Local permutations of products of Bell states and entanglement   distillation
  We present new algorithms for mixed-state multi-copy entanglement distillation for pairs of qubits. Our algorithms perform significantly better than the best known algorithms. Better algorithms can be derived that are tuned for specific initial states. The new algorithms are based on a characterization of the group of all locally realizable permutations of the 4^n possible tensor products of n Bell states. 
Jeroen Dehaene, Maarten Van den Nest, Bart De Moor, Frank Verstraete