Combinatorial optimization problems can be mapped onto ground-state-search problems of the Ising model, which can be solved efficiently with the artificial spin system. We realized a coherent Ising machine based on networked 2048 degenerate optical parametric oscillators to solve the Ising problem. The coherent Ising machine could obtain good approximate solutions for MAX-CUT problems on 2000 node graphs. (see page 40)
(left) Figure shows the structure of a G39 graph, which is a scale-free graph with 2000 nodes and 11778 edges. (right) A solution to the MAX-CUT problem on the G39 graph obtained with the coherent Ising machine in 5 ms. The red and blue nodes correspond to up and down spin states of the Ising model, respectively. The green lines show the edges between the divided subsets.