Researchers at the Delft University of Technology (Netherlands) have demonstrated that the nuclear spin of a single atom can be directly switched using radio frequency signals applied through the tip of a scanning tunneling microscope. The experimental result came as a surprise: quantum physics has long held that when the coupling between nuclear and electron spins is weak, direct manipulation of the nucleus in this way is impossible. The scientists not only disproved this belief but also showed that nuclear spin switching persists even when quantum entanglement between the nuclear and electron spins practically vanishes.
How the Experiment is Set Up
The experimental setup relied on a modified scanning tunneling microscope (STM) capable of directing electrical signals to a single atom anchored on a surface beneath the tip. The researchers improved the double magnetic resonance method, which allows independent manipulation of two quantum objects: the electron spin and the nuclear spin located in its immediate vicinity. Previously, nuclear spin was studied primarily indirectly—via the hyperfine interaction linking it to the electron spin, leading to partial quantum entanglement of the respective states. Direct control of the nuclear spin without relying on this interaction was widely believed to require energy and precision unattainable by standard experimental setups.
Two Signals Simultaneously
The key step was the simultaneous application of two radio frequency signals: the first corresponded to the resonance frequency of the electron spin switching, and the second to that of the nuclear spin. The characteristic frequencies of the nuclear spin turned out to be 100 to 1,000 times lower than those of the electron spin, making the experiment technically challenging in its own right. Nevertheless, the scientists managed to record that the nuclear spin orientation switches even in configurations where the coupling to the electron spin is weakened to the extent that the states are largely no longer entangled. This observation is the main result of the work: the nucleus responds to the microscope's electrical signals contrary to expectations based on the weak hyperfine interaction model.
The Theoretical Question Remains Open
Why the nucleus still responds to external influence despite such weak coupling to the electron is a question the experimenters are not yet ready to answer. Developing a theoretical explanation for the mechanism that allows the radio frequency signal to reach the nuclear spin without the mediation of the electron will be a task for theorists in the coming months. According to the researchers, fully interpreting the results takes time: it requires both refining the experimental model and revising established views on the limits of controllability of nuclear spin states at the single-atom level.
Significance for Quantum Technologies
The practical value of this discovery for the development of quantum computing and quantum memory cannot be overstated. Nuclear spins are significantly less sensitive to environmental fluctuations than electron spins: they maintain coherence longer and are less prone to decoherence. This means that recording quantum information onto nuclear spins will be more reliable, and storage times will be longer. In addition, qubits based on nuclear spin states potentially allow for a lower error rate in quantum computing, which is one of the main obstacles to creating scalable quantum processors. However, before transitioning from fundamental experiments to engineering prototypes, the physics of the observed effect must be understood—and, according to the researchers themselves, this will take time.
