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Quantum process
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<p>In <a href="/facts/Quantum_mechanics/BRc3Mzgr">quantum mechanics</a>, a quantum process is a somewhat ambiguous term which usually refers to the time evolution of an (<a href="/facts/Open_quantum_system/X3uw1Uln">open</a>) quantum system. Under very general assumptions, a quantum process is described by the <a href="/facts/Quantum_operation/EhCKTxgn">quantum operation</a> formalism (also known as a <i>quantum dynamical map</i>), which is a <a href="/facts/Linear_map/Q1PBKNVV">linear</a>, trace-preserving, and <a href="/facts/Completely_positive/GJGjwqgh">completely positive</a> map from the set of <a href="/facts/Density_matrix/A1XdylJa">density matrices</a> to itself. </p><p>For instance, in <a href="/facts/Quantum_process_tomography/RGTYGU8X">quantum process tomography</a>, the unknown quantum process is assumed to be a quantum operation. </p><p>However, not all quantum processes can be captured within the quantum operation formalism; in principle, the density matrix of a quantum system can undergo completely arbitrary time evolution. </p> <ul><li><a href="/facts/Michael_Nielsen/hedbpYkS">Nielsen, Michael A.</a>; <a href="/facts/Isaac_Chuang/qEgqj9tt">Chuang, Isaac L.</a> (2010). <a href="/facts/Quantum_Computation_and_Quantum_Information_(book)/l2pYpSSj"><i>Quantum Computation and Quantum Information</i></a> (2nd ed.). Cambridge: Cambridge University Press. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-1-107-00217-3. <a href="/facts/OCLC_(identifier)/Yqad8waq">OCLC</a> <a href="https://search.worldcat.org/oclc/844974180">844974180</a>.</li></ul>