Static flux bias of a flux qubit using persistent current trapping

TL;DR

This paper presents a method for stabilizing and independently controlling flux bias in superconducting qubits using on-chip persistent current trapping, avoiding external magnetic fields and resistive elements.

Contribution

It introduces a persistent current trapping technique for flux biasing that enhances stability and independence in multiqubit systems without compromising coherence.

Findings

Successful implementation of on-chip persistent current trapping for flux bias

Controlled stepwise adjustment of the trapped current

Ability to disconnect from external sources after bias setting

Abstract

Qubits based on the magnetic flux degree of freedom require a flux bias, whose stability and precision strongly affect the qubit performance, up to a point of forbidding the qubit operation. Moreover, in the perspective of multiqubit systems, it must be possible to flux-bias each qubit independently, hence avoiding the traditional use of externally generated magnetic fields in favour of on-chip techniques that minimize cross-couplings. The solution discussed in this paper exploits a persistent current, trapped in a superconducting circuit integrated on chip that can be inductively coupled with an individual qubit. The circuit does not make use of resistive elements that can be detrimental for the qubit coherence. The trapping procedure allows to control and change stepwise the amount of stored current; after that, the circuit can be completely disconnected from the external sources. We show in a practical case how this works and how to drive the bias circuit at the required value.