Physics: Atoms under the quantum magnifying glass

Using a novel quantum sensor, scientists from the Research Center Jülich and the Korean IBS Center for Quantum Nanoscience have succeeded in measuring tiny magnetic fields on an atomic scale. Similar to the imaging method of magnetic resonance tomography in medicine, materials should be examined with a resolution that has never been achieved before.

Sensors are technical measuring probes that quantitatively record the properties of materials or the environment. To do this, they use physical effects that are converted into an electrical signal that is further processed. A well-known example is the piezoelectric effect: Some solid bodies, such as quartz, change their polarization when exposed to external pressure – a directly measurable electrical voltage is created on the body, through which the triggering pressure can be precisely measured.

Quantum sensors use quantum mechanical effects, the laws that come into play on the smallest scales, in the size range of atoms. The main role of these sensors is played by electrons, those negatively charged particles that are bound to atomic nuclei and have a so-called “quantum mechanical spin”. Transferred to a picture of classical, everyday physics, the electron rotates around itself, forming a magnetic dipole. This is influenced by external magnetic fields – which can be exploited for sensors with great technical know-how.

Diamonds as sensors

One of the most advanced technologies to date are quantum sensors made of diamonds, where “errors” are not only desirable but absolutely necessary. In the artificially produced, tiny diamonds, not only similarly sized nitrogen atoms are built into the regular carbon lattice, but also “gaps”, i.e. free spaces, in the otherwise regular crystal.

If both lattice defects are close together, the electrons come together in the “gap-nitrogen defect” to form a small, rotatable magnet. If a magnetic field acts from outside, the electron magnets align themselves parallel or anti-parallel to the external magnetic field lines and according to all the rules of quantum mechanics. And then the nitrogen also makes its big appearance: If radiation of a certain frequency is directed onto the diamond, the nitrogen lights up in the defects, with the intensity of the glow depending on the quantum state of the small magnets. Recorded by a light-sensitive camera, the light can then be analyzed in detail: the glowing defects can be used as a kind of “quantum magnifying glass” to examine a sample with high spatial resolution.

Spatial triggering limited so far

In practice, a sample to be examined, such as large and complex molecules, is placed on the diamond of the quantum sensor and pushed into the strong field of a superconducting magnet. An electromagnetic signal that is now irradiated disrupts and tilts the atoms (these also have a spin!) of the sample molecules, which, under the influence of the magnetic field, align themselves properly again in parallel or anti-parallel – whereby they themselves emit characteristic electromagnetic radiation. This not only indicates the element, but also its bond and position in the molecule, as both influence how the atom spins back and radiates. If the defect in the diamond sensor is located directly under the radiating atom, the structure and composition of the molecules can be reconstructed. But the hole has a catch: the signal strength decreases quickly with distance – and since the defect is several nanometers “deep” in the diamond, spatial resolution remains limited.

Measurement with a sensor molecule

The new quantum sensor is different: Instead of a deeply built-in defect, it uses a single molecule with a fixed electron spin that is stably attached to the tip of a scanning tunneling microscope. Even that “setup” was a scientific masterpiece that was only achieved in 2018. The sensor molecule can now be brought within a few atomic distances of a sample to be measured and thus record magnetic structures with high precision – for the first time with a spatial resolution that is on the order of a single atom.

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