Editing Quantum decoherence (section)

===Depolarizing===
'''Depolarizing''' is a non-unitary transformation on a quantum system which [[Linear map|maps]] pure states to mixed states. This is a non-unitary process because any transformation that reverses this process will map states out of their respective Hilbert space thus not preserving positivity (i.e. the original [[Probability|probabilities]] are mapped to negative probabilities, which is not allowed). The 2-dimensional case of such a transformation would consist of mapping pure states on the surface of the [[Bloch sphere]] to mixed states within the Bloch sphere. This would contract the Bloch sphere by some finite amount and the reverse process would expand the Bloch sphere, which cannot happen.