor of superconducting resonators. Here, a two-temperature, power and temperature dependent model is used to evaluate resonator losses for titanium nitride and aluminum coplanar waveguide resonators with loss tangents of 10−5–10−6. This model enables a set of parameters to describe both the internal quality factor and fractional frequency shift of the resonator across a broad range of average cavity photon numbers, from 1 to 106, and temperatures between those limited by the experimental apparatus near 50 mK and thermal quasiparticle dominated. The presented model also offers potential insight into observing the temperature and power dependence of TLS–TLS interactions in these resonators.
The evolution of superconducting quantum processors is driven by the need to reduce errors and scale for fault-tolerant computation. Reducing physical qubit error rates requires further advances in the microscopic modeling and control of decoherence mechanisms in superconducting qubits. Piezoelectric interactions contribute to decoherence by mediating energy exchange between microwave photons and acoustic phonons. Centrosymmetric materials like silicon and sapphire do not display piezoelectricity and are the preferred substrates for superconducting qubits. However, the broken centrosymmetry at material interfaces may lead to piezoelectric losses in qubits. While this loss mechanism was predicted two decades ago, interface piezoelectricity has not been experimentally observed in superconducting devices. Here, we report the observation of interface piezoelectricity at an aluminum-silicon junction and show that it constitutes an important loss channel for superconducting devices. We fabricate aluminum interdigital surface acoustic wave transducers on silicon and demonstrate piezoelectric transduction from room temperature to millikelvin temperatures. We find an effective electromechanical coupling factor of K2≈2×10−5%K^2\approx 2 \times 10^{-5}\%K2≈2×10−5% comparable to weakly piezoelectric substrates. We model the impact of the measured interface piezoelectric response on superconducting qubits and find that the piezoelectric surface loss channel limits qubit quality factors to Q∼104−108Q\sim10^4-10^8Q∼104−108 for designs with different surface participation ratios and electromechanical mode matching. These results identify electromechanical surface losses as a significant dissipation channel for superconducting qubits, and show the need for heterostructure and phononic engineering to minimize errors in next-generation superconducting qubits.
Circuit quantum acoustodynamics (cQAD) devices have a wide range of applications in quantum science, all of which depend crucially on the quantum coherence of the mechanical subsystem. In this context, high-overtone bulk acoustic-wave resonators (HBARs) are particularly promising, since they have shown very high quality factors with negligible dephasing. However, the introduction of piezoelectric films, which are necessary for coupling to a superconducting circuit, can lead to additional loss channels, such as surface scattering and two-level systems (TLS). Here, we study the acoustic dissipation of HBAR resonators in cQAD systems and find that the defect density of the piezoelectric material and its interface with the bulk are limiting factors for the coherence. We measure acoustic modes with phonon lifetimes up to 400 μμμs and lifetime-limited coherence times approaching one millisecond in the quantum regime. When coupled to a superconducting qubit, this leads to a hybrid system with a large quantum coherence cooperativity of CT2=1.1×105C_{T_2}=1.1\times10^5CT2​​=1.1×105. These results represent a new milestone for the performance of cQAD devices and offer concrete paths forward for further improvements.
Abstract A quarter wavelength superconducting aluminum film coplanar waveguide resonator with a high-quality factor was designed and fabricated. Furthermore, its resonant frequency and quality as a function of temperatures and exciting powers were investigated. The experimental results showed that the resonance frequency decreases about 1% with the increase of temperature from 50 mk to 1 K, and the load quality factor of the resonator also decreases. The resonant frequency decreases with power from − 120 to − 70 dBm. The quality factor increases with increased excitation power due to the loss decreases and nonlinearity. The results are consistent with the theoretical analysis of the surface impedance model.