  The simple stationary decoherence of a two-state quantum system is discussed
from a new viewpoint of environmental entanglement. My work emphasizes that an
unconditional local state must totally be disentangled from the rest of the
universe. It has been known for long that the loss of coherence within the
given local system is gradual. Also the quantum correlations between the local
system and the rest of the universe are being destroyed gradually. I show that,
differently from local decoherence, the process of environmental
disentanglement may terminate in finite time. The time of perfect
disentanglement turns out to be on the decoherence time scale, and in a simple
case we determine the exact value of it.
