Extremal Channels for a genaral Quantum system
Abstract
Quantum channels can be mathematically represented as completely positive trace-preserving maps that act on a density matrix. A general quantum channel can be written as a convex sum of `extremal' channels. We show that for an N-level system, the extremal channel can be characterized in terms of N2-N real parameters coupled with rotations. We give a representation for N= 2, 3, 4.
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