Research

We explore and control quantum spin systems at the ultimate size limit — individual atoms and molecules on surfaces — by combining electron spin resonance (ESR) with scanning tunneling microscopy (STM). The sections below introduce the technique and the research directions it enables.

Single-Atom Electron Spin Resonance

In 2015, ESR on individual magnetic atoms on surfaces was implemented in a scanning tunneling microscope for the first time [Baumann et al., Science 350 (2015)], a major step towards quantum-coherent control of single spins.

A single magnetic atom is placed on a thin insulating film of magnesium oxide (MgO) and imaged by STM at low temperature (~1 K) in ultra-high vacuum. The MgO film decouples the atomic spin from the conduction electrons of the silver substrate underneath, protecting its magnetic properties. The spin states |0⟩ and |1⟩ of the atom are Zeeman-split by an external magnetic field. To perform ESR, a radiofrequency (RF) voltage applied to the STM junction coherently drives the spin, and the change in state population at the resonance frequency f₀ is detected through a spin-polarized tip that acts as an atomic-scale spin filter. Notably, this technique exceeds the energy resolution of conventional STM spectroscopy by roughly a factor of 1000, reaching ~10 neV [Willke et al., Sci. Adv. 4 (2018)].

Schematic of single-atom electron spin resonance in an STM and ESR spectrum — **Fig. 1** — Single-atom ESR. Magnetic atoms are deposited on bilayer MgO on a silver crystal; a radiofrequency voltage V_RF added to the junction drives the Zeeman-split spin, and the resonance appears as a change ΔI in the spin-polarized tunnel current.

Quantum Sensing

As in conventional ESR, the atom's resonance frequency f₀ is sensitive to any local magnetic field — so a single atom can serve as a quantum sensor for its magnetic environment. The combination of energy and spatial resolution of ESR-STM makes it possible to measure the weak magnetic dipole–dipole interaction between individual atoms [Choi et al., Nat. Nanotechnol. 12 (2017)]: an ESR spectrum of a sensor atom splits into two peaks that reflect the two spin states of a nearby target atom, and distance-dependent measurements follow the r⁻³ law of dipolar coupling. This determines magnetic moments with a precision better than 0.01 μ_B and revealed, for single holmium atoms, spin lifetimes of several hours [Natterer et al., Nature 543 (2017)]. At separations below ~1 nm, exchange interaction takes over [Yang et al., PRL 119 (2017)].

The magnetic STM tip itself also interacts with the atomic spin, shifting f₀ with tip position. Turning this around establishes magnetic resonance imaging (MRI) on the atomic scale [Willke et al., Nat. Phys. 15 (2019)] — mapping the 3D magnetic interaction between a surface spin and the tip with a spatial resolution one to two orders of magnitude beyond other scanning-field-gradient techniques.

Single atom as a quantum sensor and ESR spectrum with dipolar splitting — **Fig. 2** — A single Fe atom as a quantum sensor: the dipolar field of a nearby target atom splits the sensor's ESR line by Δf, encoding the target's spin state and magnetic moment.

Magnetic resonance imaging of a single atom — **Fig. 3** — Atomic-scale MRI: STM topography of a single Ti atom (left) and the spatially resolved ESR signal (right) highlighting the resonant interaction with the magnetic tip.

Single-Atom Nuclear Spins

The energy resolution of ESR-STM even resolves the hyperfine coupling of individual atoms [Willke et al., Science 362 (2018)] — the first detection of single-atom nuclear spins in an STM, making it possible to distinguish isotopes atom by atom. The hyperfine spectrum depends on the atomic-scale environment: moving an atom to a different binding site on MgO changes its spectrum, giving access to position-dependent information about the electronic ground state. Moreover, the spin-polarized tunnel current can pump the nuclear spin via spin-transfer torque, initializing it far from thermal equilibrium [Yang et al., Nat. Nanotechnol. 13 (2018)].

Hyperfine spectra of individual titanium isotopes and nuclear spin pumping in copper — **Fig. 4** — Hyperfine interaction of single atoms: distinct ESR multiplets for ⁴⁸Ti (I = 0), ⁴⁷Ti (I = 5/2) and ⁴⁹Ti (I = 7/2), and current-induced nuclear polarization of a single Cu atom.

Coherent Spin Manipulation

Using pulsed ESR, single atomic spins can be manipulated coherently: Rabi oscillations demonstrate coherent driving of the electron spin, and Hahn-echo measurements yield phase coherence times of ~200 ns, limited mostly by nearby tunneling electrons [Yang et al., Science 366 (2019); Willke et al., Sci. Adv. 4 (2018)].

Rabi oscillations and Hahn echo measurements on a single atom — **Fig. 5** — Coherent control: Rabi oscillations of a single Ti spin for different RF amplitudes, and Hahn-echo sequence with the corresponding trajectory on the Bloch sphere.

Current Directions

Building on these techniques, our group at KIT extends quantum-coherent control from single atoms to molecular spins — such as electrically controlled molecular spin switches and on-surface molecular ferrimagnets — and to spin defects in two-dimensional semiconductors. Within the ERC Starting Grant ATOMQUANT, we develop these atomic-scale platforms towards applications in quantum sensing and quantum information. See our publications for the latest results, and join us if you want to be part of it.