Abstract
We present a new way to make Ising machines, i.e., using networks of coupled self-sustaining nonlinear oscillators. Our scheme 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., adding noise, and turning subharmonic locking on and off smoothly) 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. 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 present working hardware prototypes using CMOS electronic oscillators.
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- 1.
Moreover, as we show in Sect. 3.4, our scheme is inherently resistant to variability even without such calibration.
- 2.
More generally, \(c_{ij}\)s can be any \(2\pi \)-periodic odd functions, which are better suited to practical oscillators.
- 3.
In the Ising Hamiltonian (1), \(J_{ij}\) is only defined when \(i<j\); here we assume that \(J_{ij} = J_{ji}\) for all i, j.
- 4.
More generally, we can use \(\{2k\pi ~|~k\in \mathbf {Z}\}\) and \(\{2k\pi +\pi ~|~k\in \mathbf {Z}\}\) to represent the two states for each oscillator’s phase.
- 5.
The G-set problems are available for download as set1 at http://www.optsicom.es/maxcut.
- 6.
G1\(\sim \)21 are of size 800; G22\(\sim \)42 are of size 2000; G43\(\sim \)47, G51\(\sim \)54 are of size 1000; G48\(\sim \)50 are of size 3000.
- 7.
Their results and runtime are available for download at http://www.optsicom.es/maxcut in the “Computational Experiences” section.
- 8.
Ising machines can be used on general graph colouring problems, and this four-colouring problem is chosen here for illustrative purposes. Four-colouring a planar graph is actually not NP-hard and there exist polynomial-time algorithms for it [46].
- 9.
Hawaii and Alaska are considered adjacent such that their colours will be different in the map.
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Acknowledgements
The authors would like to thank the reviewers for the useful comments and in particular anonymous reviewer No. 2 for pointing us to Ercsey-Ravasz/Toroczkai and Yin’s work on designing dynamical systems to solve NP-complete problems.
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Wang, T., Roychowdhury, J. (2019). OIM: Oscillator-Based Ising Machines for Solving Combinatorial Optimisation Problems. In: McQuillan, I., Seki, S. (eds) Unconventional Computation and Natural Computation. UCNC 2019. Lecture Notes in Computer Science(), vol 11493. Springer, Cham. https://doi.org/10.1007/978-3-030-19311-9_19
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