Decoherence of floating qubits due to capacitive coupling

TL;DR

This paper reveals that capacitive coupling to bias leads can significantly cause decoherence in floating qubits, challenging previous assumptions, and suggests design strategies to mitigate this effect for improved coherence times.

Contribution

It demonstrates that capacitive coupling to bias leads is a major decoherence source in floating qubits and proposes symmetric coupling as a mitigation strategy.

Findings

Capacitive coupling to bias leads limits qubit coherence.

Symmetric coupling design improves relaxation times.

Experimental data supports the theoretical model.

Abstract

It has often been assumed that electrically floating qubits, such as flux qubits, are immune to decoherence due to capacitive coupling. We show that capacitive coupling to bias leads can be a dominant source of dissipation, and therefore of decoherence, for such floating qubits. Classical electrostatic arguments are sufficient to get a good estimate of this source of relaxation for standard superconducting qubit designs. We show that relaxation times can be improved by designing floating qubits so they couple symmetrically to the bias leads. Observed coherence times of flux qubits with varying degrees of symmetry qualitatively support our results.