Controllable Coupling in Phase-Coupled Flux Qubits

We propose a scheme for tunable coupling of phase-coupled flux qubits. The phase-coupling scheme can provide a strong coupling strength of the order of Josephson coupling energy of Josephson junctions in the connecting loop, while the previously studied inductive coupling scheme cannot provide due to small mutual inductance and induced currents. We show that, in order to control the coupling, we need {\it two} dc-SQUID's in the connecting loop and the control fluxes threading the dc-SQUID's must be in {\it opposite} directions. The coupling strength is analytically calculated as a function of the control flux at the co-resonance point.Comment: version to appear in Phys. Rev.

Strong Coupling of a Cavity QED Architecture for a Current-biased Flux Qubit

We propose a scheme for a cavity quantum electrodynamics (QED) architecture for a current-biased superconducting flux qubit with three Josephson junctions. The qubit operation is performed by using a bias current coming from the current mode of the circuit resonator. If the phase differences of junctions are to be coupled with the bias current, the Josephson junctions should be arranged in an asymmetric way in the qubit loop. Our QED scheme provides a strong coupling between the flux qubit and the transmission line resonator of the circuit.Comment: 5 pages, 3 figure

Macroscopic Many-Qubit Interactions in Superconducting Flux Qubits

Superconducting flux qubits are considered to investigate macroscopic many-qubit interactions. Many-qubit states based on current states can be manipulated through the current-phase relation in each superconducting loop. For flux qubit systems comprised of $N$ qubit loops, a general expression of low energy Hamiltonian is presented in terms of low energy levels of qubits and macroscopic quantum tunnelings between the many-qubit states. Many-qubit interactions classified by {\em Ising type- or tunnel-}exchange interactions can be observable experimentally. Flux qubit systems can provide various artificial-spin systems to study many-body systems that cannot be found naturally.Comment: 5 pages, 1 figur