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Systematic Improvements in Transmon Qubit Coherence enabled by Niobium Surface Encapsulation



Mustafa BAL, Akshay Murthy, Francesco Crisa, Shaojiang Zhu, Florent Lecocq, Joe Aumentado, Joel Ullom, Pete Hopkins


We present a novel transmon qubit fabrication technique that yields systematic improvements in T1 coherence times. In this study, the devices are fabricated using a novel 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 3 different capping materials and different film substrates across different qubit foundries definitively demonstrates the detrimental impact that niobium oxides have on the coherence times of superconducting qubits, compared to oxides of other materials such as tantalum, aluminum or titanium nitride. The results highlight that surface encapsulated niobium qubit devices exhibit T1 coherence times 2 to 5 times longer than baseline niobium qubit devices with native niobium oxides. When capping niobium with tantalum, we obtain average qubit lifetimes above 200 microseconds. We also present a comparative structural and chemical analysis that suggests that the highly amorphous niobium suboxides potentially induce higher losses. These results are in line with high accuracy measurements of the niobium oxide loss tangent via the use of ultra-high Q superconducting radiofrequency (SRF) cavities. This new surface encapsulation strategy opens the door to further reduce dielectric losses via passivation with ambient-stable materials, while preserving fabrication and volume manufacturability thanks to the compatibility with silicon processes.
PRX Quantum


qubit, quantum information, quantum computing, superconductor, josephson junction, niobium


Bal, M. , Murthy, A. , Crisa, F. , Zhu, S. , Lecocq, F. , Aumentado, J. , Ullom, J. and Hopkins, P. (2023), Systematic Improvements in Transmon Qubit Coherence enabled by Niobium Surface Encapsulation, PRX Quantum, [online], (Accessed May 21, 2024)


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Created June 14, 2023, Updated September 8, 2023