  Many developments in science and engineering depend on tackling complex
optimizations on large scales. The challenge motivates intense search for
specific computing hardware that takes advantage from quantum features,
nonlinear dynamics, or photonics. A paradigmatic optimization problem is
finding low-energy states in classical spin systems with fully-random
interactions. To date no alternative computing platform can address such
spin-glass problems on a large scale. Here we propose and realize an optical
scalable spin-glass simulator based on spatial light modulation and multiple
light scattering. By tailoring optical transmission through a disordered
medium, we optically accelerate the computation of the ground state of large
spin networks with all-to-all random couplings. Scaling of the operation time
with the problem size demonstrates optical advantage over conventional
computing. Our results point out optical vector-matrix multiplication as a tool
for spin-glass problems and provide a general route towards large-scale
computing that exploits speed, parallelism and coherence of light.
