environmental coupling

Environmental coupling is the unavoidable interaction between a quantum system and everything around it — the air molecules that bounce off it, the photons that scatter from it, the thermal jiggling of whatever holds it in place. No real system is perfectly isolated; it is always touching a vast 'bath' of other degrees of freedom. This coupling is the doorway through which a once-private quantum state becomes entangled with the wider world.

It is this coupling that drives decoherence. When the system interacts with the environment, the two become correlated: the environment effectively records which alternative the system is in. Because that record is spread across enormous numbers of particles you cannot track or recombine, the system's own interference between alternatives is, for all practical purposes, lost. The strength and nature of the coupling set how quickly this happens and which states survive it.

Seen this way, the boundary between quantum and classical is not a sharp wall but a matter of degree, governed by how strongly something is coupled to its surroundings. Physicists who want to keep a system quantum — for a delicate experiment or a quantum computer — fight a constant battle to weaken this coupling, by cooling, shielding, and isolating their apparatus. The same coupling that makes the world look classical is the enemy of fragile quantum behaviour.