Pseudounitary freedom in the operator-sum representation

Abstract

A dynamical map is a map that takes one density operator to another. Such a map can be written in an operator-sum representation (OSR) by using a spectral decomposition. The method of construction applies to general maps, which need not be completely positive. The OSR is not unique; there is freedom to choose a different set of operators in the OSR, yet still obtain the same map. Here, we show that, whereas the freedom for completely positive maps is unitary, the freedom for maps that are not necessarily completely positive is pseudounitary.