# Faculty of Physics

### Transition Region between the Quantum Regime and Classical Physics

Quantum physics is known to work perfectly for microscopic particles such as atoms and photons; however, its predictions turn our “classical” everyday experience upside down if one applies them to truly tangible objects. For instance, one and the same object is allowed to exist simultaneously at different places, and its dynamic behaviour is determined by whether or not it is being observed. A main goal of Prof. Klaus Hornberger’s group is to investigate systems that belong to the transition region between the quantum regime and classical physics. Based on the theory of open quantum systems, the group is studying to what extent one can understand the emergence of classical physical properties and classical laws if one views quantum theory as universally valid. These questions can be studied if one deals with the dynamics of ever larger molecules and micromechanical nanoparticles as they interact with their natural environment. The increasing complexity of such objects makes it impossible to use a completely microscopic description, and makes it necessary to identify the general principles and mechanisms behind the quantum-to-classical transition. Alongside this, the group is developing experimentally realisable proposals to probe the boundary region between quantum behaviour and classical physics, and to verify quantum phenomena at levels that have not been possible up to now. Examples include the diffraction of metal clusters at gratings of light or the ground-state cooling of optically levitated nanorods.