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Controllable coupling of superconducting flux qubits

S. H. W. van der Ploeg, A. Izmalkov, Alec Maassen van den Brink, U. Huebner, M. Grajcar, E. Il'ichev, H. -G. Meyer, A. M. Zagoskin

cond-mat.supr-conarXiv:cond-mat/0605588

Abstract

We have realized controllable coupling between two three-junction flux qubits by inserting an additional coupler loop between them, containing three Josephson junctions. Two of these are shared with the qubit loops, providing strong qubit--coupler interaction. The third junction gives the coupler a nontrivial current--flux relation; its derivative (i.e., the susceptibility) determines the coupling strength J, which thus is tunable in situ via the coupler's flux bias. In the qubit regime, J was varied from ~45 (antiferromagnetic) to ~ -55 mK (ferromagnetic); in particular, J vanishes for an intermediate coupler bias. Measurements on a second sample illuminate the relation between two-qubit tunable coupling and three-qubit behavior.

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