  Although magnetism is a quantum phenomenon, its energy scale can be strong
enough to sustain technological usage at room temperature and above.
Temperature, however, leads to the decoherence which hampers quantum
information applications. Common wisdom dictates that when magnets are heated
to temperatures where thermal energy is higher than the interactions among
atomic magnetic moments, the latter become uncorrelated and classical
paramagnetic behavior is recovered. Surprisingly, our neutron scattering
experiments on Yb chains in an insulating perovskite crystal overturn this
conventional picture. We find a sharply defined spectrum of spinon excitations,
which are the fractional elementary excitations in spin-1/2 chains, that
persists to temperatures where thermal energy is more than 20 times higher than
the energy of interactions between Yb magnetic moments, $J$. The observed
sharpness of the spinon continuum boundary indicates that spinon mean free path
exceeds $\approx 35$ inter-atomic spacings at temperatures markedly above the
magnetic energy scale. Our observations reveal an interesting and highly
unusual quantum behavior in a rare earth magnet, which challenges quantum
metrology and has profound implications for spin qubit systems in quantum
information applications operating at finite temperatures.
