Conditional quantum entropy: Difference between revisions

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{{Short description|Measure of relative information in quantum information theory}}

The '''conditional quantum entropy''' is an [[entropy measure]] used in [[quantum information theory]]. It is a generalization of the [[conditional entropy]] of [[classical information theory]]. For a bipartite state <math>\rho^{AB}</math>, the conditional entropy is written <math>S(A|B)_\rho</math>, or <math>H(A|B)_\rho</math>, depending on the notation being used for the [[von Neumann entropy]]. The quantum conditional entropy was defined in terms of a conditional density operator <math> \rho_{A|B} </math> by [[Nicolas Cerf]] and [[Chris Adami]],<ref>{{Cite journal|lastlast1=Cerf|firstfirst1=N. J.|last2=Adami|first2=C.|date=1997|title=Negative Entropy and Information in Quantum Mechanics|journal=[[Physical Review Letters]]|volume=79|issue=26|pages=5194–5197|doi=10.1103/physrevlett.79.5194|arxiv=quant-ph/9512022|bibcode=1997PhRvL..79.5194C|s2cid=14834430}}</ref><ref>{{Cite journal|lastlast1=Cerf|firstfirst1=N. J.|last2=Adami|first2=C.|date=1999-08-01|title=Quantum extension of conditional probability|journal=[[Physical Review A]]|volume=60|issue=2|pages=893–897|doi=10.1103/PhysRevA.60.893|arxiv=quant-ph/9710001|bibcode=1999PhRvA..60..893C|s2cid=119451904 }}</ref> who showed that quantum conditional entropies can be negative, something that is forbidden in classical physics. The negativity of quantum conditional entropy is a sufficient criterion for quantum [[Separable state|non-separability]].

In what follows, we use the notation <math>S(\cdot)</math> for the [[von Neumann entropy]], which will simply be called "entropy".

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By analogy with the classical conditional entropy, one defines the conditional quantum entropy as <math>S(A|B)_\rho \ \stackrel{\mathrm{def}}{=}\ S(AB)_\rho - S(B)_\rho</math>.

An equivalent operational definition of the quantum conditional entropy (as a measure of the [[quantum communication]] cost or surplus when performing [[quantum state]] merging) was given by [[Michał Horodecki]], [[Jonathan Oppenheim]], and [[Andreas Winter]].<ref>{{Cite journal|lastlast1=Horodecki|firstfirst1=Michał|last2=Oppenheim|first2=Jonathan|last3=Winter|first3=Andreas|title=Partial quantum information|journal=Nature|volume=436|issue=7051|pages=673–676|arxiv=quant-ph/0505062|doi=10.1038/nature03909|bibcode=2005Natur.436..673H|year=2005|pmid=16079840|s2cid=4413693}}</ref>

==Properties==

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==References==

{{reflist}}

* {{Cite book|title=Quantum Computation and Quantum Information|last1=Nielsen|first=Michael A.|last2=Chuang|first2=Isaac L.|publisher=Cambridge University Press|year=2010|isbn=978-1-107-00217-3|edition=2nd|___location=Cambridge|pages=|oclc=844974180|author-link=Michael Nielsen|author-link2=Isaac Chuang|title-link=Quantum Computation and Quantum Information (book)}}

*{{citation|first=Mark M.|last=Wilde|arxiv=1106.1445|title=Quantum Information Theory|pages=xi-xii|year=2017|publisher=Cambridge University Press|bibcode = 2011arXiv1106.1445W |doi=10.1017/9781316809976.001|chapter=Preface to the Second Edition|isbn=9781316809976|s2cid=2515538 }}

[[Category:Quantum mechanical entropy]]