Learning aboutQuantum MechanicsThe Nitty-gritty

Credits for image original: Tewy.

Professor Frederic P. Schuller has a lecture series where he introduces Quantum Mechanics from a very precise and mathematical point of view. Definition, theorem, proof. The pure state is replaced by a density operator early on so that we can deal with mixed quantum states.

Frederic Schuller teaches QM

For a quick and condensed introduction to density operators I can recommend these lecture notes by Professor Reinhold Bertlmann.

Density Matrices by Reinhold Bertlmann