Temperature is a measure of the relative motion between many atoms.
It is not defined for single atoms, nor does it make much sense for even small numbers of atoms. It is a statistical measure of the kinetic energy of large ensemble of atoms such as what you would find in a gas. Even in a crystal lattice, the atoms jiggle about their lattice sites in an uncontrollable manner. Absolute zero is, therefore, not a physically attainable state for an ensemble of atoms for which temperature is still a good statistical measure of their kinetic energy. There would always be an irreducible internal energy of the gas due to its interaction with the physical vacuum.