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IEEE Transactions on Applied Superconductivity

Suppressing Dissipative Paths in Superconducting Coplanar Waveguide Resonators

J. M. Hornibrook, E.E. Mitchell, D. J. Reilly

Published Apr 5, 2013• 6 citations
Publisher

Abstract

The electromagnetic environment can have a significant effect on the resonant line-shape and loadedQ-factor of superconducting coplanar waveguides in the low temperature (<; 100 mK) and low microwave power regime. Here, electromagnetic simulations are used to visualize the electric field strength throughout the resonator environment, highlighting situations in which parasitic modes lead to increased dissipation. Methods used to suppress parasitic coupling include increasing the density of wire bonds and vias, reducing the width of the transmission line (whilst maintaining the characteristic impedance at 50 Ω) and establishing strong grounding to the sample enclosure. The effect of reducing the parasitic coupling on the microwave transmission spectrum is reported.

DOI
10.1109/tasc.2013.2251055

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The electromagnetic environment can have a significant effect on the resonant line-shape and loadedQ-factor of superconducting coplanar waveguides in the low temperature (<; 100 mK) and low microwave power regime. Here, electromagnetic simulations are used to visualize the electric field strength throughout the resonator environment, highlighting situations in which parasitic modes lead to increased dissipation. Methods used to suppress parasitic coupling include increasing the density of wire bonds and vias, reducing the width of the transmission line (whilst maintaining the characteristic impedance at 50 Ω) and establishing strong grounding to the sample enclosure. The effect of reducing the parasitic coupling on the microwave transmission spectrum is reported.
Parasitic electromagnetic fields are shown to strongly suppress the quality (Q)-factor of superconducting coplanar waveguide resonators via non-local dissipation in the macroscopic environment. Numerical simulation and low temperature measurements demonstrate how this parasitic loss can be reduced, establishing a Lorentzian lineshape in the resonator frequency response and yielding a loaded Q-factor of 2.4 x 10^5 for niobium devices on sapphire substrates. In addition, we report the dependence of the Q and resonance frequency shift Delta f_0 with input power and temperature in the limit where loss from two-level systems in the dielectric dominate.
We successfully designed and fabricated an absorption-type of superconducting coplanar waveguide (CPW) resonators. The resonators are made from a niobium film (about 160 nm thick) on a high-resistance Si substrate, and each resonator is fabricated as a meandered quarter-wavelength transmission line (one end is short to the ground and another end is capacitively coupled to a through feedline). With a vector network analyzer we measured the transmissions of the applied microwave through the resonators at ultra-low temperature. The obtained loaded quality factors are significantly high, i.e. up to ∼10 6 . When the temperature increases slowly from the base temperature (20 mK), the resonance frequencies of the resonators are blue shifted and the quality factors are lowered slightly. In principle, this type of device can integrate a series of CPW resonators with a common feedline, making it a promising candidate as the data bus for coupling distant solid-state qubits and the sensitive detector of single photons.
Planar superconducting microwave transmission line resonators can be operated at multiple harmonic resonance frequencies. This allows covering wide spectral regimes with high sensitivity, as it is desired, e.g., for cryogenic microwave spectroscopy. A common complication of such experiments is the presence of undesired “spurious” additional resonances, which are due to standing waves within the resonator substrate or housing box. Identifying the nature of individual resonances (“designed” vs “spurious”) can become challenging for higher frequencies or if elements with unknown material properties are included, as is common for microwave spectroscopy. Here, we discuss various experimental strategies to distinguish designed and spurious modes in coplanar superconducting resonators that are operated in a broad frequency range up to 20 GHz. These strategies include tracking resonance evolution as a function of temperature, magnetic field, and microwave power. We also demonstrate that local modification of the resonator, by applying minute amounts of dielectric or electron spin resonance-active materials, leads to characteristic signatures in the various resonance modes, depending on the local strength of the electric or magnetic microwave fields.
Superconducting microwave circuits based on coplanar waveguides (CPW) are susceptible to parasitic slotline modes which can lead to loss and decoherence. We motivate the use of superconducting airbridges as a reliable method for preventing the propagation of these modes. We describe the fabrication of these airbridges on superconducting resonators, which we use to measure the loss due to placing airbridges over CPW lines. We find that the additional loss at single photon levels is small, and decreases at higher drive powers.
We investigate analytically the coupling of a coplanar waveguide resonator to a coplanar waveguide feedline. Using a conformal mapping technique we obtain an expression for the characteristic mode impedances and coupling coefficients of an asymmetric multi-conductor transmission line. Leading order terms for the external quality factor and frequency shift are calculated. The obtained analytical results are relevant for designing circuit-QED quantum systems and frequency division multiplexing of superconducting bolometers, detectors and similar microwave-range multi-pixel devices.