Subsystem purity as an enforcer of entanglement - art. no. 050401

We show that entanglement can always arise in the interaction of an arbitra rily large system in any mixed state with a single qubit in a pure state. T his small initial purity is enough to enforce entanglement even when the to tal entropy is close to maximum. We demonstrate this feature using the Jayn es-Cummings interaction of a two-level atom in a pure state with a field in a thermal state at an arbitrarily high temperature. We find the time and t emperature variation of a lower bound on the amount of entanglement produce d and study the classical correlations quantified by the mutual information .