Hi, there. I am a little confused about the following statement in Wikipedia. and it's about the density operators. "...As mentioned above, a state of a quantum system is given by a unit vector in a Hilbert space. More generally, if one has a large number of copies of the same system, then the state of this ensemble is described by a density matrix, ..." Q: what's the copes of the same system. e.g. (a) One atom's entire energy levels. or (b) a large number of atoms. Which one, (a) or (b) could be the satiation that the density operator is describing. for me, (a) is reasonable.