Quantum paraelectricity in the Kitaev quantum spin liquid candidates H3LiIr2O6 and D3LiIr2O6

H${}_{3}$LiIr${}_{2}$O${}_{6}$ shows no magnetic order down to lowest temperatures, raising hopes for a true Kitaev spin liquid. Its structure reveals H bonds between honeycomb layers and theory has proposed hydrogen disorder to affect the magnetic exchange. Using dielectric spectroscopy, the authors report here dipolar relaxations in H${}_{3}$LiIr${}_{2}$O${}_{6}$ and D${}_{3}$LiIr${}_{2}$O${}_{6}$, mirroring the dynamics of protons or deuterons within the double-well potentials of the H bonds. Under cooling, thermal activation crosses over into quantum tunneling, establishing a quantum paraelectric state with quasistatic disorder. This supports the realization of the long-pursued Kitaev spin liquid.