A General Fidelity Limit for Quantum Channels
  We derive a general limit on the fidelity of a quantum channel conveying an ensemble of pure states. Unlike previous results, this limit applies to arbitrary coding and decoding schemes, including nonunitary decoding. This establishes the converse of the quantum noiseless coding theorem for all such schemes. 
Howard Barnum, Christopher A. Fuchs, Richard Jozsa, Benjamin   Schumacher