Quantum process

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;^[1][2] in principle, the density matrix of a quantum system can undergo completely arbitrary time evolution.

References

1. Philip Pechukas, "Reduced Dynamics Need Not Be Completely Positive", Phys. Rev. Lett. 73, 1060 (1994).
2. Anil Shaji and E.C.G. Sudarshan "Who's Afraid of not Completely Positive Maps?", Physics Letters A Volume 341, 20 June 2005, Pages 48–54

• M. Nielsen and I. Chuang, Quantum Computation and Quantum Information, Cambridge University Press, 2000