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arXiv:1101.4218 · Applied Physics Letters

Introduction of a dc bias into a high-Q superconducting microwave cavity

Fei Chen, A. J. Sirois, Raymond W. Simmonds, A. J. Rimberg

Published Mar 28, 2011• 48 citations
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Abstract

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.

DOI
10.1063/1.3573824

Related papers

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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.

arXiv:1508.05770•Applied Physics Letters•Nov 9, 2015•17 citations
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.
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.
We report a technique that can noninvasively add multiple DC wires into a 3D superconducting microwave cavity for electronic devices that require DC electrical terminals. We studied the influence of our DC lines on the cavity performance systematically. We found that the quality factor of the cavity is reduced if any of the components of the electrical wires cross the cavity equipotential planes. Using this technique, we were able to incorporate a quantum dot (QD) device into a 3D cavity. We then controlled and measured the QD transport signal using the DC lines. We have also studied the heating effects of the QD by the microwave photons in the cavity.
We report on a technique for applying a DC bias in a 3D microwave cavity. We achieve this by isolating the two halves of the cavity with a dielectric and directly using them as DC electrodes. As a proof of concept, we embed a variable capacitance diode in the cavity and tune the resonant frequency with a DC voltage, demonstrating the incorporation of a DC bias into the 3D cavity with no measurable change in its quality factor at room temperature. We also characterize the architecture at millikelvin temperatures and show that the split cavity design maintains a quality factor Qi ∼ 8.8 × 105, making it promising for future quantum applications.
Abstract Three-dimensional microwave waveguide cavities are essential tools for many cavity quantum electrodynamics experiments. However, the need to control quantum emitters with dc magnetic fields inside the cavity often limits such experiments to normal-conducting cavities with relatively low quality factors of about 10 4 . Similarly, controlling quantum emitters with dc electric fields in normal- and superconducting waveguide cavities has so far been difficult, because the insertion of dc electrodes has strongly limited the quality factor. Here, we present a method to apply dc electric fields within a superconducting waveguide cavity, which is based on the insertion of dc electrodes at the nodes of the microwave electric field. Moreover, we present a method to apply dc magnetic fields within the same cavity by trapping the magnetic flux in holes positioned in facing walls of the cavity. We demonstrate that the TE 301 mode of such a superconducting, rectangular cavity made from niobium maintains a high internal quality factor of Q int ≃ 1.7 × 10 6 at the few photon level and a base temperature of 3 K. A cloud of Rydberg atoms coupled to the microwave electric field of the cavity is used to probe the applied dc electric and magnetic fields via the quadratic Stark effect and the Zeeman effect, respectively.
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.
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.