The origin of anomalous non-linear microwave absorption in Josephson junction qubits: mysterious nature of two level systems or their dynamic interaction?

TL;DR

This paper investigates the cause of anomalous microwave absorption in Josephson qubits, challenging the idea of TLS interactions and proposing a power law distribution of TLS dipole moments as an explanation.

Contribution

It demonstrates that TLS dynamic interactions cannot explain the observed loss tangent behavior and introduces a power law distribution model for TLS dipole moments as a novel explanation.

Findings

TLS interactions do not account for the observed loss tangent dependence.

A power law distribution of TLS dipole moments explains the anomalous absorption.

The model aligns with recent measurements and theoretical TLS formation models.

Abstract

Quantum two-level systems (TLSs) commonly found at low temperature in amorphous and disordered materials are responsible for decoherence in superconducting Josephson junction qubits particularly because they absorb energy of coherent qubit oscillations in the microwave frequency range. In planar Josephson resonators with oxide interfaces this absorption is characterized by an anomalously weak loss tangent dependence on the field in the non-linear regime that conflicts with the theoretical expectations and the observations in amorphous dielectrics. It was recently suggested that this anomalous absorption is due to TLS dynamic interactions. Here we show that such interactions cannot lead to the observed loss-tangent field dependence and suggest the alternative explanation assuming that TLS dipole moments $p$ are distributed according to the specific power law $P(p) \propto 1/p^{3-\eta}$ ($0\leq \eta <1$). This assumption, indeed, results in the observed loss tangent behavior. The hypothesis of a power law distribution is supported both by the recent measurements of individual TLS dipole moments and the theoretical model of TLS formation due to the long-range dipole-dipole interaction, thus connecting the anomalous absorption with the possible solution of the long-standing problem of the nature of TLSs.