# Quantum process

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{{Short description|Time evolution of quantum systems}}
In [quantum mechanics](/source/quantum_mechanics), a '''quantum process''' is a somewhat ambiguous term which usually refers to the time evolution of an ([open](/source/open_quantum_system)) quantum system.  Under very general assumptions, a quantum process is described by the [quantum operation](/source/quantum_operation) formalism (also known as a ''quantum dynamical map''), which is a [linear](/source/linear_map), trace-preserving, and [completely positive](/source/completely_positive) map from the set of [density matrices](/source/density_matrix) to itself.

For instance, in [quantum process tomography](/source/quantum_process_tomography), the unknown quantum process is assumed to be a quantum operation.

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.

== References ==
<references/>

* {{Cite book|last1=Nielsen|first1=Michael A.|authorlink1=Michael Nielsen|last2=Chuang|first2=Isaac L.|authorlink2=Isaac Chuang|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)}}

Category:Quantum mechanics

{{Quantum-stub}}

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