Decoherence

The term decoherence is used in many fields of (quantum) physics to describe the disappearance or absence of certain superpositions of quantum states. Decoherence is a consequence of the unavoidable interaction of virtually all physical systems with their environment. In particular, macroscopic objects must be strongly entangled if quantum theory is universally valid [1,2]. Decoherence then explains within quantum theory why macroscopic objects seem to possess their familiar classical properties. No additional classical concepts are required for a consistent quantum description. Decoherence explains, for example, why particles appear localized in space (hence there is no need for an additional particle concept). Contradictory levels of description (classical and quantum) are no longer needed, instead a consistent description in terms of a universal ► wave function can be pursued.

The basic mechanism of decoherence is the unavoidable and generally irreversible disappearance of certain phase relations from the states of (local) systems by interaction with their environment according to the ► Schrödinger equation. Equivalently, decoherence describes irreversibly increasing entanglement as a consequence of a unitary global dynamics. Phase relations between certain states of a system are preserved globally (because of the assumed unitarity), but are no longer locally accessible, thus leading to apparent non-unitarity — or, in other words — to an apparent violation of the quantum ► superposition principle. This non-unitarity can be described as a disappearance of non-diagonal (in a certain basis) elements of the ► density matrix characterizing the local system. The two most important consequences of decoherence are suppression of interference and the selection of a set of preferred (dynamically stable) states.