  Recently, the coherent Ising machine (CIM) as a degenerate optical parametric
oscillator (DOPO) network has been researched to solve Ising combinatorial
optimization problems. We formulate a theoretical model for the CIM with
discrete-time measurement feedback processes, and perform numerical simulations
for the simplest network, composed of two degenerate optical parametric
oscillator pulses with the anti-ferromagnetic mutual coupling. We evaluate the
extent to which quantum coherence exists during the optimization process.
