On states of quantum theory

TL;DR

This paper explores the structure of generalized quantum states within $C^{*}$-algebras, distinguishes between normal and singular states, and applies these concepts to quantum information theory, especially covariant cloners.

Contribution

It introduces a new approach to decompose bounded linear functionals into quantum states using GNS construction and analyzes the nature of covariant quantum channels.

Findings

Optimal covariant cloners cannot have a singular component

Normal states can be represented by density operators

The GNS construction aids in decomposing linear functionals into quantum states

Abstract

In this paper the generalized quantum states, i.e. positive and normalized linear functionals on $C^{*}$-algebras, are studied. Firstly, we study normal states, i.e. states which are represented by density operators, and singular states, i.e. states can not be represented by density operators. It is given an approach to the resolution of bounded linear functionals into quantum states by applying the GNS construction, i.e. the fundamental result of Gelfand, Neumark and Segal on the representation theory of $C^{*}$-algebras, and theory of projections. Secondly, it is given an application in quantum information theory. We study covariant cloners, i.e. quantum channels in the Heisenberg and the Schr\"{o}dinger pictures which are covariant by shifting, and it is shown that the optimal cloners can not have a singular component. Finally, we discuss on the representation of pure states in the sense of the Gelfand-Pettis integral. We also give physical interpretations and examples in different sections of the present work.