Against all expectations, researchers control the magnetic core of a single atom

Controlling the Magnetic Core of a Single Atom

In a surprising breakthrough, a team from Delft University of Technology demonstrated direct manipulation of the nuclear spin—the magnetic core—of an individual atom. Using a refined double magnetic resonance technique combined with a scanning tunneling microscope (STM), they were able to flip the nucleus’s spin with radio-frequency signals, even when the electron and nuclear spins were largely disentangled. This finding challenges the conventional view that nuclear spins are immune to the electrical fields generated by an STM and opens new avenues for quantum information storage.

From Electron Spins to Nuclear Spins

Magnetism in everyday metals such as iron arises from the collective alignment of electron spins. Within each atom, however, both electrons and the nucleus possess intrinsic angular momentum, or "spin," that behaves like tiny compass needles. While electron spins are relatively easy to detect and control, nuclear spins are far weaker and traditionally considered more isolated from external perturbations. The Delft group, led by Sander Otte, has long used STM‑based techniques to probe electron spins at the single‑atom level. Their latest work extends this capability to the nucleus itself.

Double Magnetic Resonance on a Single Atom

The researchers adapted an established method called double magnetic resonance, which normally addresses ensembles of spins, for use on a solitary atom. By delivering two synchronized radio-frequency signals—one tuned to the electron spin transition and another to the nuclear spin transition—they could target each spin independently. The nuclear‑spin signal operates at frequencies a hundred to a thousand times lower than the electron‑spin signal, akin to playing a low‑pitched string on a guitar while the higher‑pitched string represents the electron.

When the team applied both signals simultaneously, they observed that the nuclear spin could be flipped even though the hyperfine interaction (the coupling between electron and nucleus) was minimal. "According to our simple understanding, this should not really happen," Otte noted, emphasizing that the nucleus should not respond to the STM’s electric field under conventional models.

Implications for Quantum Information

Nuclear spins are attractive candidates for quantum bits (qubits) because their weak interaction with the environment makes them exceptionally stable—an essential property for long‑term quantum memory. Direct control, as demonstrated here, could enable the writing and reading of quantum information stored in a nucleus without disturbing neighboring electron spins. While the underlying mechanism that allows the STM’s radio waves to affect the nucleus remains unresolved, the experimental proof‑of‑concept suggests a new toolbox for quantum technologies.

Looking Ahead

The Delft experiment raises more questions than it answers. Future work will need to clarify how the STM’s electric field couples to the nuclear spin and whether similar control can be achieved in other atomic species or solid‑state platforms. As Otte puts it, "Back to the drawing board, then— that is really how science works. There is always something new left to figure out."

For now, the ability to address the magnetic core of a single atom stands as a striking reminder that even the most well‑studied quantum systems can still surprise us, and that the frontier of quantum control continues to expand beyond expectations.