Identification of Two-Level Systems in Superconducting Qubits from Experimental Characterizations to Theoretical Calculations

Superconducting qubits are promising for quantum computing, but their performance is often affected by decoherence caused by two-level system (TLS) defects. One critical source of these defects is the interface between the superconducting metal layer and the silicon substrate. Potential causes of TLS defects at this interface include atomic steps, grain boundaries from the metal layer, atomic mixing, or strain fields linked to these microstructures. However, these kinds of defects are difficult to observe directly because of their size, which is at the sub-nanometer scale.

We investigated the interface between the aluminum superconducting layer and the silicon substrate in a Josephson junction using aberration-corrected scanning transmission electron microscopy (STEM) and 3D structure refinement. The Fully Automated Nanoscale To Atomistic Structure from Theory and eXperiments (FANTASTX) software determines atomic structures for agreement with experiment and low total energy computed by density functional theory (DFT). The relaxed, experiment-consistent atomic models were interrogated for TLS defects, several of which were identified at the Al/Si interface. The structural character, density, and energy splitting of these defects will be discussed.

This work was supported by DOE DE-SC0020313. Partially prepared by LLNL under Contract DE-AC52-07NA27344. MKYC & VSCK acknowledge the support from the BES SUFD Early Career award. Work performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.