Scientists have shown that a tiny electric voltage applied beneath a single magnetic atom can change its spin direction by acting on the electrons of the surrounding metal surface. A theoretical reciprocity relation links the torque on the spin to how the local electron density changes when the spin rotates, turning the electric field drive into a property that can be measured.

The torque produced by a local potential ρ(R) is given by τ(R)=v∂n(R;ˆm)/∂ˆm, where v is the applied voltage, n the electron density and ˆm the spin direction. The relation predicts that only spins with specific symmetry can be driven; spin‑½ atoms and the central transition of half‑integer spins remain “dark”.

On a normal metal the same hybridization that provides the electric‑field drive also fast‑relaxes the spin, leading to broad linewidths. On a superconductor the energy gap protects the spin: the hybridization does not limit coherence, and the substrate‑induced linewidth drops below 0.1 MHz while a modest RF voltage (tens of mV) generates Rabi frequencies of several MHz with a sharp STM tip.

However, the method requires substantial spin‑orbit coupling and an orbital‑selective couplings between the tip and the atom. Without these, or for point‑contact coupling, the drive vanishes. Moreover, when the Yu–Shiba–Rusinov (YSR) bound state lies close to the superconducting gap (0.2–0.5 Δ), the substrate‑limited linewidth is small enough for coherent control, but nearby quasiparticle poisoning can still limit coherence.

These findings highlight that strong coupling to the substrate, normally a source of decoherence, becomes useful for electrically controlling atomic spins when the surface is superconducting and the spin–orbit interaction is present.