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

We propose an effective method to produce four-qubit χ-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.