Quantum tomographic cryptography with Bell diagonal states:   non-equivalence of classical and quantum distillation protocols
  We present a generalized tomographic quantum key distribution protocol in which the two parties share a Bell diagonal mixed state of two qubits. We show that if an eavesdropper performs a coherent measurement on many quantum ancilla states simultaneously, classical methods of secure key distillation are less effective than quantum entanglement distillation protocols. We also show that certain Bell diagonal states are resistant to any attempt of incoherent eavesdropping. 
Dagomir Kaszlikowski, Jenn Yang Lim, D. K. L. Oi, Frederick H.   Willeboordse, Ajay Gopinathan, L. C. Kwek