Realization of an n-qubit controlled-U gate with superconducting quantum   interference devices or atoms in cavity QED
  We propose an approach to realize an $n$-qubit controlled-$U$ gate with superconducting quantum interference devices (SQUIDs) in cavity QED. In this approach, the two lowest levels of a SQUID represent the two logical states of a qubit while a higher-energy intermediate level serves the gate manipulation. Our method operates essentially by creating a single photon through one of the control SQUIDs and then performing an arbitrary unitrary transformation on the target SQUID with the assistance of the cavity photon. In addition, we show that the method can be applied to implement an $n $% -qubit controlled-$U$ gate with atomic qubits in cavity QED. 
Chui-Ping Yang, Siyuan Han