  We demonstrate by an explicit model calculation that the decay of
entanglement of two two-state systems (two qubits) is governed by the product
of the factors that measure the degree of decoherence of each of the qubits,
subject to independent sources of quantum noise. This demonstrates an important
physical property that separated open quantum systems can evolve quantum
mechanically on time scales larger than the times for which they remain
entangled.
