In quantum physics, fermions are particles with half-integer spin, named after Enrico Fermi, the father of quantum theory and the developer of the first nuclear reactor.

There are two types of elementary fermions: quarks and leptons. As fermions are often approximately conserved, they are believed to be the constituents of matter, while bosons - particles with integer spin - transmit forces.

Now, a French team from the Charles Fabry Laboratory at the Institut d'optique (CNRS), collaborating with a team from the Free University in Amsterdam, has shown that fermions tend to avoid each other and cannot "travel" in close proximity, a major event in our knowledge of phenomena at a quantum scale.

Since long ago, the theory of quantum mechanics stated that the fermions could not travel in close proximity. For instance, this means that in a stream of identical particles, the distance between them will be always higher than a given value, named the "correlation length".

What the French-Dutch team has done was to demonstrate that this "anti-bunching" property is real, something that has never been possible to demonstrate before. It's like the particles reject each other, even if interactions between them are negligible. In fact, this "anti-bunching" is a result of quantum interferences which impede the probability of finding two very close particles.

The research team made a comparison between the behavior of fermions and that of bosons, in case of similar conditions. In the case of the bosons, the same interferences induced the contrary to a "bunching" effect: an increased probability of finding two particles together.

The experiments were made employing the same system (which ensured identical conditions) on two helium isotopes. In this case, the researchers proved the correlation length of fermions, found to be close to a millimeter.

Even if this effect had been anticipated, its demonstration is a breakthrough in our ability to detect correlations between atoms and assessing the behavior of matter in quantum physics.