Solving combinatorial optimisation problems using oscillator based Ising machines

We present OIM (Oscillator Ising Machines), a new way to make Ising machines using networks of coupled self-sustaining nonlinear oscillators. OIM is theoretically rooted in a novel result that establishes that the phase dynamics of coupled oscillator systems, under the influence of subharmonic injection locking, are governed by a Lyapunov function that is closely related to the Ising Hamiltonian of the coupling graph. As a result, the dynamics of such oscillator networks evolve naturally to local minima of the Lyapunov function. Two simple additional steps (i.e., turning subharmonic locking on and off smoothly, and adding noise) enable the network to find excellent solutions of Ising problems. We demonstrate our method on Ising versions of the MAX-CUT and graph colouring problems, showing that it improves on previously published results on several problems in the G benchmark set. Using synthetic problems with known global minima, we also present initial scaling results. Our scheme, which is amenable to realisation using many kinds of oscillators from different physical domains, is particularly well suited for CMOS IC implementation, offering significant practical advantages over previous techniques for making Ising machines. We report working hardware prototypes using CMOS electronic oscillators.