Antimatter interferometry is one of the Top 10 Breakthroughs of the Year

Acknowledgement by the Physics World magazine of the experimentation that involves researchers at Politecnico di Milano.


The double slit interference experiment carried out for the first time using single antielectrons was chosen as one of the Top 10 Breakthroughs of the Year in Physics. This prestigious recognition is assigned by the Physics World magazine each year, to the 10 most revolutionary steps forward in the world of Physics.

The experiment was carried out by researchers from the Politecnico di Milano, the INFN [National Institute of Nuclear Physics], the Università degli Studi di Milano, and the Albert Einstein Center (AEC) for Fundamental Physics, and the High Energy Physics Laboratory (LHEP) at Bern University.

The team achieved success for the first time in the sophisticated undertaking of running the experiment using single positrons. This experiment made it possible to demonstrate that the positron, that is the antiparticle that corresponds to the electron, has a dual nature: it is both a wave and a particle. This characteristic was found by observing the interference of antimatter waves with single positrons for the first time, and confirms that the laws of quantum mechanics also apply to antimatter.

Great success is not only achieved in great laboratories. This result comes from tenacious, enthusiastic work done by a small team of passionate researchers

Rafael Ferragut, Head of the L-NESS Positron Laboratory of the Politecnico di Milano in Como that hosted the experiment comments:

How is the experiment carried out?

In this experiment, based on the interferometry technique, when the antimatter “waves” generated by a single positron interfere constructively, they collapse and localize at a point, behaving like a single particle, and are detected as such. This demonstrates directly, for the first time, that the wave-particle dualism also applies to antimatter.

This is a version, done using single particles of antimatter, of the classical double slit interference experiment, conducted for the first time with photons by Thomas young, then proposed at a conceptual level with single particles by Albert Einstein, and done using single electrons by Gian Franco Missiroli, Pier Giorgio Merli, and Giulio Pozzi, and published in 1976.

A source launches a beam of particles towards a detector. Gratings are positioned along the path, with two slits through which the particles pass. In the particles were to behave like particles, they would travel along a straight line and would produce a shape on the detector that is the same as the slits. But if the particles have a wave nature, a striped image appears on the detector, with various maximums and minimums, which correspond to the slits. This new image is generated by interference of the waves that pass through the slits.

From a conceptual point of view, in order to interpret the result of the experiment, one must consider that a single particle is propagated in space as a periodic vibration as well, that is as a wave: this concept was introduced by Louis de Broglie in 1923. From a technical point of view, to achieve this, the researchers designed and set up an extremely accurate, very high precision piece of equipment.

The experiment has three main elements: the beam, the interferometer, and the detector. The beam of individual positrons, the energy in which is well determined, is collimated to improve its parallelism quality. The interferometer is made up of two series of micrometric slits, with a high degree of parallelism and periodicity. The first series of slits is used to make the single waves coherent.  Subsequently, the waves propagate over a certain distance in space, until they reach a second series of slits, where they form secondary wave fronts. These waves interfere with each other constructively or destructively, and for an interference figure or diagram on the emulsions positioned further away.

The originality of using an asymmetrical configuration for the interferometer makes it possible to have an enlargement five times the periodicity of the first slit. This means that, the periodicity in the interference figure obtained on the emulsions was about 6 micrometers. This was an extremely high accuracy task.

For two years data was gathered, and at the same time improvements were made to the interferometer, until the resonance could be seen, with a high visibility signal. For each measurement, a statistic of about twenty million positrons on the emulsions was accumulated, one at a time, over a period of about 8 days. The visibility trend of the fringes in relation to the energy unequivocally demonstrates the quantum nature of the interference.

Now the long-term goal of the experiment is to use the interferometer’s extraordinary accuracy to measure the matter-antimatter gravitational interaction