  We show theoretically that multipartite entanglement is generated on a
massive scale in the spectrum, or optical frequency comb, of a single optical
parametric oscillator (OPO) emitting well above threshold. In this system, the
quantum dynamics of the strongly depleted pump field are responsible for the
onset of the entanglement by correlating the two-mode squeezed,
bipartite-entangled pairs of OPO signal fields. (Such pairs are independent of
one another in the undepleted, classical pump approximation.) We verify the
multipartite nature of the entanglement by evaluating the van Loock-Furusawa
criterion for a particular set of entanglement witnesses deduced from physical
considerations.
