Quantum Mechanics on the Macro-Scale
Florian Marquardt, University of Erlangen
Quantum mechanics is one of the cornerstones of our scientific understanding of nature. It explains the behaviour of fundamental particles, the chemical bond, the structure and color of materials, photosynthesis, and much more. It underlies applications ranging from lasers over semiconductor chips to nuclear magnetic resonance imaging. Still, the interpretation of quantum mechanics has been puzzling scientists and philosophers ever since its inception. How is one to think of an object that behaves sometimes like a particle, localized at a point in space, and at other times like a wave, spread out over space and able to show interference effects? Initially, quantum effects were observed only in large ensembles of microscopic particles, like electrons or atoms, in effect hiding some of the more troubling aspects. Nevertheless, the laws of quantum physics should also apply to single objects. The mystery was illustrated by Schrödinger's striking "Gedankenexperiment" of a cat which could be in a quantum superposition of dead and alive. Modern high-precision experiments are now routinely controlling individual quantum systems. One goal is to verify quantum theory in ever-larger objects and to understand the emergence of the more familiar laws of classical motion. A crucial ingredient is decoherence, i.e. the loss of quantum interference effects induced by fluctuating forces. One nice example is provided by experiments that study the interference of ever-larger molecules, possibly ranging up to viruses in the future. Another example are "nanomechanical" objects, i.e. beams and other vibrating structures fabricated on the nanometre scale using the tools of computer-chip production. There, one also tries to create quantum superpositions, now involving many billions of atoms.