  The three essential pillars of magnetic data storage devices are readability,
writeability, and stability. However, these requirements compete as magnetic
domain sizes reach the fundamental limit of single atoms and molecules. The
proven magnetic bistability of individual holmium atoms on magnesium oxide
appeared to operate within this magnetic trilemma, sacrificing writeability for
unprecedented stability. Using the magnetic stray field created by the tip of a
spin-polarized scanning tunneling microscope (SP-STM), we controllably move the
Ho state into the quantum regime, allowing us to write its state via the
quantum tunneling of magnetization (QTM). We find that the hyperfine
interaction causes both the excellent magnetic bistability, even at zero
applied magnetic field, and the avoided level crossings which we use to control
the magnetic state via QTM. We explore how to use such a system to realize a
high-fidelity single atom NOT gate (inverter). Our approach reveals the
prospect of combining the best traits of the classical and quantum worlds for
next generation data storage.
