Systematic Improvements in Transmon Qubit Coherence Enabled by Niobium Surface Encapsulation

• DOI: 10.48550/arxiv.2304.13257
• Publication Date: 2023-01-01
• Authors: Mustafa Bal; Akshay A. Murthy; Shaojiang Zhu; Francesco Crisa; Xinyuan You; Ziwen Huang; Tanay Roy; Jaeyel Lee; David Van Zanten; Roman Pilipenko; Ivan Nekrashevich; Daniel Bafia; Yulia Krasnikova; Cameron Kopas; Ella Lachman; Duncan Miller; Josh Mutus; Matthew J. Reagor; Hilal Cansizoglu; Jayss Marshall; David P. Pappas; Kim Vu; Kameshwar Yadavalli; Jin‐Su Oh; Lin Zhou; Matthew J. Kramer; Florent Lecocq; Dominic P. Goronzy; Carlos G. Torres‐...; G. C. Pritchard; Vinayak P. Dravid; James M. Rondinelli; Michael J. Bedzyk; Mark C. Hersam; J. F. Zasadzinski; Jens Koch; J. A. Sauls; Alexander Romanenko; Anna Grassellino
• Keywords: Niobium nitride; Transmon; Passivation

We present a novel transmon qubit fabrication technique that yields systematic improvements in T$_1$ relaxation times. fabricate devices using an encapsulation strategy involves passivating the surface of niobium and thereby preventing formation its lossy oxide. By maintaining same superconducting metal only varying structure, this comparative investigation examining different capping materials, such as tantalum, aluminum, titanium nitride, gold, film substrates across foundries definitively demonstrates detrimental impact oxides have on coherence times qubits, compared to native aluminum or nitride. Our surface-encapsulated exhibit 2 5 longer than baseline with oxides. When we obtain median lifetimes above 300 microseconds, maximum values up 600 represent highest date for qubits prepared both sapphire silicon. structural chemical analysis suggests why amorphous may induce higher losses other These results are line high-accuracy measurements oxide loss tangent obtained ultra-high Q radiofrequency (SRF) cavities. This new enables even further reduction dielectric via passivation ambient-stable while preserving scalable manufacturability thanks compatibility silicon processes.