Nuclear magnetic resonance on a single atom with a local probe

The nuclear spin is a prime candidate for quantum information applications due to its weak coupling to the environment and inherently long coherence times. However, this weak coupling also challenges the addressability of the nuclear spin. Here we demonstrate nuclear magnetic resonance (NMR) on a single on-surface atom using a local scanning probe. We employ an electron-nuclear double resonance measurement scheme and resolve nuclear spin transitions of a single 47Ti isotope with a nuclear spin of I = 5/2. The quadrupole interaction enables to resolve multiple NMR transitions, which are consistent with our eigenenergy calculations. Our experimental results indicate that the nuclear spin can be driven efficiently irrespective of its hybridization with the electron spin, which is required for direct control of the nuclear spin in the long-lifetime regime. This investigation of NMR on a single atom in a platform with atomic-scale control is a valuable development for other platforms deploying nuclear spins for characterization techniques or quantum information technology.

💡 Research Summary

In this work the authors demonstrate nuclear magnetic resonance (NMR) on a single on‑surface atom by employing a scanning tunneling microscope (STM) equipped with electron‑nuclear double resonance (ENDOR) capabilities. The system under study consists of individual 47Ti atoms (nuclear spin I = 5/2) adsorbed on oxygen sites of a bilayer MgO film grown on Ag(100). An external magnetic field is applied perpendicular to the surface, lifting the degeneracy of the Ti electron spin (S = 1/2) and allowing conventional electron‑spin resonance (ESR) to be driven by a GHz‑frequency radio‑frequency (RF) voltage applied to the tunnel junction. The ESR spectrum of 47Ti exhibits six hyperfine‑split peaks, each corresponding to a different nuclear spin projection mI, whereas isotopes without nuclear spin display a single peak. Importantly, the relative heights of the ESR peaks directly reflect the time‑averaged occupation probabilities of the nuclear spin states, providing a built‑in readout of the nuclear spin population.

To address the central challenge of independently controlling the nuclear spin—i.e., without relying on the electron‑nuclear hybridization that shortens nuclear lifetimes—the authors introduce a second RF source in the MHz range. By simultaneously applying the GHz ESR drive and the MHz NMR drive, they monitor changes in the ESR peak amplitudes while sweeping the NMR frequency. When the NMR frequency matches a nuclear transition, the corresponding ESR peaks equalize in height, indicating that the nuclear populations of the two involved mI levels have been saturated. This ENDOR scheme thus converts the ESR signal into a spectroscopic probe of the nuclear spin.

The quadrupole interaction of 47Ti (I = 5/2) splits each hyperfine line into two distinct NMR transitions, allowing the authors to resolve multiple nuclear resonances. Experimentally they observe transitions near 50 MHz and 85 MHz (−5/2 ↔ −3/2) and near 65 MHz and 75 MHz (−3/2 ↔ −1/2), each appearing for both electron‑spin orientations (↑ and ↓). By fitting the measured transition energies to an effective spin Hamiltonian that includes Zeeman, hyperfine, and quadrupole terms, they extract a hyperfine coupling A = 132.1 ± 0.4 MHz and a quadrupole coupling Q = −2.8 ± 0.8 MHz, values consistent with earlier bulk and surface studies. The nuclear g‑factor derived from the magnetic‑field dependence of the NMR lines is gN = 0.37 ± 0.04, slightly larger than the bulk value of 0.315, likely reflecting the altered electric‑field environment at the surface.

A systematic magnetic‑field study (200 mT – 1.4 T) reveals that, unlike the hybridization‑dependent NMR‑like features observed in pure ESR measurements, the ENDOR signals persist even when the electron‑nuclear mixing is reduced by more than an order of magnitude. This demonstrates that the nuclear spin can be driven efficiently without involving the electron spin, a prerequisite for preserving the long nuclear coherence times. The authors also explore the dependence of the ENDOR signal on the amplitude of the ESR drive voltage, confirming that the relative populations of the electron‑spin ground (↑) and excited (↓) states can be tuned, which in turn modifies the intensity of the nuclear transitions associated with each electron spin manifold.

To elucidate the driving mechanism, three possibilities are considered: (i) modulation of the hyperfine interaction, (ii) modulation of the quadrupole interaction via the RF electric field, and (iii) an effective oscillating magnetic field generated by the RF electric field (e.g., through piezoelectric displacement of the atom, modulation of the tunnelling barrier, or time‑dependent spin‑orbit coupling). The first two mechanisms are inconsistent with the observed MHz frequencies and selection‑rule constraints. The third mechanism, an RF‑induced effective magnetic field, aligns with the known driving of electron spins in ESR‑STM and can account for the observed nuclear transitions across the full magnetic‑field range. Because this mechanism does not rely on material‑specific constants, it suggests that STM‑based nuclear spin control could be extended to other nuclear species and host surfaces.

In summary, the paper achieves three major milestones: (1) it provides the first clear demonstration of NMR on a single atom using a local probe, (2) it shows that quadrupole interactions enable the resolution of multiple nuclear transitions and allow selective addressing of nuclear spin states, and (3) it establishes that nuclear spins can be driven independently of the electron spin, preserving their intrinsic long coherence times. This platform opens new avenues for atomic‑scale quantum simulation, high‑resolution magnetic sensing, and the development of nuclear‑spin‑based quantum memories, all within a system that offers sub‑angstrom spatial control and the ability to engineer custom spin Hamiltonians atom by atom.