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Applied Physics Letters

Applying a direct current bias to superconducting microwave resonators by using superconducting quarter wavelength band stop filters

Shaoxiong Li, Jan B. Kycia

Published Jun 17, 2013• 18 citations
Publisher

Abstract

We have designed a superconducting quarter wavelength band stop filter which provides a high attenuation at approximately 7 GHz. Such filters were used as dc bias lines applied to whole wavelength and half wavelength superconducting resonators at two different extremes: voltage node and voltage antinode. The results obtained indicate that this filter can apply dc bias to arbitrary length superconducting coplanar waveguide resonators while maintaining high quality factor, Q∼15 000−18 000.

DOI
10.1063/1.4808364

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IEEE International Superconductive Electronics Conference (ISEC)•Jul 1, 2013•0 citations
We have designed a superconducting quarter wavelength band stop filter which provides a high attenuation at approximately 7 GHz. Such filters were used as dc bias lines applied to whole wavelength and half wavelength superconducting resonators at two different extremes: voltage node and voltage antinode. The results obtained indicate that this filter can apply dc bias to arbitrary length superconducting coplanar waveguide resonators while maintaining high quality factor, Q∼15 000−18 000.
We present the design of a reflective stop-band filter based on quasi-lumped elements that can be utilized to introduce large dc and low-frequency voltage biases into a low-loss superconducting coplanar waveguide (CPW) cavity. Transmission measurements of the filter are seen to be in good agreement with simulations and demonstrate insertion losses greater than 20 dB20\,{\rm dB}20dB in the range of 3 to 10 GHz{\rm 3\,to\,10\,GHz}3to10GHz. Moreover, transmission measurements of the CPW's fundamental mode demonstrate that loaded quality factors exceeding 10510^5105 can be achieved with this design for dc voltages as large as 20 V{\rm 20\,V}20V and for the cavity operated in the single-photon regime. This makes the design suitable for use in a number of applications including qubit-coupled mechanical systems and circuit QED.
Superconducting microwave resonators based on coplanar waveguides (CPWs), which allow for on-chip implementation, have a wide variety of uses, from the coupling of qubits to the detection of photons from interstellar clouds. With the integration of a bias circuitry, the already versatile resonator will become even more so. However, applying a DC voltage or current bias to the resonator without significantly degrading its quality factor Q is no trivial task. One of the first superconducting microwave resonators with the ability to allow for the application of a DC bias, made use of symmetry points. In order to not rely on those symmetry points, our group came up with a different design that used a shunted capacitor instead. However, this design needs a more extensive and in particular more complicated fabrication process. As the groups focus is now completely away from the original design, we go back in this thesis, to honestly evaluate such a resonator by applying a bias to its center. We present a design with the cavity length l to be a full wavelength (l = λ) terminated on both ends to ground, fed in by a capacitively coupled AC-feedline at λ/4 and a DC-bias line at its center (λ/2). After the first fabrication trial we were unable to successfully judge the performance of the microwave resonator as it suffered from bad internal loss, leading to a low internal Q ~ 300 at 4 K. The only thing that this showed us, is that its design is perhaps not as straightforward. On the other hand, interesting results specific to the design were obtained from QUCS simulations (using a combination of lumped and distributed elements). These simulations showed that the quality of the resonator is highly sensitive to the position of its galvanically connected bias line, with respect to the center of the cavity (voltage node). Where Qint drops down to 10 % of its maximum for < 0.1 mm (= l/320) away from the center. Furthermore, we saw that asymmetry in the ports causes the maximum Qint (voltage node) to be off-center by 9 μm, and that adding an extra feedline in symmetry to the existing one puts it back on-center. Demonstrating the voltage node susceptibility to asymmetries of the cavity mode. Despite the unpromising signs the results show, suggestions have been put forward about better isolation of the bias line that have the potential to still make the design attractive. While these isolations still baring the traits of a faster and less complex fabrication.
In many hybrid quantum systems, a superconducting circuit is required, which combines DC-control with a coplanar waveguide (CPW) microwave resonator. The strategy thus far for applying a DC voltage or current bias to microwave resonators has been to apply the bias through a symmetry point in such a way that it appears as an open circuit for certain frequencies. Here, we introduce a microwave coupler for superconducting CPW cavities in the form of a large shunt capacitance to ground. Such a coupler acts as a broadband mirror for microwaves while providing galvanic connection to the center conductor of the resonator. We demonstrate this approach with a two-port λ/4-transmission resonator with linewidths in the MHz regime (Q∼103) that shows no spurious resonances and apply a voltage bias up to 80 V without affecting the quality factor of the resonator. This resonator coupling architecture, which is simple to engineer, fabricate, and analyse, could have many potential applications in experiments involving superconducting hybrid circuits.
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.
We report a technique for applying a dc voltage or current bias to the center conductor of a high-quality factor superconducting microwave cavity without significantly disturbing selected cavity modes. This is accomplished by incorporating dc bias lines into the cavity at specific locations. The measured S-matrix parameters of the system are in good agreement with theoretical predictions and simulations. We find that at 4 K the quality factor of the cavity degrades by less than 1% under the application of a dc bias.
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.