Surface Engineering by Atomic Layer Deposition and heat treatment for high coherence superconducting qubits

Over the years, coherence times for superconducting qubits have seen great progress. However, the fabrication of sufficiently reliable qubit circuits for practical quantum computing requires extending the quantum coherence lifetimes. These lifetimes are mainly limited by dissipations caused by two-level system (TLS) defects found in the amorphous native oxide of the superconducting film and neighboring dielectric regions. In this context, our work aims to increase the coherence time of superconducting qubits by reducing the density of two-level system (TLS) defects present in the native oxides covering the superconductor's surface. To achieve this, we propose an original approach that combines the passivation of the superconductor’s surface with films deposited by Atomic Layer Deposition (ALD), which inherently have lower TLS defect densities, and thermal treatments designed to dissolve the initially present lossy native oxides. This surface engineering approach has already demonstrated its effectiveness in reducing TLS defects in 3D niobium resonators. By applying an ALD-deposited Al₂O₃ film followed by thermal treatment to a 1.3 GHz niobium resonator, we significantly extended their photon lifetimes. This experiment resulted in an almost native oxide-free surface while the ALD-deposited Al₂O₃ film successfully protected the Nb surface from re-oxidation [1,2]. In a more recent study [3], using a well-tailored thermal treatment, we were able to create an air-stable nano-crystalline NbO layer on top of the niobium surface, resulting in a very high quality factor (~ 10¹¹) at the TLS dominated regime, which correlates with a more than ten times lower TLS density on the niobium surface. This passivation layer can result in a tenfold enhancement of coherence time if applied on to a niobium superconducting qubits. These preliminary results highlight the potential of our method to mitigate surface-related losses and enhance coherence lifetimes of superconducting qubits.