In the course of the project two time-multiplexed Ising-machine (IM) concepts were developed, preserving the strengths of Coherent Ising Machines (dense connectivity, fast time-to-solution parameter) while key limitations to miniaturisation and thermal stability were removed. The first IM was designed with propagating spinwaves. For this, a ring oscillator was built, in which artificial spins are represented with spin-wave RF pulses propagating in a 10-mm-long 5-μm-thick Yttrium Iron Garnet (YIG) film and having the phases 0 and 180 degrees relative to the reference signal. A ring oscillator comprises a phase-sensitive amplifier which binarises the phases. The couplings are implemented by delayed pulse injection, following the CIM time-multiplexing principle. The prototype supports an 8-spin MAX-CUT instance and finds solutions in less than 4 μs with approximately 7 μJ energy per run. This proof-of-concept established the basic operation, quantified the dispersion-limited spin capacity of a single track, and outlined a CMOS-compatible scaling path (hundreds to ~10^5 spins via multi-track waveguides and integrated amplification).
To overcome the spin-wave dispersion and limits of YIG, surface acoustic wave (SAW)-based ring oscillator with intrinsically linear dispersion and high thermal stability was developed. The system implements a fully programmable, all-to-all 50-spin Ising machine using a commercially available SAW delay line, microwave phase-sensitive amplification, and an FPGA measurement-and-feedback block analogous to that used in state-of-the-art CIMs. Single-run compute time is 10 ms; total power is 1.82 W; energy per solution is 18.2 mJ; and the figure of merit reaches 55 solutions/s/W. Thanks to operation at 320 MHz and short acoustic delay line the thermal stability is improved by 4–5 orders of magnitude relative to optical CIMs and by 1–2 orders relative to SWIM, enabling stable room-temperature operation without precision thermostats or phase locked loop (PLL) systems.
For system characterization, a comprehensive benchmarking was performed with 170 random 50-spin MAX-CUT instances (BiqMac) across densities 0.1–0.9 with 500 runs per instance. For 99%-accurate solutions, success probabilities are high, with an optimum-coupling operating point yielding approximately 84% average success at density 0.5 and reducing the 99% time-to-solution from 597 ms to 25 ms. Exact-solution rates are comparable to reported 100-spin CIM behaviour, with notably stronger performance at high graph density.
