Physics Colloquium: Scanning Tunneling Microscopy and Spectroscopy of Defects in Superconducting Qubits

Prof. Maria Iavarone, Temple University

Abstract: Superconducting qubits are among the most promising platforms for quantum computing technologies given their fidelity and scalability. Over the past two decades, the quantum coherence of superconducting qubits has increased by orders of magnitude because of improvements in qubit design, fabrication methods, and materials quality. However, the microscopic mechanisms of qubit decoherence are still poorly understood. Non-equilibrium quasiparticles and two-level systems still contribute to decoherence. It is believed that these sources of noise are found in atomic scale defects and at different interfaces. Further progress requires deeper understanding and mitigating the underlying mechanisms. As part of the Superconducting Quantum Materials and Systems (SQMS), we have undertaken a coordinated approach using state-of-the-art materials characterization techniques to identify defects in superconducting qubits.

Scanning Tunneling Microscopy and Spectroscopy (STM/STS) allows to map electronic properties of materials down to atomic scale. In this talk I will focus on the characterization of Nb films used in superconducting qubits. Two main film interfaces can host possible sources of decoherence and determine the performance in its applications. Our studies show that the superconducting properties, grain size and type of defects in the films are affected by the interface with the substrate. Furthermore, a few nanometers thick native complex oxide film exists on the Nb surface. Our studies of this interface using different surface treatments such ultra-high vacuum annealing and Ar sputtering show how the quasiparticle density of states of the films at the top interface is affected by defects located in the top oxide layers and offers pathways to its mitigation.