Endohedral fullerenes trapping atoms and molecules with magnetic and electric moments have a potential to be used for quantum computing. A C₆₀ fullerene trapping a Li⁺ cation has an electric monopole moment, and forms a rock-salt-type cubic crystal, [Li@C₆₀](PF₆). The dielectric response and quantum motion of the Li⁺ cations in the crystal at low temperature were revealed by the dielectric and X-ray diffraction measurements [1]. The Li⁺ cations are moving inside the C₆₀ cages by the hopping and tunneling motion between the potential wells under 20 hexagons above 100 K. The Li⁺ motion is suppressed to be localized at two equivalent positions on the threefold inversion axis with decreasing temperature below 100 K. The simultaneous occupation of the Li⁺ cation at the two positions is mainly achieved by the tunneling motion. The hopping motion is slowing down and hindered by the energy barriers between the wells. The tunneling motion is also hindered below T_C = 24 K by the antiferroelectric interactions among local electric dipole moments formed between the Li⁺ cations inside and the PF₆⁻ anions outside the C₆₀ cages. The tunneling motion and intermolecular interaction of the Li⁺ cation found in [Li@C₆₀](PF₆) show that Li⁺@C₆₀ has a potential to be used as a qubit in a quantum computer using electric dipole moments.

[1] S. Aoyagi, A. Tokumitu, K. Sugimoto, H. Okada, N. Hoshino, and T. Akutagawa, J. Phys. Soc. Jpn. 85, 094605 (2016)