A mixture of states is an convex combination of states, i.e.
| ρ = | ∑ | pjρj |
| j |
where
| ∑ | pj = 1 |
| j |
and the 
are real.
Physically this corresponds having state ρj with probability pj.
All mixed states can be written as a mixture of pure states.
See also
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