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IEEE Transactions on Applied Superconductivity

Microwave Losses in YBCO Coplanar Waveguide Resonators at Low Power and Millikelvin Range

M. Arzeo, Floriana Lombardi, Thilo Bauch

Published Oct 24, 2014• 6 citations
Publisher

Abstract

We have investigated the temperature dependence of microwave losses in the low-power limit and millikelvin temperature range of a thin-film YBa2Cu3O7-x(YBCO) λ/2 coplanar waveguide resonator patterned on a (La0.3Sr0.7) (Al0.65Ta0.35)O3(LSAT) substrate covered by a CeO2seed layer. The unloaded quality factor is mainly governed by the surface resistance of the YBCO film and the dielectric losses caused by resonant absorption due to a bath of two-level fluctuators in the dielectrics. The value of the unloaded quality factor at 20 mK and low power, i.e., Q0≃2000, allows for the realization of YBCO microwave quantum circuits implementing biepitaxial grain-boundary Josephson junctions.

DOI
10.1109/tasc.2014.2364777

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We have investigated the temperature dependence of microwave losses in the low-power limit and millikelvin temperature range of a thin-film YBa2Cu3O7-x(YBCO) λ/2 coplanar waveguide resonator patterned on a (La0.3Sr0.7) (Al0.65Ta0.35)O3(LSAT) substrate covered by a CeO2seed layer. The unloaded quality factor is mainly governed by the surface resistance of the YBCO film and the dielectric losses caused by resonant absorption due to a bath of two-level fluctuators in the dielectrics. The value of the unloaded quality factor at 20 mK and low power, i.e., Q0≃2000, allows for the realization of YBCO microwave quantum circuits implementing biepitaxial grain-boundary Josephson junctions.
We have investigated both the temperature and the power dependence of microwave losses for various dielectrics commonly used as substrates for the growth of high critical temperature superconductor thin films. We present measurement of niobium superconducting λ∕2 coplanar waveguide resonators, fabricated on MgO, LaAlO3, and (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT), at the millikelvin temperature range and at low input power. By comparing our results with the two-level system model, we have discriminated among different dominant loss mechanisms. LSAT has shown the best results as regards the dielectric losses in the investigated regimes.
We have investigated both the temperature and the power dependence of microwave losses for various dielectrics commonly used as substrates for the growth of High critical Temperature Superconductor thin films. We present measurement of niobium superconducting λ/2λ/2λ/2 coplanar waveguide resonators, fabricated on La0.3Sr0.7Al0.65Ta0.35O3La_{0.3}Sr_{0.7}Al_{0.65}Ta_{0.35}O_3La0.3​Sr0.7​Al0.65​Ta0.35​O3​ (LSAT), MgOMgOMgO (MgO) and LaAlO3LaAlO_3LaAlO3​ (LAO), at the millikelvin temperature range and at low input power. By comparing our results with the Two-Level System (TLS) model, we have discriminated among different dominant loss mechanisms. LSAT has shown the best results as regards the dielectric losses in the investigated regimes.
We report the microwave response of coplanar-waveguide (CPW) resonators fabricated from YBa2Cu3O7−δ\mathrm{YBa_2Cu_3O_{7-δ}}YBa2​Cu3​O7−δ​ (YBCO) thin films over temperatures from approximately 70 mK70~\mathrm{mK}70 mK to 40 K40~\mathrm{K}40 K. The resonators exhibit internal quality factors QiQ_\mathrm{i}Qi​ in the range of 4×1034\times10^34×103 to 10410^4104 at 70 mK, which increase to a maximum of approximately 8×1038\times10^38×103 to 1.2×1041.2\times10^41.2×104 near 6 K6~\mathrm{K}6 K. At low temperatures, both QiQ_\mathrm{i}Qi​ and the fractional shift of the resonance frequency Δfr/frΔf_\mathrm{r}/f_\mathrm{r}Δfr​/fr​ increases with temperature, qualitatively resembling behavior commonly associated with two-level-system (TLS) defects. However, neither response saturates on the temperature scale set by the resonator frequency, and the loss exhibits no observable microwave-power dependence. We show that low-temperature frequency upturn may be better described by an additional paramagnetic response associated with defect-induced local moments or Andreev bound states, while the low-temperature loss follows an approximately logarithmic temperature dependence whose microscopic origin remains unresolved. These measurements establish the millikelvin performance of patterned YBCO resonators and show that their low-temperature response cannot be understood within the conventional TLS framework alone.
The loss in superconducting microwave resonators at low-photon numbers and low temperatures is not well understood but has implications for achievable coherence times in superconducting qubits. We have fabricated single-layer resonators with a high quality factor by patterning a superconducting aluminum film on a sapphire substrate. Four resonator geometries were studied with resonant frequencies ranging from 5 to 7 GHz: a quasi-lumped element resonator, a coplanar strip waveguide resonator, and two hybrid designs that contain both a coplanar strip and a quasi-lumped element. Transmitted power measurements were taken at 30 mK as a function of frequency and probe power. We find that the resonator loss, expressed as the inverse of the internal quality factor, decreases slowly over four decades of photon number in a manner not merely explained by loss from a conventional uniform spatial distribution of two-level systems in an oxide layer on the superconducting surfaces of the resonator.
We report our setup and measurement results of the quality factor Q of superconducting microwave resonators at mK temperature in a dilution refrigerator. The power dependence of the superconducting resonators' Q was measured down to the single photon level. Measurements were performed on two types of microwave resonators. First, a half-wavelength coplanar waveguide (CPW) resonator made of niobium film on an oxidized high-resistivity silicon substrate was measured. Then a rectangular box resonator made of machined superconducting aluminum alloy was measured. While the CPW chip resonator's Q value showed noticeable decrease at low microwave power, the box resonator's Q showed little power dependence. This setup is intended to perform characterization of various microwave resonators, which will serve as key components in superconducting qubit in circuit QED architecture.
Superconducting microwave circuits with and without Josephson inductances are the Swiss Army knife for many experiments and technologies from quantum information science to astrophysical particle detectors. Despite a large variety of existing circuit types, thin film materials and Josephson junction technologies, a flexible and reliable platform for high-magnetic-field and high-temperature applications is yet to be found. In this manuscript, we investigate coplanar waveguide cavities made of the high-temperature cuprate superconductor YBa2_22​Cu3_33​O7_77​ (YBCO), integrated with Josephson inductances and quantum interferometers that are generated by the controlled local irradiation of the YBCO with a focused helium ion beam. We obtain strongly flux-tunable microwave resonators, which not only display periodic interferometer oscillations of resonance frequency and decay rate, but also a superimposed Fraunhofer-like modulation pattern. The latter originates from the magnetic field tuning of the individual Josephson junction critical currents due to the out-of-plane junction barriers. It allows adjusting resonance frequency and flux responsivity independently of each other, potentially enabling tunable microwave circuits with low sensitivity to external magnetic-field noise over a broad range of frequencies. Finally, we investigate the temperature dependence of the cavities, show that they have promising characteristics up to 14 \,K, and present a model for the junction-induced cavity losses.