Wednesday 27th January  10h00: Časlav Brukner (IQOQI, Fakultät für Physik, Wien, Austria).
The quantum to classical transition and the complexity of Schrödinger-cat states.

Abstract:

The descriptions of the quantum realm and the macroscopic classical world differ significantly not only in their mathematical formulations but also in their foundational concepts. When and how physical systems stop to behave quantumly and begin to behave classically is still heavily debated in the physics community. We have developed a novel approach to understand this transition, which neither needs to refer to the environment of a system (decoherence) nor to change the quantum laws itself (collapse models) but puts the stress on the limits of observability of quantum phenomena due to restricted precision of our measurement apparatuses. I will first demonstrate that under realistic conditions of every-day life, where we are only able to perform coarse-grained measurements, macroscopic realism and the classical Newtonian laws can emerge out of the full quantum laws: the system’s time evolution governed by the Schrödinger equation and the state projection induced by measurements. I will then demonstrate that nevertheless there exist ”non-classical” Hamiltonians which are in conflict with macroscopic realism even under the coarse-grained measurements. Such Hamiltonians normally build up Schrödinger-cat state superpositions. I will argue that these Hamiltonians are unlikely to be realized in nature because of their high computational complexity.

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