title: Controllable coupling of superconducting flux qubits.
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 approximately 45 (antiferromagnetic) to approximately -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.
authors.affiliation: Institute for Physical High Technology, P.O. Box 100239, D-07702 Jena, Germany.
authors.forename: S H W
authors.initials: SH
authors.lastname: van der Ploeg
authors.forename: A
authors.initials: A
authors.lastname: Izmalkov
authors.forename: Alec Maassen
authors.initials: AM
authors.lastname: van den Brink
authors.forename: U
authors.initials: U
authors.lastname: Hübner
authors.forename: M
authors.initials: M
authors.lastname: Grajcar
authors.forename: E
authors.initials: E
authors.lastname: Il'ichev
authors.forename: H-G
authors.initials: HG
authors.lastname: Meyer
authors.forename: A M
authors.initials: AM
authors.lastname: Zagoskin