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IEEE Transactions on Applied Superconductivity

Coupling of Microwave Resonators to Feed Lines

G. Hammer, S. Wünsch, M. Roesch, K. Ilin, E. Crocoll, M. Siegel

Published Jun 1, 2009• 6 citations
Publisher

Abstract

Superconducting transmission line resonators can be used to read out different sensors, e.g. flux qubit, transition edge sensor, or as detection element in kinetic inductance detectors. For both applications, high loaded quality factors of the microwave resonators are required, so that it is possible to detect small deviations in their resonance frequency. Thus high loaded quality factors lead to a high sensitive readout.

DOI
10.1109/tasc.2009.2018476

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Superconducting transmission line resonators can be used to read out different sensors, e.g. flux qubit, transition edge sensor, or as detection element in kinetic inductance detectors. For both applications, high loaded quality factors of the microwave resonators are required, so that it is possible to detect small deviations in their resonance frequency. Thus high loaded quality factors lead to a high sensitive readout.
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.
Summary form only given, as follows. Thin-film superconducting microwave resonators play an important role in large superconducting cameras for applications ranging from cosmology to nuclear non-proliferation. The high quality factor and low loss of these devices make it possible to couple many resonators to a single feedline, enabling frequency-division multiplexing of large arrays. We review the use of superconducting microwave resonators for both microwave kinetic inductance detectors and superconducting transition-edge sensors.
Superconducting microwave resonators have the potential to revolutionize submillimeter and far-infrared astronomy, and with it our understanding of the universe. The field of low-temperature detector technology has reached a point where extremely sensitive devices like transition-edge sensors are now capable of detecting radiation limited by the background noise of the universe. However, the size of these detector arrays are limited to only a few thousand pixels. This is because of the cost and complexity of fabricating large-scale arrays of these detectors that can reach up to 10 lithographic levels on chip, and the complicated SQUID-based multiplexing circuitry and wiring for readout of each detector. In order to make substantial progress, next- generation ground-based telescopes such as CCAT or future space telescopes require focal planes with large-scale detector arrays of 104–106 pixels. Arrays using microwave kinetic inductance detectors (MKID) are a potential solution. These arrays can be easily made with a single layer of superconducting metal film deposited on a silicon substrate and pattered using conventional optical lithography. Furthermore, MKIDs are inherently multiplexable in the frequency domain, allowing ∼ 103 detectors to be read out using a single coaxial transmission line and cryogenic amplifier, drastically reducing cost and complexity. An MKID uses the change in the microwave surface impedance of a superconducting thin-film microresonator to detect photons. Absorption of photons in the superconductor breaks Cooper pairs into quasiparticles, changing the complex surface impedance, which results in a perturbation of resonator frequency and quality factor. For excitation and readout, the resonator is weakly coupled to a transmission line. The complex amplitude of a microwave probe signal tuned on-resonance and transmitted on the feedline past the resonator is perturbed as photons are absorbed in the superconductor. The perturbation can be detected using a cryogenic amplifier and subsequent homodyne mixing at room temperature. In an array of MKIDs, all the resonators are coupled to a shared feedline and are tuned to slightly different frequencies. They can be read out simultaneously using a comb of frequencies generated and measured using digital techniques. This thesis documents an effort to demonstrate the basic operation of ∼ 256 pixel arrays of lumped-element MKIDs made from superconducting TiNx on silicon. The resonators are designed and simulated for optimum operation. Various properties of the resonators and arrays are measured and compared to theoretical expectations. A particularly exciting observation is the extremely high quality factors (∼ 3 × 107) of our TiNx resonators which is essential for ultra-high sensitivity. The arrays are tightly packed both in space and in frequency which is desirable for larger full-size arrays. However, this can cause a serious problem in terms of microwave crosstalk between neighboring pixels. We show that by properly designing the resonator geometry, crosstalk can be eliminated; this is supported by our measurement results. We also tackle the problem of excess frequency noise in MKIDs. Intrinsic noise in the form of an excess resonance frequency jitter exists in planar superconducting resonators that are made on dielectric substrates. We conclusively show that this noise is due to fluctuations of the resonator capacitance. In turn, the capacitance fluctuations are thought to be driven by two-level system (TLS) fluctuators in a thin layer on the surface of the device. With a modified resonator design we demonstrate with measurements that this noise can be substantially reduced. An optimized version of this resonator was designed for the multiwavelength submillimeter kinetic inductance camera (MUSIC) instrument for the Caltech Submillimeter Observatory.
Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.
Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.
We report our development on design, fabrication and characterization of various superconducting resonators for quantum device applications. The resonators are developed mainly for superconducting qubit implementation in circuit QED structure, but future application will include photon detectors such as microwave kinetic inductance detectors. First we designed and fabricated half-wavelength coplanar waveguide resonators with niobium thin film on a high resistive silicon wafer, with the fundamental frequencies in the range of 1 to 8 GHz. The loaded quality factors are controlled by the coupling capacitors, and the lower bound of the intrinsic quality factor was measured down to 7 mK in the dilution refrigerator. We have also designed quarter-wavelength resonators and lumped LC resonators coupled to a transmission line, and characterized multiple resonators in a single transmission line. For the 3D transmon qubit application, we designed and fabricated a rectangular box resonator with superconducting aluminum. The intrinsic quality factor of the box resonator exceeds three million at 7 mK. We will also present basic results on the integration of a transmon qubit in the superconducting resonator.