This article has a style difficult to understand for readers interested in the subject. If you have ability, please edit it, helps to make it more accessible to the general public, without removing technical details of interest to specialists. Quantum mechanics is a formal theory, ie describing formal quantities (not physical) as the state vector, called a wave function representations of continuous basis, or the density matrix. These quantities, for a formalism or a given interpretation, correspond to physical observables. Consequently, the quantum state is a purely mathematical and abstract, and a source of difficulties in dealing with the theory for the first time. Especially, the quantum state is the state where you can find, since observing a quantum object is always obtained an eigenvalue for that observable, although the state of the system is not their own state for that observable. The first two states are pure quantum states, ie can be described by a vector "ket" Dirac, while the latter is a mixed quantum state, namely a statistical mixture of pure states. A mixed state also requires a statistical description of the quantum description. This is achieved with the density matrix, which extends quantum mechanics to quantum statistical mechanics. Text is available under the Creative Commons Attribution ShareAlike 3.0, additional terms may apply. See Terms of Use for more information.

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