Generation of energy-entangled W states via parametric fluorescence in   integrated devices
  Tripartite entangled states, such as GHZ and W states, are typically generated by manipulating two pairs of polarization-entangled photons in bulk optics. Here we propose a scheme to generate W states that are entangled in the energy degree of freedom in an integrated optical circuit. Our approach employs photon pairs generated by spontaneous four-wave mixing (SFWM) in a micro- ring resonator. We also present a feasible procedure for demonstrating the generation of such a state, and we compare polarization-entangled and energy-entangled schemes for the preparation of W states. 
M. Menotti, L. Maccone, J. E. Sipe, and M. Liscidini
quant-ph