Distinctness of ensembles having the same density matrix and the nature   of liquid NMR quantum computing
  We show that differently constructed ensembles having the same density matrix may be physically distinguished by observing fluctuations of some observables. An explicit expression for fluctuations of an observable in an ensemble is given. This result challenges Peres's fundamental postulate and seems to be contrary to the widely-spread belief that ensembles with the same density matrix are physically identical. This leads us to suggest that the current liquid NMR quantum computing is truly quantum-mechanical in nature. 
Gui-Lu Long, Yi-Fan Zhou, Jia-Qi Jin, and Yang Sun