## Fixed points of quantum operations.(English)Zbl 1060.81009

Summary: Quantum operations frequently occur in quantum measurement theory, quantum probability, quantum computation, and quantum information theory. If an operator $$A$$ is invariant under a quantum operation $$\phi$$, we call $$A$$ a $$\phi$$-fixed point. Physically, the $$\phi$$-fixed points are the operators that are not disturbed by the action of $$\phi$$. Our main purpose is to answer the following question. If $$A$$ is a $$\phi$$-fixed point, is $$A$$ compatible with the operation elements of $$\phi$$? We show in general that the answer is no and we give some sufficient conditions under which the answer is yes. Our results follow from some general theorems concerning completely positive maps and injectivity of operator systems and von Neumann algebras.

### MSC:

 81R15 Operator algebra methods applied to problems in quantum theory 81P15 Quantum measurement theory, state operations, state preparations 47H10 Fixed-point theorems 47N50 Applications of operator theory in the physical sciences
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### References:

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