The magnetic quantum states of holmium single atom magnets on MgO(100) have proven extremely robust when exposed to high magnetic fields and temperatures up to 35 K. Here we address the stability of Ho at small magnetic fields, where the hyperfine interaction creates several avoided level crossings. Using spin-polarized scanning tunneling microscopy, we demonstrate quantum state control via Landau-Zener tunneling and stable magnetization at zero field. Our observations indicate a total spin ground state of $J_z=\pm 8$. Combined quantum and classical control render Ho a promising qubit candidate.

Figure (a) Schematic of SP-STM tip (green) centered above a Ho atom on an MgO thin film on Ag(100) (STM image recorded at $I_t=104$ pA, $V=-130$ mV, white scale bar 2 nm). We read the holmium's magnetic state, Up or Down, before and after its evolution by switching it at a tunnel voltage above the switching threshold $V_2>V_t=73$ mV (left and right panels). (b) Current-time traces showing two-state switching ($I_{\mathrm{set}}=100$ pA, $V_2=-130$ mV, $T=4.7$ K, feedback open). When at tunnel distances, the antiferromagnetic tip exposes the atom to a stray field, $B_0$; when sufficiently retracted the field is zero. In the example shown, the Up state is preserved for 16 min at $V_1<V_t$ and $B=B_0$.

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