Quanteninformation: Fundamentales, Übergang zum Klassischen

Quantum physics is in conflict with the paradigm of macroscopic realism. The first objective of this project is to deliver a novel theoretical approach to macroscopic realism and classical physics within quantum theory. While it not at variance with decoherence, it differs conceptually from it, in putting stress on limits of "observability" of quantum effects of macroscopic objects. The crucial point is that, as the number of degrees of freedom of the system increases, the requirement on precision of our measurement apparatuses such that quantum effects can still be observed, increases correspondingly. In view that the ultimate resources in the laboratory (or even in the whole universe) impose a funda-mental limit on measurement accuracy and complexity of quantum state preparation, this experimental difficulty turns into a principal impossibility. As a consequence, classical physical laws follow from quantum laws with the restriction of coarse-grained measurements. The second objective of the proposal is to identify the non-classical key ingredients that give rise to the enhanced quantum computational power. The main idea is that the full power of quantum compu-tation lies both in temporal correlations of a genuine quantum nature and in spatial correlation due to entanglement. While all classical algorithms satisfy constraints on their (classical) temporal correla-tions, their violation in quantum algorithms requires additional information transfer along the algo-rithm to be simulated classically. The final objective is providing a definite answer to the question in which sense quantum game strategies are `better` than classical ones. In the standard setting of games the players make their decisions by applying local operations to their physical systems which are then transported to the location of the referee who performs the measurement and calculates the players` final pay-off. The fundamental idea is that classical local operations on bits impose general constraints on the probabilities for the outcomes of the referee`s measurement, which can be violated in quantum games by using local quantum operations and entangled qubits.