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Hans Mooij, Kavli Institute of Nanoscience, Delft University of Technology and KITP, Santa Barbara
Quantum tunneling of fluxoids (vortices without flux quantization) is possible with superconducting loops that contain small Josephson junctions. The flux qubit is a loop with three junctions that is magnetically biased at half a flux quantum. A quantum superposition occurs of two macroscopic classical states which differ by one fluxoid in the loop and which have opposite circulating current. The flux qubit has good coherence relative to the quantum operation time. Readout is by means of a SQUID detector, quantum non-demolition measurement with high fidelity has been achieved. Flux qubits are coupled by inductive coupling; the interaction is strong enough that a controlled-not gate can straightforwardly be implemented. Very strong coupling to a harmonic oscillator can be realized.
Fluxoids moving in two-dimensional Josephson junction arrays are bosonic quantum particles that move in a band. By tailoring the potential, it was possible to observe Anderson as well as Mott localization in one dimension. Interacting flux qubits can, in a natural way, be arranged into two-dimensional arrays that can be used for quantum simulations. The potential of such schemes will be explored.