Improvement of the superconducting qubit lifetime with the surface passivation technology

It is well known that the two-level systems (TLSs) existing in the amorphous surface/interface oxides of the metal pads are the main decoherence source of the superconducting quantum devices. There are a few ways to remove the surface oxides in order to improve the microwave loss; One of the straightforward methods is to encapsulate the metal pads with another low loss thin metal, such as Ta or TiN, which has less oxidation in the atmosphere. In this work, we present our measurements on the Nb-based superconducting transmon qubits with different surface passivation technologies. We clearly show the improvement of the qubit coherence if carefully choosing the encapsulating materials, which is indicative that the surface TLS losses are effectively suppressed with encapsulation. We benchmark the qubit energy relaxation and discuss the instabilities of the TLSs resonating near the qubit frequency.