Physicists at Delft University of Technology’s Kavli Institute of Nanoscience have used a scanning tunneling microscope to independently control the spins of an electron and a nucleus within a single titanium atom. The team achieved direct nuclear magnetic resonance (NMR) control without relying on electron-nuclear spin hybridization, a result published in Nature Communications.
Delft researchers flip nuclear spins with radio waves
The experiment utilized an isolated 47Ti atom placed on bilayer MgO/Ag(100). By employing an Electron-Nuclear Double Resonance (ENDOR) scheme at an ultra-low temperature of 0.4 K, the team targeted an S = 1/2 electronic spin and an I = 5/2 nuclear spin. This configuration created a six-level system where the nuclear state could be manipulated independently of the electronic readout channel.
To isolate the nuclear spin, the researchers scaled magnetic fields between 200 mT and 1,400 mT. This high-field regime suppressed electron-nuclear hybridization by approximately 10 times, according to the study. While the electronic channel operated in the GHz range, the 47Ti nucleus was driven by local radio-frequency fields between 50 and 85 MHz.
Did you know? Atomic nuclei have magnetic moments roughly three orders of magnitude smaller than electrons. This makes them harder to manipulate but significantly better shielded from environmental noise.
Technical parameters of the single-atom ENDOR architecture
The research team used quadrupole splitting (Q = −2.8 ± 0.8 MHz) to lift transition degeneracy, which allowed them to address nuclear spin sublevels independently. The primary drive was an RF electric-field-induced oscillating magnetic field, removing the need for system-dependent hyperfine or quadrupole modulation.
| Specification | Experimental Value |
|---|---|
| Platform | Isolated 47Ti on bilayer MgO/Ag(100) |
| Magnetic Field (Bz) | 200 mT to 1.4 Tesla |
| Nuclear Drive Range | 50–85 MHz |
| Operating Temperature | 0.4 K |
Unexpected results challenge physics assumptions
The ability to flip the nuclear spin while it remained largely disentangled from the electron surprised the researchers. Physicists previously believed this nuclear spin could not be manipulated directly because it should not be sensitive to the electrical signals of a scanning tunneling microscope.
The nucleus’s shielding traditionally made this level of control seem improbable. Lead researcher Sander Otte stated that the strongest magnetism comes from electrons, but the nuclear spin’s shielding is what makes it valuable for quantum information storage.
Because the nuclear spin is better protected from external interference, surface relaxation lifetimes can reach the order of seconds. This contrasts with previous STM methods that required low fields (≤ 100 mT) to create strong hyperfine hybridization, a process that often collapsed the nucleus’s environmental isolation.
Future requirements for atomic quantum memory
The current demonstration used continuous RF driving to verify that isolated nuclear spins can be driven by local fields. However, the team notes that achieving full coherent quantum gate control will require a transition to pulsed-wave sequencing.
The platform suggests that atomic-scale nuclear qudits—quantum units with more than two states—could be integrated into engineered surface lattices for use as quantum memory registers.
Pro tip: In quantum computing, a “qudit” differs from a “qubit” by having multiple possible states (d > 2), allowing for higher-dimensional information processing within a single physical site.
Nuclear spin manipulation queries
Why is the nuclear spin more stable than the electronic spin?
Nuclear spins have magnetic moments about 1,000 times smaller than electrons, which shields them from environmental noise and allows for longer relaxation lifetimes, sometimes lasting seconds.
What is the role of the 1.4 Tesla magnetic field in this experiment?
The high magnetic field ensures the electronic Zeeman energy dominates the system, which suppresses electron-nuclear hybridization by about 10 times and preserves the intrinsic lifetime of the nucleus.
How does the ENDOR scheme differ from standard NMR?
The Electron-Nuclear Double Resonance (ENDOR) scheme uses a dual-frequency approach—GHz for the electron and MHz for the nucleus—to separate the readout channel from the manipulation of the nuclear state.
Regarding the unexpected nature of the results, Sander Otte conceded, “Back to the drawing board, then. That is really how science works. There is always something new left to figure out.”