Four-qubit device with mixed couplings
  We present the first experimental results on a device with more than two superconducting qubits. The circuit consists of four three-junction flux qubits, with simultaneous ferro- and antiferromagnetic coupling implemented using shared Josephson junctions. Its response, which is dominated by the ground state, is characterized using low-frequency impedance measurement with a superconducting tank circuit coupled to the qubits. The results are found to be in excellent agreement with the quantum-mechanical predictions. 
M. Grajcar, A. Izmalkov, S.H.W. van der Ploeg, S. Linzen, T. Plecenik,   Th. Wagner, U. Huebner, E. Il'ichev, H.-G. Meyer, A...