Dekoherenssille

Dekoherenssille (Finnish term for decoherence) is the process in quantum mechanics by which a system loses its quantum coherence due to interaction with its environment. It describes how coherent superpositions become effectively classical when the system becomes entangled with many degrees of freedom and the observable interference terms become unobservable.

Mechanism: When a quantum system is not isolated, its state becomes entangled with environmental degrees of freedom. By focusing on the system alone and ignoring the environment (tracing out), the system's density matrix loses its off-diagonal elements in a preferred basis—the pointer basis—leading to classical statistical mixtures rather than coherent superpositions. The decoherence timescales can be extremely short for macroscopic objects, often far shorter than measurement times.

Significance: Decoherence provides a framework to understand the emergence of classical properties without invoking a fundamental collapse. It is distinct from dissipation; it suppresses interference but does not by itself select a single outcome. In quantum information, decoherence is a major source of error, motivating isolation, error correction, and fault-tolerant designs. The concept underpins theories like einselection and quantum Darwinism.

History and terminology: Although the underlying ideas date to Zeh and Zurek in the 1980s and 1990s, decoherence has become a standard part of open quantum systems theory. In Finnish-language literature, dekoherenssille is used to denote the decoherence process or its role "for decoherence".

See also: open quantum systems, decoherence theory, density matrix, quantum measurement problem, Einselection.