  We derive bounds on the entanglement of formation of states of a 4xN
bipartite system using two entanglement monotones constructed from operational
separability criteria. The bounds are used simultaneously as constraints on the
entanglement of formation. One monotone is the negativity, which is based on
the Peres positive-partial-transpose criterion. For the other, we formulate a
monotone based on a separability criterion introduced by Breuer (H.-P. Breuer,
e-print quant-ph/0605036).

Quantum Physics