Quantum decoherence is the term accepted and used in quantum mechanics to explain how an entangled quantum state can give rise to a classical (non-interlaced) physical state. In other words, a physical system, under certain specific conditions, ceases to exhibit quantum effects and exhibits a typically classical behavior, without the counterintuitive effects typical of quantum mechanics.
The name comes from the technical fact that decoherence is manifested mathematically by the loss of coherence of the relative complex phase of the linear combinations that define the state. Thus quantum decoherence would explain why on a large scale classical physics ignoring quantum effects is a good explanation of the world's behavior.
For example, in the case of Schrödinger's cat experiment, the interaction of cat particles with the environment could produce decoherence and make the combination of "living cat" and "dead cat" lose coherence and become a Classical state and therefore after a period of time of ħ² (10⁻⁶⁵ s) the cat was inside the box actually alive or dead, but not in an overlap of both. Consistency is thus very important in explaining why many macroscopic physical systems have such a different behavior from systems exhibiting quantum effects.
The problem of quantum decoherence is a complementary explanation to the problem of quantum state reduction, which is the main interpretive difficulty within the quantum mechanics problem. For this reason this section reviews historical positions on the problem of state reduction, and why ultimately quantum states for macroscopic bodies lack some of the typical properties of quantum states (entanglement, quantum interference, Reduction of the state, etc.).
The usefulness of decoherence lies in its application to the analysis of probabilities, initial and final states after interaction with the environment. In particular, this formalism allows us to see how the quantum interference terms are annulled after the occurrence of the decoherence analyzed in the previous section.
Image: Electron accumulation over time in Young's experiment, interference patterns are a typically quantum effect associated with a pure state (coherent state), in the presence of quantum decoherence the pattern of interference would disappear.
Credits: In a place in the cosmos