Observation of giant two-level systems in a granular superconductor

TL;DR

This study investigates two-level systems in granular aluminum superconducting films, revealing strongly coupled TLS with unusually large electric dipole moments, especially in higher resistivity films, impacting quantum circuit applications.

Contribution

The paper provides the first experimental evidence of large dipole moment TLS in granular superconductors and explores their dependence on film resistivity and disorder.

Findings

Large electric dipole moments up to 30 eA detected in TLS.

Higher resistivity films show more large dipole moment TLS.

Insights into TLS behavior in granular superconductors for quantum applications.

Abstract

Disordered thin films are a common choice of material for superconducting, high impedance circuits used in quantum information or particle detector physics. A wide selection of materials with different levels of granularity are available, but, despite low microwave losses being reported for some, the high degree of disorder always implies the presence of intrinsic defects. Prominently, quantum circuits are prone to interact with two-level systems (TLS), typically originating from solid state defects in the dielectric parts of the circuit, like surface oxides or tunneling barriers. We present an experimental investigation of TLS in granular aluminum thin films under applied mechanical strain and electric fields. The analysis reveals a class of strongly coupled TLS having electric dipole moments up to 30 eA, an order of magnitude larger than dipole moments commonly reported for solid state defects. Notably, these large dipole moments appear more often in films with a higher resistivity. Our observations shed new light on granular superconductors and may have implications for their usage as a quantum circuit material.