Ultrastrong coupling in circuit quantum electrodynamics systems not only provides a platform to study the quantum Rabi model, but it can also facilitate the implementation of quantumlogic operations via high-lying resonator states. In this regime, quantum manifolds with different excitation numbers are intrinsically connected via the counter-rotating interactions, which can result in multi-photon processes. Recent experiments have demonstrated ultrastrong coupling in superconducting qubits electromagnetically coupled to superconducting resonators. Here we report the experimental observation of multiphoton sideband transitions of a superconducting flux qubit coupled to a coplanar waveguide resonator in the ultrastrong coupling regime. With a coupling strength reaching about 10% of the fundamental frequency of the resonator, we obtain clear signatures of higher-order red-sideband transitions and the first-order blue-sideband transition in a transmission spectroscopic measurement. This study advances the understanding of driven ultrastrongly-coupled systems.
We develop a theory for the quantum circuit consisting of a superconducting loop interrupted by four Josephson junctions and pierced by a magnetic flux (either static or time-dependent).In addition to the similarity with the typical three-junction flux qubit, we demonstrate the difference of the four-junction circuit from its three-junction analogue, especially its distinct advantages over the latter. Moreover, the four-junction circuit in the phase regime is also investigated. Our theory provides a tool to explore the physical properties of this four-junction superconducting circuit.
We study a hybrid quantum system consisting of spin ensembles and superconducting flux qubits, where each spin ensemble is realized using the NV centers in a diamond crystal and thenearestneighbor spin ensembles are effectively coupled via a flux qubit. We show that the coupling strengths between flux qubits and spin ensembles can reach the strong and even ultrastrong coupling regimes by either engineering the hybrid structure in advance or tuning the excitation frequencies of spin ensembles via external magnetic fields. When extending the hybrid structure to an array with equal coupling strengths, we find that in the strong coupling regime, the hybrid array is reduced to a tight-binding model of a 1D bosonic lattice. In the ultrastrong coupling regime, it exhibits quasi-particle excitations separated from the ground state by an energy gap. Moreover, these quasiparticle excitations and the ground state are stable under a certain condition which is tunable via the external magnetic field. This may provide an experimentally accessible method to probe the instability of the system.