Lattice-Renormalized Tunneling Models for Superconducting Qubit Materials

TL;DR

This paper introduces a lattice-renormalized formalism for modeling tunneling two-level systems in superconducting qubit materials, improving accuracy in predicting TLS behavior and interactions with lattice phonons.

Contribution

The authors develop a novel lattice-renormalized formalism derived from the nuclear Hamiltonian, enabling better modeling of TLS in superconducting materials compared to previous methods.

Findings

Bound experimental tunnel splittings for hydrogen-based TLS.

Identified strong anharmonic phonon couplings affecting TLS.

Provided insights into defect-induced decoherence in qubits.

Abstract

We present a lattice-renormalized formalism for configurational tunneling two-level systems (TLS) that overcomes limitations of minimum-energy-path and light-particle models. Derived from the nuclear Hamiltonian, our formulation introduces composite phonon coordinates to capture lattice distortions between degenerate potential wells. This approach resolves deficiencies in prior models and enables accurate computation of tunnel splittings and excitation spectra for hydrogen-based TLS in bcc Nb. Our results bound experimental tunnel splittings and reveal strong anharmonic couplings between tunneling atoms and lattice phonons, establishing a direct link between TLS dynamics and phonon-mediated strain interactions. The formalism further generalizes to multi-level systems (MLS), providing insight into defect-induced decoherence in superconducting qubits and guiding strategies for materials design to suppress TLS-related loss.