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<p><b>New page</b></p><div>{{Short description|Time evolution of quantum systems}}<br />
In [[Physics:Quantum mechanics|quantum mechanics]], a '''quantum process''' is a somewhat ambiguous term which usually refers to the time evolution of an ([[Physics:Open quantum system|open]]) quantum system. Under very general assumptions, a quantum process is described by the [[Physics:Quantum operation|quantum operation]] formalism (also known as a ''quantum dynamical map''), which is a [[Linear map|linear]], trace-preserving, and completely positive map from the set of [[Density matrix|density matrices]] to itself.<br />
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For instance, in quantum process tomography, the unknown quantum process is assumed to be a quantum operation.<br />
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However, not all quantum processes can be captured within the quantum operation formalism;<ref>{{cite journal|last1=Pechukas|first1=Philip|title=Reduced Dynamics Need Not Be Completely Positive|journal=Physical Review Letters|volume=73|issue=8|year=1994|pages=1060–1062|issn=0031-9007|doi=10.1103/PhysRevLett.73.1060|pmid=10057614 |bibcode=1994PhRvL..73.1060P }}</ref><ref>{{cite journal | last1=Shaji | first1=Anil | last2=Sudarshan | first2=E.C.G. | title=Who's afraid of not completely positive maps? | journal=Physics Letters A | publisher=Elsevier BV | volume=341 | issue=1–4 | year=2005 | issn=0375-9601 | doi=10.1016/j.physleta.2005.04.029 | pages=48–54| bibcode=2005PhLA..341...48S }}</ref> in principle, the density matrix of a quantum system can undergo completely arbitrary time evolution.<br />
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== References ==<br />
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* {{Cite book|last1=Nielsen|first1=Michael A.|last2=Chuang|first2=Isaac L.|title=Quantum Computation and Quantum Information|publisher=Cambridge University Press|location=Cambridge|year=2010|edition=2nd|oclc=844974180|isbn=978-1-107-00217-3|title-link=Quantum Computation and Quantum Information (book)}}<br />
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[[Category:Quantum mechanics]]<br />
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{{Sourceattribution|Quantum process}}</div>imported>WikiHarold