In the world known from everyday experience, mechanical systems have in principle perfectly known properties. For example, they are obviously always located at some fixed location at a given time. In the world of quantum mechanics, i.e., the world of individual atoms and molecules, objects must by no means be in only one place at a given time. To a certain extent, they can be in several places simultaneously, by being in certain superposition states. Since the 1980s, researchers have been investigating this seemingly paradoxical observation: They are trying to understand the exact transition between the classical world and quantum mechanics. After all, macroscopic mechanical systems also consist of atoms – which means that the laws of physics that apply on a small scale must also apply on the large scale and hence macroscopic objects. That is why it is very amazing that the two physical theories describe nature in such radically different ways.

It is now known that it is again the quantum properties are essentially responsible for the apparently classical behavior of objects in the physical world. However, in a way the interactions between macroscopic systems and their environment are so strong that the subtle quantum properties are less noticeable in the macroscopic system as such. The precise mechanism leading to this decoherence, as it is called, it not in all detail adequately investigated and illustrated until know.

In their research project, the scientists at Freie Universität Berlin and the University of Vienna developed a setting that allows for fresh insights into the interface of the two worlds that describe nature in such different ways. They experimentally observed the light emitted from a cavity one mirror of which constituted a very small mechanical oscillating object. By statistically analyzing this emitted light, they were able to draw profound conclusions about the precise interactions responsible for the emergence of effectively classical properties. The first research results are surprising: The physicists encountered amazing memory effects in the mechanical motion and hence the mirrors can not simply be described as damped mechanical motion, as is usually done. These intricate memory effects lead to highly unorthodox ways of decoherence - yet again leading to classical behavior.

Going a step further, the new knowledge the researchers gained about the dynamics of mechanical systems can be used in the quantum technologies, for example in metrology, which is the science of accurate measurement, here using quantum effects and very small devices. This only works, needless to say, if the dynamics are understood precisely. The findings published in the prestigious journal Nature Communications are a significant contribution in this direction.

The research project was sponsored by several EU programs (RAQUEL, SIQS, AQuS, MNOS, ITNcQOM, IQOEMS, Marie Curie) as well as the European Research Council (TAQ), the German Federal Ministry of Education and Research, the Austrian Science Fund, and the Alexander von Humboldt Foundation.