Observation of giant two-level systems in a granular superconductor

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.

arXiv comment14 pages, 9 figures

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• Spin Environment of a Superconducting Qubit in High Magnetic Fields
• Quantum Coherence in Superconducting Vortex States
• Properties of two level systems in current-carrying superconductors
• Tuning of superconducting properties with disorder in NbxSn nanocrystalline thin films
• Non-equilibrium Dynamics of Two-level Systems directly after Cryogenic Alternating Bias
• Stacked Josephson junctions for quantum circuit applications