A Resource Efficient Source of Multi-photon Polarization Entanglement
  Current photon entangling schemes require resources that grow with the photon number. We present a new approach that generates quantum entanglement between many photons, using only a single source of entangled photon pairs. The different spatial modes, one for each photon as required by other schemes, are replaced by different time slots of only two spatial modes. States of any number of photons are generated with the same setup, solving the scalability problem caused by the previous need for extra resources. Consequently, entangled photon states of larger numbers than before are practically realizable. 
E. Megidish, T. Shacham, A. Halevy, L. Dovrat, and H. S. Eisenberg
quant-ph