For classical-quantum states, the conditional entropy can change by at most epsilon log2(d_B-1) + h2(epsilon) under a trace-distance perturbation epsilon, and this bound cannot be improved.
Uniform continuity bounds for information characteristics of quantum channels depending on input dimension and on input energy
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abstract
We obtain continuity bounds for basic information characteristics of quantum channels depending on their input dimension (if it is finite) and on the input energy bound (if the input dimension is infinite). We pay a special attention to the case of a multi-mode quantum oscillator as an input system. First we prove continuity bounds for the output conditional mutual information for a single channel and for $n$ copies of a channel. Then we obtain estimates for variation of the output Holevo quantity with respect to simultaneous variations of a channel and of an input ensemble. As a result tight and close-to-tight continuity bounds for basic capacities of quantum channels depending on the input dimension are obtained. They complement the Leung-Smith continuity bounds depending on the output dimension. Finally, we obtain tight and close-to-tight continuity bounds for basic capacities of infinite-dimensional energy-constrained channels with respect to the energy-constrained Bures distance generating the strong convergence of quantum channels.
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quant-ph 1years
2019 1verdicts
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Optimal uniform continuity bound for conditional entropy of classical--quantum states
For classical-quantum states, the conditional entropy can change by at most epsilon log2(d_B-1) + h2(epsilon) under a trace-distance perturbation epsilon, and this bound cannot be improved.