Beyond trace class -- Tensor products of Hilbert spaces and operator ideals in quantum physics

TL;DR

This paper explores the deep connections between tensor products of Hilbert spaces, operator ideals, and their foundational role in quantum physics, especially in algebraic quantum field theory and quantum information.

Contribution

It establishes a canonical isometric isomorphism between different tensor product constructions and highlights the significance of Banach operator ideals in quantum theory.

Findings

Proves isometric isomorphism between tensor product spaces (Theorem 3.8)

Revisits the role of trace class and Hilbert-Schmidt operators in quantum physics

Provides a linear algebraic description of quantum teleportation

Abstract

Starting from the meaning of the conjugate of a complex Hilbert space, including a related application of the theorem of Fr\'{e}chet-Riesz (by which an analysis of semilinear operators can be reduced to - linear - operator theory) to a revisit of applications of nuclear and absolutely $p$-summing operators in algebraic quantum field theory in the sense of Araki, Haag and Kastler ($p=2$) and more recently in the framework of general probabilistic spaces ($p=1$), we will outline that Banach operator ideals in the sense of Pietsch, or equivalently tensor products of Banach spaces in the sense of Grothendieck are even lurking in the foundations and philosophy of quantum physics and quantum information theory. In particular, we concentrate on their importance in algebraic quantum field theory. In doing so, we establish a canonical isometric isomorphism between the Hilbert spaces $H\otimes_2 (K \otimes_2 L)$ and $(H \otimes_2 K) \otimes_2 L$ (Theorem 3.8) and revisit the role of trace class operators. A few applications are specified, including the appropriateness of the class of Hilbert-Schmidt operators and an implied Banach operator ideal representation of the tensor product of two complex Hilbert spaces $H \otimes_2 K$ (Proposition 3.4) and a purely linear algebraic description of the quantum teleportation process (Example 3.10).