Concurrence of quasi pure quantum states

Entangled states constitute one of the most fundamental differences between quantum and classical mechanics. The objects of prime interest in most experiments are pure entangled states. Therefore, experimental setups are designed such that unavoidable environment coupling is minimised, in order to keep the system state as pure as possible. On the theoretical side, most efforts are concerned with arbitrary mixed states – so far with limited success. Hardly any entanglement measure can be calculated for systems larger than bipartite two-level systems – the smallest ones of interest.