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Journal of Physics Conference Series

Properties of high-quality coplanar waveguide resonators for QIP and detector applications

T. Lindström, J E Healey, M. S. Colclough, C H Webster, C. M. Muirhead, Alexander Ya. Tzalenchuk

Published Mar 1, 2009• 1 citation
Publisher

Abstract

We have fabricated and characterized λ/2 thin-film coplanar waveguide resonators made from niobium deposited on sapphire and SiO 2 /Si substrates. The samples have been characterized at temperatures down to 30 mK. In this work we discuss two important properties of these resonators: The tunability of the centre frequency using a weak applied magnetic field, and the RF-power dependence of the quality factor.

DOI
10.1088/1742-6596/150/5/052140

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We have fabricated and characterized λ/2 thin-film coplanar waveguide resonators made from niobium deposited on sapphire and SiO 2 /Si substrates. The samples have been characterized at temperatures down to 30 mK. In this work we discuss two important properties of these resonators: The tunability of the centre frequency using a weak applied magnetic field, and the RF-power dependence of the quality factor.
The sensitivity of a kinetic inductance detector is determined by the performance of the cavity circuit and therefore a high quality factor and low phase noise are required. We developed superconducting niobium coplanar waveguide resonators for kinetic inductance detectors at a resonance frequency of 6 GHz. The length of the resonators is adjusted to a quarter of a wavelength which corresponds to ≈ 5.3 mm in our geometry. Different layouts with coplanar waveguide transmission line topology for the resonator were designed and simulated. Some parameters of the device fabrication process were varied. The niobium thin films were deposited by magnetron sputtering on sapphire substrates kept at ambient temperature. The resonators were patterned by photolithography and reactive ion etching and measured at helium temperatures. Because of the quarter wave geometry with one shorted end the reflection parameter was measured. We have studied the influence of the coupling capacitances on quality factor. The optimization includes microwave simulations and implementation in different resonator structures. The measurements of optimized devices showed a very high quality factor above one million (10 6 ). The obtained results and perspective on further development of resonators will be discussed.
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.
The use of a transmission line resonator offers the possibility for different sensor readout methods, e.g. flux qubit and kinetic inductance detectors. Both cases require a high quality factor, Q L , of the cavity circuit to detect small deviations in the resonance frequency. Various superconducting niobium coplanar waveguide resonators of λ/4 and λ/2 wavelength for 3 and 6 GHz were developed. Different layouts for the coplanar resonators were designed, simulated and measured on common silicon and sapphire substrates at 4.2 K. We found a very good agreement between simulation and measurement results, with Q L between 10 4 and 10 5 at helium temperatures. The presented results show the dependences on the chosen cross section geometries, the coupling factor and the complex layout of the transmission line.
We characterize niobium nitride (NbN) λ/2 coplanar waveguide resonators, which were fabricated from a 10-nm-thick film on silicon dioxide grown by sputter deposition. For films grown at 180 °C, we report a superconducting critical temperature of 7.4 K associated with a normal square resistance of 1 kΩ, leading to a kinetic inductance of 192 pH/◻. We fabricated resonators with a characteristic impedance up to 4.1 kΩ and internal quality factors Qi>104 in the single photon regime at zero magnetic field. Moreover, in the many photon regime, the resonators present a high magnetic field resilience with Qi>104 in a 6 T in-plane magnetic field and in a 300 mT out-of-plane magnetic field. These findings make such resonators a compelling choice for circuit quantum electrodynamics experiments involving quantum systems with small electric dipole moments operated in finite magnetic fields.
Abstract We demonstrate the operation of superconducting coplanar microwave resonators in a very large frequency range up to 50 GHz. The resonators are fabricated from niobium thin films on sapphire substrates and have fundamental frequencies of 5 GHz or 10 GHz. We study numerous harmonics of the resonators at temperatures between 1.5 K and 6 K, and we determine quality factors of up to 22 600 at 1.5 K. As an example for spectroscopy applications of such resonators we detect the superconducting transition of a bulk tin sample at multiple probing frequencies.
We describe measurements on microwave coplanar resonators designed for quantum bit experiments. Resonators have been patterned onto sapphire and silicon substrates, and quality factors in excess of a million have been observed. The resonant frequency shows a high sensitivity to magnetic field applied perpendicular to the plane of the film, with a quadratic dependence for the fundamental, second, and third harmonics. Frequency shift of hundreds of linewidths can be obtained with no change in the quality factor.