We investigate the interplay of superradiant phase transition (SPT) and energy band physics in an extended Dicke-Hubbard lattice whose unit cell consists of a Dicke model coupled toan atomless cavity. We found in such a periodic lattice the critical point that occurs in a single Dicke model becomes a critical region that is periodically changing with the wavenumber k. In the weak-coupling normal phase of the system we observed a flat band and its corresponding localization that can be controlled by the ground-state SPT. Our work builds the connection between flat band physics and SPT, which may fundamentally broaden the regimes of many-body theory and quantum optics.
We investigate theoretically the ground-state property of a two-dimensional array of superconducting circuits including the on-site superconducting qubits (SQs) with weak anharmonicity.In particular, we analyse the influence of this anharmonicity on the Mott insulator to superfluid quantum phase transition. The complete ground-state phase diagrams are presented under the mean field approximation. Interestingly, the anharmonicity of SQs affects the Mott lobes enormously, and the single excitation Mott lobe disappears when the anharmonicity become zero. Our results can be used to guide the implementations of quantum simulations using the superconducting circuits, which have nice integrating and flexibility.
We propose how to realize high-fidelity quantum storage using a hybrid
quantum architecture including two coupled flux qubits and a nitrogen-vacancy
center ensemble (NVE). One of theflux qubits is considered as the quantum
computing processor and the NVE serves as the quantum memory. By separating the
computing and memory units, the influence of the quantum computing process on
the quantum memory can be effectively eliminated, and hence the quantum storage
of an arbitrary quantum state of the computing qubit could be achieved with
high fidelity. Furthermore the present proposal is robust with respect to
fluctuations of the system parameters, and it is experimentally feasibile with
currently available technology.
We propose an experimentally realizable hybrid quantum circuit for achieving
a strong coupling between a spin ensemble and a transmission-line resonator via
a superconducting flux qubitused as a data bus. The resulting coupling can be
used to transfer quantum information between the spin ensemble and the
resonator. More importantly, in contrast to the direct coupling without a data
bus, our approach requires far less spins to achieve a strong coupling between
the spin ensemble and the resonator (e.g., 3 to 4 orders of magnitude less).
This drastic reduction of the number of spins in the ensemble can greatly
improve the quantum coherence of the spin ensemble. This proposed hybrid
quantum circuit could enable a long-time quantum memory when storing
information in the spin ensemble.