For the development of multi-qubit registers, an essential ingredient is coupling _tunability_. In particular, if the qubit-qubit interaction can be tuned continuously between two values with opposite signs, it can be naturally switched off--a great advantage for quantum-gate proposals. Conversely, the ability to have both ferro- and antiferromagnetic couplings is crucial for implementing the Ising-glass Hamiltonians naturally arising in adiabatic quantum computing. Elaborating on proposals originally made for Josephson charge qubits, one coupling strategy is to replace the inductance between two superconducting flux qubits by an effective ("quantum") inductance, which is tunable via an external flux bias. |
I will discuss two experiments which have realized this idea, one with small-area (three-junction) qubits coupled through shared Josephson junctions, and one involving magnetically coupled large-area (rf-SQUID) flux qubits. Both provide a clear demonstration of sign tunability; in addition, the former experiment is manifestly in the qubit regime, while the latter features a high degree of control, with individually tunable tunneling barriers and a dedicated readout SQUID for each qubit.