An Approximate Parallel Annealing Ising Machine for Solving Traveling Salesman Problems

Annealing-based Ising machines have emerged as high-performance solvers for combinatorial optimization problems (COPs). As a typical COP with constraints imposed on the solution, traveling salesman problems (TSPs) are difficult to solve using conventional methods. To address this challenge, we design an approximate parallel annealing Ising machine (APAIM) based on an improved parallel annealing algorithm. In this design, adders are reused in the local field accumulator units (LAUs) with half-precision floating-point representation of the coefficients in the Ising model. The momentum scaling factor is approximated by a linear, incremental function to save hardware. To improve the solution quality, a buffer-based energy calculation unit selects the best solution among the found candidate results in multiple iterations. Finally, approximate adders are applied in the design for improving the speed of accumulation in the LAUs. The design and synthesis of a 64-spin APAIM show the potential of this methodology in efficiently solving complicated constrained COPs.