“…Simpler schemes, where tripartite entanglement exists in the three output fields directly from above threshold optical parametric oscillator (OPO) [13] or type II SHG [14,15], have been predicted and experimentally confirmed [16]. Quadripartite entanglement generated from cascaded second-harmonic generation [17] and in an opto-mechanical system are also suggested [18]. However, for quadripartite entanglement, to our knowledge, quasiperiodic optical superlattice is as an important candidate [19,20].…”