High-temperature quantum coherence in a rare-earth spin chain

At high temperatures, quantum effects are generally considered unimportant, giving way to classical behavior. In magnetic systems, when thermal energies exceed the interaction strength between atomic magnetic moments, the spins typically become uncorrelated, resulting in classical paramagnetism. This thermal decoherence of quantum spins is a major hindrance to quantum information applications of spin systems. Remarkably, our neutron scattering experiments on Yb chains in an insulating perovskite crystal defy these conventional expectations. We observe a sharply defined spinon continuum, a hallmark of fractionalized excitations in one-dimensional quantum magnets, persisting to temperatures well above the energy scale of Yb-Yb interactions. The observed sharpness of the spinon continuum's dispersive upper boundary indicates a spinon mean free path exceeding ≈ 35 inter-atomic spacings at temperatures more than an order of magnitude above the interaction energy scale. We thus discover an important and highly unique quantum behavior, which expands the realm of quantumness to high temperatures where entropy-governed classical behaviors were previously believed to dominate. These results have profound implications for spin systems in quantum information applications operating at finite temperatures and inspire new developments in quantum metrology.