Practical Implementations of Twirl Operations
  Twirl operations, which convert impure singlet states into Werner states, play an important role in many schemes for entanglement purification. In this paper we describe strategies for implementing twirl operations, with an emphasis on methods suitable for ensemble quantum information processors such as nuclear magnetic resonance (NMR) quantum computers. We implement our twirl operation on a general two-spin mixed state using liquid state NMR techniques, demonstrating that we can obtain the singlet Werner state with high fidelity. 
M. S. Anwar, L. Xiao, A. J. Short, J. A. Jones, D. Blazina, S. B.   Duckett and H. A. Carteret