Permutation asymmetry inducing entanglement between degrees of freedom   in multiphoton states
  We describe and examine entanglement between different degrees of freedom in multiphoton states based on the permutation properties. From the state description, the entanglement comes from the permutation asymmetry. According to the different permutation properties, the multiphoton states can be divided into several parts. It will help to deal with the multiphoton interference, which can be used as the measurement of the entanglement. 
F.W. Sun, B.H. Liu, C.W. Wong, G.C. Guo