What process causes a quantum system to lose coherence by interacting with its environment?

Quantum decoherence is the process by which a quantum system loses observable coherence through interaction with its environment.

An isolated quantum state can maintain a superposition, allowing alternative possibilities to interfere. When the system interacts with surrounding particles, electromagnetic fields, measuring devices, or thermal fluctuations, information about that state becomes correlated with the environment. The system then behaves as though its phase relationships have been lost.

Decoherence helps explain why macroscopic objects appear classical even though they are made of quantum constituents. It does not mean that quantum information is destroyed from the viewpoint of the complete system: the information is distributed into environmental degrees of freedom and becomes practically inaccessible. This differs from wave-function collapse, which is an interpretation-dependent description of measurement outcomes.

Decoherence is especially important in quantum computing. Noise shortens a qubit's coherence time, making it harder to preserve superpositions and entanglement long enough to complete a computation.