Static flux bias of a flux qubit using persistent current trapping

Qubits based on the magnetic flux degree of freedom require a flux bias, the stability and precision of which strongly affect the qubit performance, up to a point of forbidding the qubit operation. Moreover, in 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 which can be detrimental for qubit coherence. The trapping procedure allows us 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.