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Bulletin of the American Physical Society

Decoupling a superconducting qubit from dielectric loss and other sources of linear dissipation

Yen‐Hsiang Lin, BaoLong Nguyen, Nick Grabon, Jon Migue, Vladimir Manucharyan

Published Mar 15, 2017• 0 citations
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Quantum processors using superconducting qubits suffer from dielectric loss leading to noise and dissipation. Qubits are usually designed as large capacitor pads connected to a non-linear Josephson junction (or SQUID) by a superconducting thin metal wiring. Here, we report on finite-element simulation and experimental results confirming that more than 50% of surface loss in transmon qubits can originate from Josephson junctions wiring and can limit qubit relaxation time. We experimentally extracted dielectric loss tangents of qubit elements and showed that dominant surface loss of wiring can occur for real qubits designs. Finally, we experimentally demonstrate up to 20% improvement in qubit quality factor by wiring design optimization.
In a world transitioning to the era of second quantum revolution, avoiding processes such as dissipation, which annihilates the quantum state unnecessarily, is crucial. Therefore, understanding the sources of loss responsible for the relaxation in a given quantum system is highly relevant. Since superconducting quantum circuits are the favorite contenders for quantum computing, two directions are followed in this work to study the sources of loss in such circuits.1.Superconducting high quality factor resonators have several applications in quantum technology. They are also the best test beds to study losses. In this work, the study of losses is carried out by realizing Aluminum resonators of different mean free path and capacitances. The behavior of the internal quality factor as a function of applied microwave power and temperature is investigated. The study focuses on the quasiparticle and dielectric losses in the resonators which have been argued to be the main cause of losses in superconducting quantum circuits. A comparative analysis of ground plane loss is also carried out in order to estimate the losses caused by the metallic ground plane. 2.Fluxonium qubit is one of the state of the art superconducting qubits. Their relaxation times are as high as 8-10 milliseconds in 3D architecture. To achieve large scalability, the qubits must be fabricated using 2D architecture. The change in architecture places new constraints on traditional sources of loss while also introducing new ones. The sources of loss causing qubit relaxation in 2D Fluxonium qubit are the area of emphasis in this work. Furthermore, the causes of noise in such a qubit are examined in order to understand the limitations on the coherence of the qubit.
Josephson junctions form the essential non-linearity for almost all superconducting qubits. The junction is formed when two superconducting electrodes come within ∼1 nm of each other. Although the capacitance of these electrodes is a small fraction of the total qubit capacitance, the nearby electric fields are more concentrated in dielectric surfaces and can contribute substantially to the total dissipation. We have developed a technique to experimentally investigate the effect of these electrodes on the quality of superconducting devices. We use λ/4 coplanar waveguide resonators to emulate lumped qubit capacitors. We add a variable number of these electrodes to the capacitive end of these resonators and measure how the additional loss scales with the number of electrodes. We then reduce this loss with fabrication techniques that limit the amount of lossy dielectrics. We then use these techniques for the fabrication of Xmon qubits on a silicon substrate to improve their energy relaxation times by a factor of 5.