Creating permanent single atom magnets and possibly single atom qubits

The magnetization of a single surface adsorbed atom is described by the occupation of its magnetic quantum states. In the absence of an external magnetic field, and in the presence of the crystal field created by the binding of the atom to its nearest neighbors, typically two of these states are degenerate, e.g., the spin up and the spin down state, and the atom is in a superposition of both. From this consideration it is clear that it is far from trivial to find adsorbate/substrate systems for which one of these states can be conserved for a macroscopic time, i.e., for which magnetic information storage in the smallest unit of matter is possible. We have identified 3 such systems using the rare earth atoms Ho and Dy, and will report on the conditions under which the atoms exhibit stable magnetic quantum states. A fundamental understanding of their surprising stability boils down to knowing the mechanisms driving their relaxation. These are electron and phonon scattering, but also the magnetic coupling of the 5d 6s levels to the 4f electrons. We close by discussing perspectives for single atom qubits with long magnetic coherence times.