Systematic Improvements in Transmon Qubit Coherence Enabled by Niobium Surface Encapsulation
arXiv (Cornell University)(2023)
摘要
We present a novel transmon qubit fabrication technique that yields
systematic improvements in T_1 relaxation times. We fabricate devices using
an encapsulation strategy that involves passivating the surface of niobium and
thereby preventing the formation of its lossy surface oxide. By maintaining the
same superconducting metal and only varying the surface structure, this
comparative investigation examining different capping materials, such as
tantalum, aluminum, titanium nitride, and gold, and film substrates across
different qubit foundries definitively demonstrates the detrimental impact that
niobium oxides have on the coherence times of superconducting qubits, compared
to native oxides of tantalum, aluminum or titanium nitride. Our
surface-encapsulated niobium qubit devices exhibit T_1 relaxation times 2 to
5 times longer than baseline niobium qubit devices with native niobium oxides.
When capping niobium with tantalum, we obtain median qubit lifetimes above 300
microseconds, with maximum values up to 600 microseconds, that represent the
highest lifetimes to date for superconducting qubits prepared on both sapphire
and silicon. Our comparative structural and chemical analysis suggests why
amorphous niobium oxides may induce higher losses compared to other amorphous
oxides. These results are in line with high-accuracy measurements of the
niobium oxide loss tangent obtained with ultra-high Q superconducting
radiofrequency (SRF) cavities. This new surface encapsulation strategy enables
even further reduction of dielectric losses via passivation with ambient-stable
materials, while preserving fabrication and scalable manufacturability thanks
to the compatibility with silicon processes.
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关键词
transmon qubit coherence,niobium
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