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Min entropy : ウィキペディア英語版
Min entropy
The min entropy, in information theory, is the smallest of the Rényi family of entropies, corresponding to the most conservative way of measuring the unpredictability of a set of outcomes, as the negative logarithm of the probability of the ''most likely'' outcome. The various Rényi entropies are all equal for a uniform distribution, but measure the unpredictability of a nonuniform distribution in different ways. The min entropy is never greater than the ordinary or Shannon entropy (which measures the average unpredictability of the outcomes) and that in turn is never greater than the Hartley or max entropy, defined as the logarithm of the ''number'' of outcomes.
As with the classical Shannon entropy and its quantum generalization, the von Neumann entropy, one can define a conditional versions of min entropy. The conditional quantum min entropy is a one-shot, or conservative, analog of conditional quantum entropy.
To interpret a conditional information measure, suppose Alice and Bob were to share a bipartite quantum state \rho_. Alice has access to system A and Bob to system B. The conditional entropy measures the average uncertainty Bob has about Alice's state upon sampling from his own system. The min entropy can be interpreted as the distance of a state from a maximally entangled state.
This concept is useful in quantum cryptography, in the context of privacy amplification (See for example 〔Vazirani, Umesh, and Thomas Vidick. "Fully device independent quantum key distribution." (2012)〕).
== Definitions ==

Definition: Let \rho_ be a bipartite density operator on the space \mathcal_A \otimes \mathcal_B. The min-entropy of A conditioned on B is defined to be
:::H_(A|B)_ \equiv -\inf_D_(\rho_||I_A \otimes \sigma_B)
where the infimum ranges over all density operators \sigma_B on the space \mathcal_B. The measure D_ is the maximum relative entropy defined as
:::D_(\rho||\sigma) = \inf_\
The smooth min entropy is defined in terms of the min entropy.
:::H_^(A|B)_ = \sup_ H_(A|B)_
where the sup and inf range over density operators \rho'_ which are \epsilon-close to \rho_
. This measure of \epsilon-close is defined in terms of the purified distance
:::P(\rho,\sigma) = \sqrt
where F(\rho,\sigma) is the fidelity measure.
These quantities can be seen as generalizations of the von Neumann entropy. Indeed, the von Neumann entropy can be expressed as
:::S(A|B)_ = \lim_\lim_\fracH_^(A^n|B^n)_^(A|B)_ \geq H_^(A|BC)_~.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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