Selinger's CPM construction and nuclear ideals

Abstract

Monoidal categories have recently been seen to be appropriate structures for establishing abstract axioms for quantum physics. The tensor product in a monoidal category corresponds to the creation of a composite system obtained by joining two independent quantum systems. This correspondence is formalized in the work of Abramsky-Coecke and Selinger. From this point of view, the tensor product in a monoidal category is abstracting the tensor product of Hilbert spaces. The notion of a dagger compact closed category axiomatizes further structure in the Hilbert space category. In particular, the dagger is an abstraction of the familiar Hilbert adjoint. Peter Selinger associates to each dagger compact closed category C its category of completely positive maps, denoted CPM(C). He proves that the resulting category is again a dagger compact closed category. We seek a similar result for tensored †-categories equipped with nuclear ideals. We establish an appropriate notion of completely positive maps in a nuclear ideal. We then define a CPM construction for tensored †-categories equipped with nuclear ideals. Analogous to Selinger's construction, given a nuclear ideal N for a tensored †-category C, our construction yields its category of completely positive maps, CPMN(C). We prove that CPM N(C). is again a tensored †-category. In the process, we also verify that our completely positive maps properly extend Selinger's notion of CPMs. In particular, they preserve nuclear-positive matrices, which are generalizations of von Neumann's positive density operators. As well, we characterize the completely positive maps for the nuclear ideal of Hilbert-Schmidt operators for Hilb and the nuclear ideal of finite relations for LFR. Our construction abstracts the passage from finite-dimensional Hilbert spaces to the category of all Hilbert spaces.