Protective Measurement, Postseletion and the Heisenberg Representation
  Classical ergodicity retains its meaning in the quantum realm when the employed measurement is protective. This unique measuring technique is reexamined in the case of post-selection, giving rise to novel insights studied in the Heisenberg representation. Quantum statistical mechanics is then briefly described in terms of two-state density operators. 
Yakir Aharonov, Eliahu Cohen
quant-ph