Enhanced coherence in superconducting circuits via band engineering

TL;DR

This paper demonstrates how band engineering with higher-harmonic Josephson elements can create flat bands and Dirac points in superconducting circuits, leading to noise-insensitive quantum states and improved coherence.

Contribution

It introduces the use of $ ext{cos}(2 heta)$ Josephson elements for tailored spectra with flat bands and Dirac points, enhancing coherence and noise robustness in superconducting qubits.

Findings

Flat bands enable noise-insensitive quantum states.

Band engineering can improve qubit coherence.

Proposed flux qutrit exhibits a spin-one Dirac spectrum.

Abstract

In superconducting circuits interrupted by Josephson junctions, the dependence of the energy spectrum on offset charges on different islands is $2e$ periodic through the Aharonov-Casher effect and resembles a crystal band structure that reflects the symmetries of the Josephson potential. We show that higher-harmonic Josephson elements described by a $\cos(2\varphi)$ energy-phase relation provide an increased freedom to tailor the shape of the Josephson potential and design spectra featuring multiplets of flat bands and Dirac points in the charge Brillouin zone. Flat bands provide noise-insensitive quantum states, and band engineering can help improve the coherence of the system. We discuss a modified version of a flux qubit that achieves in principle no decoherence from charge noise and introduce a flux qutrit that shows a spin-one Dirac spectrum and is simultaneously quote robust to both charge and flux noise.