Experimentally, a mixed ensemble might be realized as follows. Consider a "black box" apparatus that spits electrons towards an observer. The electrons' Hilbert spaces are identical. The apparatus might produce electrons that are all in the same state; in this case, the electrons received by the observer are then a pure ensemble. However, the apparatus could produce electrons in different states. For example, it could produce two populations of electrons: one with state formula_24 with spins aligned in the positive direction, and the other with state formula_25 with spins aligned in the negative direction. Generally, this is a mixed ensemble, as there can be any number of populations, each corresponding to a different state.