Producing and Distinguishing x-Type Four-Qubit States in Flux Qubits

We propose an effective method to produce four-qubit x-type entangled states by using flux qubits coupled to an LC circuit which acts as a quantum data bus (QDB).In our scheme,the interaction is mediated by the exchange of virtual rather than real photons because of the large detuning between flux qubits and QDB,and then QDB-induced loss can be effectively avoided.The experimental feasibility of the scheme is also presented.