  A two-site Ising model is implemented as an injection-locked laser network
consisting of a single master laser and two mutually coupled slave lasers. We
observed ferromagnetic and antiferromagnetic orders in the in-phase and
out-of-phase couplings between the two slave lasers. Their phase difference is
locked to either 0 or p even if the coupling path is continuously modulated.
The system automatically selects the oscillation frequency to satisfy the
in-phase or out-of-phase coupling condition, when the mutual coupling dominates
over the injection-locking by the master laser.
