  Traditional magnetic sub-Kelvin cooling relies on the nearly free local
moments in hydrate paramagnetic salts, whose utility is hampered by the dilute
magnetic ions and low thermal conductivity. Here we propose to use instead
fractional excitations inherent to quantum spin liquids (QSLs) as an
alternative, which are sensitive to external fields and can induce a very
distinctive magnetocaloric effect. With state-of-the-art tensor-network
approach, we compute low-temperature properties of Kitaev honeycomb model. For
the ferromagnetic case, strong demagnetization cooling effect is observed due
to the nearly free $Z_2$ vortices via spin fractionalization, described by a
paramagnetic equation of state with a renormalized Curie constant. For the
antiferromagnetic Kitaev case, we uncover an intermediate-field gapless QSL
phase with very large spin entropy, possibly due to the emergence of spinon
Fermi surface. Potential realization of topological excitation cooling in
Kitaev materials is also discussed, which may offer a promising pathway to
circumvent existing limitations in the paramagnetic hydrates.
