quantum process

{{Short description|Time evolution of quantum systems}}

In quantum mechanics, a quantum process is a somewhat ambiguous term which usually refers to the time evolution of an (open) quantum system. Under very general assumptions, a quantum process is described by the quantum operation formalism (also known as a quantum dynamical map), which is a linear, trace-preserving, and completely positive map from the set of density matrices to itself.

For instance, in 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;{{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 }}{{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 }} in principle, the density matrix of a quantum system can undergo completely arbitrary time evolution.