Quantum Optics, Quantum Nanophysics, Quantum Information > Research > Quantum Information Theory > Novel theory how the classical world emerges from quantum mechanics

Novel theory how the classical world emerges from quantum mechanics

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 and philosophical consequences. When and how physical systems stop to behave quantumly and begin to behave classically is still heavily debated in the physics community and subject to theoretical and experimental research.

Conceptually different from the decoherence program, we present a novel theoretical approach to macroscopic realism and classical physics within quantum theory. It focuses on the limits of observability of quantum effects of macroscopic objects, i.e., on the required precision of our measurement apparatuses such that quantum phenomena can still be observed. First, we demonstrate that for unrestricted measurement accuracy a violation of macrorealism is possible for arbitrarily large systems. Then we show for a certain time evolution that under the restriction of coarse-grained measurements not only macrorealism but even the classical Newtonian laws emerge out of the Schrödinger equation and the projection postulate. This resolves the apparent impossibility of how classical realism and deterministic laws can emerge out of fundamentally random quantum events.

Finally, we demonstrate that there exist non-classical time evolutions which allow to see a violation of macroscopic realism even under classical coarse-grained measurements. The question why we then do not see such violations arises again. We suggest that the reason for this is that non-classical time evolutions are of high computational complexity. Figuratively, this means that if nature spontaneously “chooses” a time evolution, it is much more likely that a low complex, i.e. a classical, time evolution is realized and thus our every-day world appears classical under coarse-grained measurements.

Relevant publications:

J. Kofler and C. Brukner
Classical world arising out of quantum physics under the restriction of coarse-grained measurements
Physical Review Letters 99 (2007) 180403

J. Kofler and C. Brukner
Conditions for quantum violation of macroscopic realism 
Physical Review Letters 101 (2008) 090403

Media coverage:

Under the magnifying glass of sharp measurements Albert Einstein sees a strange and colorful quantum picture of the face next to him. Its abstractness is symbolized by Pablo Picasso’s “Head of a Reading Woman”. Under an every-day coarse-grained view the classical appearance of Charlie Chaplin emerges.

The two graphs on the top show the quantum states of a Schrödinger cat-like superposition of a spin pointing to the north and to the south (left) and a classical fifty-fifty mixture in which half of the spins is along north and the other half is along south (right). Under every-day coarse-grained measurements both states have the same classical description in terms of a fifty-fifty probability distribution (bottom left and right). However, there exist non-classical time evolutions producing time-dependent superposition states that allow to violate macroscopic realism even under coarse-grained measurements.