在量子力学里，开放量子系统的量子相干性会因为与外在环境发生量子纠缠而随着时间逐渐丧失，这效应称为量子退相干（英语：Quantum decoherence），又称为量子去相干。量子退相干是量子系统与环境因量子纠缠而产生的后果。由于量子相干性而产生的干涉现象会因为量子退相干而变得消失无踪。量子退相干促使系统的量子行为变迁成为经典行为，这过程称为「量子至经典变迁」（quantum-to-classical transition）。德国物理学者汉斯·泽贺最先于1970年提出量子退相干的概念。自1980年以来，量子退相干已成为热门研究论题。

In quantum mechanics, quantum decoherence is the loss of coherence or ordering of the phase angles between the components of a system in a quantum superposition. One consequence of this dephasing is classical or probabilistically additive behavior. Decoherence occurs when a system interacts with its environment in a thermodynamically irreversible way. This prevents different elements in the quantum superposition of the total system's wavefunction from interfering with each other. Decoherence was first introduced in 1970 by the German physicist H. Dieter Zeh and has been a subject of active research since the 1980s.