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## Main Question or Discussion Point

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

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.

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.