Weak measurement of the Dirac distribution
  Recent work [J.S. Lundeen et al. Nature, 474, 188 (2011)] directly measured the wavefunction by weakly measuring a variable followed by a normal (i.e. `strong') measurement of the complementary variable. We generalize this method to mixed states by considering the weak measurement of the product of the two observables, which, as a non-Hermitian operator, is normally unobservable. This generalized method provides mixed states an operational definition related to the operator representation proposed by Dirac. Uniquely, it can be performed `in situ', determing the quantum state without destroying it. 
Jeff S. Lundeen and Charles Bamber
quant-ph