Illustration: Right panel adapted from .
(Left) Schematic representation of the frequency comb (OFC) generated in an optical parametric oscillator (OPO), showing modes 1–12 (out of millions). The OPO contains a nonlinear crystal which has been engineered to couple multiple pump and OFC modes simultaneously, such that pumping the OPO at different frequencies (red and green) entangles pairs of modes in the OFC, as indicated. The simple graphs below show how each of the modes is entangled, e.g., 1 is connected to 12 by the green pump, but 12 is connected to 7 by the red pump, which in turn connects back to 6. (Right) A toroidal cluster state, with substructure, encoded in an optical frequency comb and universal for quantum computation. Circled region at left shows four macro-nodes, each of which contains four physical nodes (magnified view at right), showing the extensive amount of entanglement that is possible with optical frequency combs in an optical parameter oscillator