Quantum annealing of a well-defined frustrated magnet

Abstract

Quantum annealing, which involves quantum mechanical tunnelling among possible solutions, has state-of-the-art applications not only in quickly finding the lowest-energy configuration of a complex system [1, 2], but also in quantum computing [3, 4]. Despite recent progress in quantum simulators, processors, and algorithms [5, 9], well-defined materials that exhibit many-body quantum annealing phenomena are still unavailable in the real world. Here we report a single-crystal study of spin dynamics and heat-transport properties of the frustrated magnet α-CoV2O6, consisting of a triangular arrangement of ferromagnetic Ising spin chains without evident structural disorder [10-12]. We observe apparent quantum annealing phenomena resulting from time-reversal symmetry breaking in a tiny transverse field. For instance, below～ 1 K and in a longitudinal magnetic field of～2 T, the system exhibits no indication of approaching the lowest-energy state for at least 15 hours in zero transverse field, but quickly converges towards that configuration with a nearly temperature-independent relaxation time of ~10 seconds in a transverse field of ~3.5 mK. Our large-scale classical and quantum Monte Carlo simulations show good agreement with the experimental results, and suggest that a tiny transverse field can profoundly enhance quantum spin fluctuations around domain walls, triggering rapid quantum annealing process from metastable Kosterlitz-Thouless phases, at low temperatures.