Compact vacuum gap transmon qubits: Selective and sensitive probes for superconductor surface losses

• URL: http://arxiv.org/abs/2206.14104v3
• Date: Mon, 11 Jul 2022 19:12:24 GMT
• Title: Compact vacuum gap transmon qubits: Selective and sensitive probes for superconductor surface losses
• Authors: M. Zemlicka, E. Redchenko, M. Peruzzo, F. Hassani, A. Trioni, S. Barzanjeh, J. M. Fink
• Abstract summary: State-of-the-art transmon qubits rely on large capacitors which systematically improves their coherence. We present transmon qubits with sizes as low as 36$ times $39$ mu$m$2$ with $gtrsim$100 nm wide vacuum gap capacitors.
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• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: State-of-the-art transmon qubits rely on large capacitors which systematically improves their coherence due to reduced surface loss participation. However, this approach increases both the footprint and the parasitic cross-coupling and is ultimately limited by radiation losses - a potential roadblock for scaling up quantum processors to millions of qubits. In this work we present transmon qubits with sizes as low as 36$ \times $39$ \mu$m$^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 $20 \mu \mathrm{s}^{-2}$, which is on par with the highest $T_1$ devices relying on larger geometries and expected to improve substantially for lower loss superconductors like NbTiN, TiN or Ta.

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