New approach brings scientists one step closer to practical quantum computing MCS Menu Quantum computers have arisen as a possible solution for highly complex mathematical problems, offering “quantum advantage” over classical computers in certain cases. The Quantum Approximate Optimization Algorithm (QAOA) is a leading candidate for realizing this advantage, and some success has been achieved for small problems. But demonstrations on large problems have remained too computationally costly to run on classical computers and current quantum hardware. Researchers from JPMorganChase and the U.S. Department of Energy’s (DOE) Argonne National Laboratory have now found a way to make such calculations possible. Their results were published in Physical Review Letters. Overcoming the computational barrier The team focused on the Sherrington–Kirkpatrick (SK) model, a disordered system with random interactions between every pair of variables. “As problem size grows, the SK model’s optimal value is known in the large-system limit. This value provides a baseline to measure how well QAOA is performing,” said Jeffrey Larson, a computational mathematician in Argonne’s Mathematics and Computer Science division and a coauthor of the paper. The researchers discovered that in the infinite-size limit, a high-depth QAOA state for the SK model converges mathematically to a single quantum spin coupled to bosonic modes. By translating the problem into this “spin-boson” system, the team replaced the costly calculations with much simpler simulations using matrix product state simulations. For this problem setting, Larson developed a novel optimization method for identifying high-quality QAOA parameters. The simulations and parameter optimization were performed on the supercomputers at the Argonne Leadership Computing Facility (ALCF) and DOE’s National Energy Research Scientific Computing Center. The researchers were able to use these facilities through a grant from the DOE’s Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. The ALCF is a DOE Office of Science