Tunable gradiometric flux qubits in circuit-QED experiments

Gradiometric flux qubits with tunable minimal transition frequency have strong potential for scalable quantum circuits as well as for the realisation of various coupling schemes between qubits and coplanar waveguide resonators. We have systematically studied the tuning of the minimal transition frequency, the so called qubit gap, of single qubits depending on fabrication parameters. In addition we investigate how the special geometry of these qubits allows us to switch between the ultrastrong-coupling regime and negligible coupling strength. We discuss the application of tunable gradiometric flux qubits in establishing strong σx coupling between a resonator mode and the qubit enabling the simulation of relativistic quantum systems.