  In recent years, superconducting qubits show great potential in quantum
computation. Hence, microwave photons become very interesting qubits for
quantum information processing assisted by superconducting quantum computation.
Here, we present the first protocol for the entanglement concentration on
microwave photons, resorting to the cross-Kerr effect in circuit quantum
electrodynamics (QED). Two superconducting transmission line resonators (TLRs)
coupled to superconducting molecule with the N-type level structure induce the
effective cross-Kerr effect for realizing the quantum nondemolition (QND)
measurement on microwave photons. With this device, we present a two-qubit
polarization parity QND detector on the photon states of the superconducting
TLRs, which can be used to concentrate efficiently the nonlocal non-maximally
entangled states of microwave photons assisted by several linear microwave
elements. This protocol has a high efficiency and it may be useful for
solid-state quantum information processing assisted by microwave photons.
