In Landau’s Statistical Physics (part 1) , section 5, he writes:" In particular, it would be quite incorrect to suppose that the description by means of the density matrix signifies that the subsystem can be found in various ψ states with various probabilities and that the averaging is over these probabilities."
However to my knowledge what Landau opposes is exactly the physical interpretation of a density matrix. What is it that I am missing?
edit: I am not confusing the probabilistic property inherent to a pure quantum state with that of the mixed state. Still, I am under the impression that the density matrix is a characterization of the constitution of the mixed state; for instance, we could use a density matrix to describe an ensemble of systems made up with 70% of state A and 30% of state B (This example comes from Sakurai's Modern Quantum Mechanics(2e), page 180). But is this not what Landau calls incorrect?
Could it be that Landau uses the density matrix to describe a subsystem which is quite determined in a way unknown to us (since we only have incomplete information) and in the above example the matrix is used in a closed and completely described system which is probabilistic in nature? (I am ignoring here the probabilistic property in quantum physics itself as it is present in both cases.)