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arXiv:1409.3245 · IEEE Transactions on Applied Superconductivity

Copper Waveguide Cavities With Reduced Surface Loss for Coupling to Superconducting Qubits

Daniela F. Bogorin, Douglas McClure, Matthew Ware, B. L. T. Plourde

Published Jun 30, 2014• 5 citations
arXivHTMLPDF

Abstract

Significant improvements in superconducting qubit coherence times have been achieved recently with 3-D microwave waveguide cavities coupled to transmon qubits. While many of the measurements in this direction have utilized superconducting aluminum cavities, other recent work has involved qubits coupled to copper cavities with coherence times approaching 0.1 ms. The copper provides a good path for thermalizing the cavity walls and qubit chip, although the substantial cavity loss makes conventional dispersive qubit measurements challenging. We are exploring various approaches for improving the quality factor of 3-D copper cavities, including electropolishing and coating with superconducting layers of tin. We have characterized these cavities on multiple cooldowns and found tin plating to be robust. In addition, we have performed coherence measurements on transmon qubits in these cavities and observed promising performance.

DOI
10.1109/tasc.2014.2330522

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Significant improvements in superconducting qubit coherence times have been achieved recently with 3-D microwave waveguide cavities coupled to transmon qubits. While many of the measurements in this direction have utilized superconducting aluminum cavities, other recent work has involved qubits coupled to copper cavities with coherence times approaching 0.1 ms. The copper provides a good path for thermalizing the cavity walls and qubit chip, although the substantial cavity loss makes conventional dispersive qubit measurements challenging. We are exploring various approaches for improving the quality factor of 3-D copper cavities, including electropolishing and coating with superconducting layers of tin. We have characterized these cavities on multiple cooldowns and found tin plating to be robust. In addition, we have performed coherence measurements on transmon qubits in these cavities and observed promising performance.
The recent implementation of three-dimensional microwave cavities coupled to superconducting transmon qubits has led to dramatic improvements in qubit coherence times [1]. Besides the superconducting aluminum cavities that have been used in many such measurements, other recent experiments have utilized copper cavities [2] with coherence times now approaching 0.1 ms. We are investigating the effects on the quality factor for three-dimensional copper cavities that have the cavity-wall surfaces electropolished and coated with a superconducting tin layer. The copper base provides a good path for thermalizing the cavity walls and qubit chip, while the surface treatment reduces the microwave loss. We have measured a superconducting transmon qubit in such a cavity and we report our low-temperature coherence results on the same qubit and cavity measured in two different labs.
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Niobium and aluminum have been widely used to realize superconducting resonators and qubits. However, the native oxide at the metal air interface limits the devices results. This layer is highly disordered, and it induces losses limiting the performances of the resonators or qubits. Recently, tantalum led to great improvement in qubit relaxation time. This improvement has been attributed to a thinner and less disordered oxide layer compared to Nb and Al. This study is aimed to limit the formation of the oxide layer, by the introduction of novel superconducting materials such as rhenium or by encapsulating the Nb surface with other superconducting materials such as Ta, Al, Re or other non-superconducting materials such as Au. To better understand the results, we are realizing these studies on coplanar waveguide resonators where the relation between surface losses and quality factor is clear. We are also analyzing different fabrication techniques to completely characterize our devices, linking material properties to the performances. In this work we are also varying the participation ratio of the devices in order to extract with higher fidelity and precision the losses introduced by the involved interfaces.
State-of-the-art transmon qubits rely on large capacitors which systematically improves their coherence due to reduced surface loss participation. However, this approach increases both the footprint and the parasitic cross-coupling and is ultimately limited by radiation losses - a potential roadblock for scaling up quantum processors to millions of qubits. In this work we present transmon qubits with sizes as low as 36$ \times 393939 μmmm^2$ with $\gtrsim$100 nm wide vacuum gap capacitors that are micro-machined from commercial silicon-on-insulator wafers and shadow evaporated with aluminum. After the release in HF vapor we achieve a vacuum participation ratio up to 99.6\% in an in-plane design that is compatible with standard coplanar circuits. Qubit relaxation time measurements for small gaps with high vacuum electric fields of up to 22 V/m reveal a double exponential decay indicating comparably strong coupling to long-lived two-level-systems (TLS). The exceptionally high selectivity of $>$20 dB to the superconductor-vacuum surface allows to precisely back out the sub-single-photon dielectric loss tangent of aluminum oxide exposed to ambient conditions. In terms of future scaling potential we achieve a qubit quality factor by footprint area of 20μs−220 μ\mathrm{s}^{-2}20μs−2, which is on par with the highest T1T_1T1​ devices relying on larger geometries and expected to improve substantially for lower loss superconductors like NbTiN, TiN or Ta.
Niobium offers the benefit of increased operating temperatures and frequencies for Josephson junctions, which are the core component of superconducting devices. However, existing niobium processes are limited by more complicated fabrication methods and higher losses than now-standard aluminum junctions. Combining recent trilayer fabrication advancements, methods to remove lossy dielectrics and modern superconducting qubit design, we revisit niobium trilayer junctions and fabricate all-niobium transmons using only optical lithography. We characterize devices in the microwave domain, measuring coherence times up to $62\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{s}$ and an average qubit quality factor above 105{10}^{5}105: much closer to state-of-the-art aluminum-junction devices. We find the higher superconducting gap energy also results in reduced quasiparticle sensitivity above 0.16 K, where aluminum-junction performance deteriorates. Our low-loss junction process is readily applied to standard optical-based foundry processes, opening alternative avenues for direct integration and scalability, and paves the way for higher-temperature and higher-frequency quantum devices.