Evidence for coherent quantum phase-slips across a Josephson junction array

TL;DR

This paper demonstrates the experimental observation of coherent quantum phase-slips across a Josephson junction array, revealing their impact on energy level shifts and linewidths in a superconducting circuit, influenced by the Aharonov-Casher effect.

Contribution

The study provides the first experimental evidence of coherent quantum phase-slips in a Josephson junction array and analyzes their effects on circuit energy levels and dephasing.

Findings

Resolved phase-slip induced linewidths of about 100 kHz.

Observed flux-dependent dephasing in fluxonium qubit.

Confirmed interference effects due to the Aharonov-Casher effect.

Abstract

Superconducting order in a sufficiently narrow and infinitely long wire is destroyed at zero temperature by quantum fluctuations, which induce $2\pi$ slips of the phase of the order parameter. However, in a finite-length wire coherent quantum phase-slips would manifest themselves simply as shifts of energy levels in the excitations spectrum of an electrical circuit incorporating this wire. The higher the phase-slips probability amplitude, the larger are the shifts. Phase-slips occurring at different locations along the wire interfere with each other. Due to the Aharonov-Casher effect, the resulting full amplitude of a phase-slip depends on the offset charges surrounding the wire. Slow temporal fluctuations of the offset charges make the phase-slips amplitudes random functions of time, and therefore turn energy levels shifts into linewidths. We experimentally observed this effect on a long Josephson junction array acting as a "slippery" wire. The slip-induced linewidths, despite being only of order 100 kHz, were resolved from the flux-dependent dephasing of the fluxonium qubit.