A quantum wave in a helium dimer recorded for the first time

Anyone entering the world of quantum physics must prepare for several things unknown in the everyday world: Noble gases form compounds, atoms behave like particles and waves at the same time, and events that are mutually exclusive in the macroscopic world happen at the same time.

In the world of quantum physics, Reinhard Dörner and his team are working with molecules that – in the sense of most textbooks – should not exist: Helium compounds with two atoms, known as helium dimers. Helium is called a noble gas precisely because it does not form any compounds. However, if the gas is cooled to only 10 degrees above absolute zero (minus 273 ° C) and then pumped through a small nozzle into a vacuum chamber, which makes it even colder, then – very rarely – such helium dimers are formed. These are unparalleled, the weakest bound stable molecules in the universe, and the two atoms in the molecule are extremely distant from each other. While a chemical compound of two atoms usually measures about 1 angstrom (0.1 nanometer), helium dimers measure on average 50 times more, i. 52 angstroms.

Scientists from Frankfurt irradiated such helium dimers with an extremely powerful laser flash, which slightly twisted the bond between two helium atoms. That was enough to separate the two atoms. Then they saw – for the first time – a helium atom fly away like a wave and filmed it on film.

According to quantum physics, objects behave like a particle and a wave at the same time, something best known from light particles (photons), which on the one hand super superimpose like waves where they can accumulate a resistor that goes out (interference), but on the other , for example, a “solar wind” can propel spacecraft with its solar sails.

The fact that the researchers were able to observe and record a helium atom flying like a wave in their laser experiment is due to the fact that the helium atom flew away with only a certain probability: with a 98 percent probability it was still attached to its other helium partner, with with a 2 percent probability that he flew away. These two waves of helium atoms – Here it comes, quantum physics! – superimpose and their interference could be measured.

The measurement of such ‘quantum waves’ can be extended to quantum systems with several partners, such as a helium trimmer composed of three helium atoms. The helium trimmer is interesting because it can create what is called the “exotic Efimov state,” says Maxim Kunitsky, the study’s first author. We have discovered the Efim state in a helium trimmer. The laser pulse radiation method we have now developed could allow us to observe the formation and decay of Efim systems in the future and thus better understand quantum-physical systems that are difficult to approach experimentally. ”