  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.
