Over-100µs tunable planar transmons: epitaxial Josephson Junctions and design optimization*

Since superconducting qubits discovery over twenty years ago energy relaxation times has been improved by several orders. Josephson junctions, a crucial nonlinear component of superconducting qubits, are still fabricated from two aluminum polycrystalline electrodes with lossy amorphous aluminum oxide in between. Here we demonstrate our results in reducing two-level states (TLS) in amorphous oxides by means of Josephson Junctions optimization, thus reducing the participation of the lossy materials and interfaces. We carry out a comparative analysis of tunable X-mon qubit lifetimes based on various Josephson Junctions design types, demonstrating several times coherence improvement. From the other hand, we propose novel superconducting qubits fabrication technique based on Josephson junction epitaxial growth with inorganic masks. To experimentally test the proposed approaches we demonstrate tunable X-mon qubits with coherence over 100 microseconds.

*Devices were fabricated at the BMSTU Nanofabrication Facility (Functional Micro/Nanosystems, FMNS REC, ID 74300).