Quantum process explained

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. Pechukas. Philip. Reduced Dynamics Need Not Be Completely Positive. Physical Review Letters. 73. 8. 1994. 1060–1062. 0031-9007. 10.1103/PhysRevLett.73.1060. 10057614 . 1994PhRvL..73.1060P .
  2. Shaji . Anil . Sudarshan . E.C.G. . Who's afraid of not completely positive maps? . Physics Letters A . Elsevier BV . 341 . 1–4 . 2005 . 0375-9601 . 10.1016/j.physleta.2005.04.029 . 48–54. 2005PhLA..341...48S .