Atomic-scale quantum sensing

We use the manipulation capabilities of scanning probe microscopes to fabricate molecular quantum sensors on the probe tips to detect the tiny electric and magnetic fields of quantum systems at the atomic level.

Quantum sensors turn the extreme vulnerability of quantum systems to external perturbations into an asset: interactions with physical quantities lead to a change in the transition energy between quantum states, allowing a sensitive and quantitative measurement of the interaction strength. An example of such a quantum system is the spin of an electron in a magnetic field, which has two possible states: Spin up or spin down. The two states are separated by an energy difference that depends on the strength of the magnetic field. The stronger the magnetic field, the larger the energy difference. By precisely measuring the energy difference between these two spin states, it is therefore possible to draw direct conclusions about the strength of the magnetic field.

Recently, we developed a quantum sensor that can detect electric and magnetic fields from single atoms with a spatial resolution better than 0.1 nm [1]. To this end, we attached a molecular spin, that is, a molecule with an unpaired electron, to the apex of the metallic tip of a scanning tunneling microscope (STM). Typically, the lifetime of a spin in direct contact with a metal is very short and cannot be controlled. In our approach, we brought a planar molecule (PTCDA) into a special configuration [2] on the tip by precise atomic-scale manipulation, thus decoupling the molecular spin. In this configuration, the molecule is a spin 1/2 system and serves as a two-level quantum system in a magnetic field due to the Zeeman effect [3]. The ground state is split into spin-up and spin-down states, and the energy difference between these states depends on the strength of the magnetic field. Using electron spin resonance (ESR) in the STM, we were able to detect the energy difference between the states with an energy resolution of ~100 neV. This allowed us to determine the magnetic field of a single atom (only a few atomic distances away from the sensor) that caused the change in spin states [1].

We use these atomic-scale quantum sensors to resolve spins in emerging quantum materials, such as 2D materials. In addition, we are working to increase the sensitivity of our quantum sensors by a factor of about 1000, which would allow us to sense nuclear spins at the atomic scale. This work is carried out in a unique millikelvin STM/AFM that was develop in the Quantum Nanoscience department (PGI-3) of the Peter Grünberg Institute at Forschungszentrum Jülich [4][5].

References

[1] T. Esat, D. Borodin, J. Oh, A. J. Heinrich, F. S. Tautz, Y. Bae, R Temirov, A quantum sensor for atomic-scale electric and magnetic fields. Nat. Nanotechnol. (2024)

[2] T. Esat, N. Friedrich, F. S. Tautz, R. Temirov, A standing molecule as a single-electron field emitter. Nature 558, 573–576 (2018)

[3] T. Esat, M. Ternes, R. Temirov, F. S. Tautz, Electron spin secluded inside a bottom-up assembled standing metal-molecule nanostructure. Phys. Rev. Research 5, 033200 (2023)

[4] T. Esat, P. Borgens, X. Yang, P. Coenen, V. Cherepanov, A. Raccanelli, F. S. Tautz, R. Temirov, A millikelvin scanning tunneling microscope in ultra-high vacuum with adiabatic demagnetization refrigeration. Review of Scientific Instruments. 92, 063701 (2021)

[5] T. Esat, X. Yang, F. Mustafayev, H. Soltner, F. S. Tautz, R. Temirov, Determining the temperature of a millikelvin scanning tunnelling microscope junction. Commun Phys. 6, 81 (2023)