## All unital qubit channels are 4-noisy operations

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We show that any unital qubit channel can be implemented by letting the input system interact unitarily with a four-dimensional environment in the maximally mixed state and then tracing out the environment. We also provide an example where the dimension of such an environment has to be at least 3.

Originalsprog Engelsk Letters in Mathematical Physics 109 1 1-9 0377-9017 https://doi.org/10.1007/s11005-018-1104-x Udgivet - 2019