  Simulating a network of Ising spins with physical systems is now emerging as
a promising approach for solving mathematically intractable problems. Here we
report a large-scale network of artificial spins based on degenerate optical
parametric oscillators (DOPO), paving the way towards a photonic Ising machine
capable of solving difficult combinatorial optimization problems. We generated
>10,000 time-division-multiplexed DOPOs using dual-pump four-wave mixing (FWM)
in a highly nonlinear fibre (HNLF) placed in a fibre cavity. Using those DOPOs,
a one-dimensional (1D) Ising model was simulated by introducing
nearest-neighbour optical coupling. We observed the formation of spin domains
and found that the domain size diverged near the DOPO threshold, which suggests
that the DOPO network can simulate the behavior of low-temperature Ising spins.
