what is system I

What is it about?

Decomposing systems into subsystems provides different structures, i.e. subsystems of a composite system. Classically this procedure is either trivial or artificial. In the context of quantum decoherence it emphasizes relativity of the concept of physical system as well as relativity of 'quantum entanglement'. Conceptually, it bases the so-called parallel occurrence of decoherence. Operationally, it sheds new light on dealing with composite systems.

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Why is it important?

Realistic physical systems are composite and also open. Quantum mechanically the concept of 'system' (subsystem) is relative: it can be observed only by a proper choice of measured observables. Certain observations do not reveal anything of some subsystems of a composite system. This is a feature of entanglement relativity: amount of entanglement differs for different structures (decompositions) of a composite system. So, entanglement refers to a structure, not to a composite system itself.