  The entanglement of assistance quantifies the amount of entanglement that can
be concentrated among a group of spatially-separated parties using the
assistance of some auxiliary system. In this paper we study the entanglement of
assistance in the simplest scenario of three qubits. We consider how much
entanglement is lost when the helper party becomes uncoupled by local
measurement and classical communication. For three-qubit pure states, it is
found that lossless decoupling is always possible when entanglement is
quantified by the maximum probability of locally generating a Bell state.
However, with respect to most other entanglement measures, such as concurrence
and entanglement entropy, a lossless decoupling of the helper is possible only
for a special family of states that we fully characterize.

Quantum Physics