  Time-multiplexed Spinwave Ising Machines (SWIMs) have unveiled a route
towards miniaturized, low-cost, and low-power solvers of combinatorial
optimization problems. While the number of supported spins is limited by the
nonlinearity of the spinwave dispersion, other collective excitations, such as
surface acoustic waves (SAWs), offer a linear dispersion. Here, we demonstrate
an all-to-all, fully FPGA reprogrammable, 50-spin surface acoustic wave-based
Ising machine (SAWIM), using a 50-mm-long Lithium Niobate SAW delay line,
off-the-shelf microwave components, and a low-cost FPGA. The SAWIM can solve
any 50-spin MAX-CUT problem, with arbitrary coupling matrices, in less than 340
$\mu$s consuming only 0.62 mJ, corresponding to close to 3000 solutions per
second and a figure of merit of 1610 solutions/W/s. We compare the SAWIM
computational results with those of a 100-spin optical Coherent Ising machine
and find a higher probability of solution. Moreover, we demonstrate that there
is an optimum overall coupling strength between spins at which the probability
of the exact solution reaches 100%. The SAWIM illustrates the general merits of
solid state wave-based time-multiplexed Ising machines in the microwave domain
as versatile platforms for commercially feasible high-performance solvers of
combinatorial optimization problems.
