Gradiometric flux qubits with tunable gap
For gradiometric three-Josephson-junction flux qubits, we perform a
systematic study on the tuning of the minimal transition frequency, the
so-called qubit gap. By replacing one of the qubit’s Josephson junctions by a
dc SQUID, the critical current of this SQUID and, in turn, the qubit gap can be
tuned in situ by a control flux threading the SQUID loop. We present
spectroscopic measurements demonstrating a well-defined controllability of the
qubit gap between zero and more than 10 GHz. In the future, this enables one to
tune the qubit into and out of resonance with other superconducting quantum
circuits, while operating the qubit at its symmetry point with optimal
dephasing properties. The experimental data agree very well with model
calculations based on the full qubit Hamiltonian. From a numerical fit, we
determine the Josephson coupling and the charging energies of the qubit
junctions. The derived values agree well with those measured for other
junctions fabricated on the same chip. We also demonstrate the biasing of
gradiometric flux qubits near the symmetry point by trapping an odd number of
flux quanta in the gradiometer loop. In this way, we study the effect of the
significant kinetic inductance, thereby obtaining valuable information for the
qubit design.