# quantum operator algebras in quantum field theories

## 0.1 Introduction

This is a topic entry that introduces quantum operator algebras and presents concisely the important roles they play in quantum field theories.

###### Definition 0.1.

Quantum operator algebras (QOA) in quantum field theories are defined as the algebras of observable operators, and as such, they are also related to the von Neumann algebra; quantum operators are usually defined on Hilbert spaces, or in some QFTs on Hilbert space bundles or other similar families of spaces.

###### Remark 0.1.

Representations of Banach $*$-algebras (that are defined on Hilbert spaces) are closely related to C* -algebra representations which provide a useful approach to defining quantum space-times.

## 0.2 Quantum operator algebras in quantum field theories: QOA Role in QFTs

Important examples of quantum operators are: the Hamiltonian operator (or Schrödinger operator), the position and momentum operators, Casimir operators, unitary operators and spin operators. The observable operators are also . More general operators were recently defined, such as Prigogine’s superoperators.

Another development in quantum theories was the introduction of Frechét nuclear spaces or ‘rigged’ Hilbert spaces (Hilbert bundles). The following sections define several types of quantum operator algebras that provide the foundation of modern quantum field theories in mathematical physics.

### 0.2.1 Quantum groups; quantum operator algebras and related symmetries.

Quantum theories adopted a new lease of life post 1955 when von Neumann beautifully re-formulated quantum mechanics (QM) and quantum theories (QT) in the mathematically rigorous context of Hilbert spaces and operator algebras defined over such spaces. From a current physics perspective, von Neumann’ s approach to quantum mechanics has however done much more: it has not only paved the way to expanding the role of symmetry in physics, as for example with the Wigner-Eckhart theorem and its applications, but also revealed the fundamental importance in quantum physics of the state space geometry of quantum operator algebras.

## 0.3 Basic mathematical definitions in QOA:

### 0.3.1 Von Neumann algebra

Let $\mathcal{H}$ denote a complex (separable) Hilbert space. A von Neumann algebra $\mathcal{A}$ acting on $\mathcal{H}$ is a subset of the algebra of all bounded operators $\mathcal{L}(\mathcal{H})$ such that:

• (i) $\mathcal{A}$ is closed under the adjoint operation (with the adjoint of an element $T$ denoted by $T^{*}$).

• (ii) $\mathcal{A}$ equals its bicommutant, namely:

 $\mathcal{A}=\{A\in\mathcal{L}(\mathcal{H}):\forall B\in\mathcal{L}(\mathcal{H}% ),\forall C\in\mathcal{A},~{}(BC=CB)\Rightarrow(AB=BA)\}~{}.$ (0.1)

If one calls a commutant of a set $\mathcal{A}$ the special set of bounded operators on $\mathcal{L}(\mathcal{H})$ which commute with all elements in $\mathcal{A}$, then this second condition implies that the commutant of the commutant of $\mathcal{A}$ is again the set $\mathcal{A}$.

On the other hand, a von Neumann algebra $\mathcal{A}$ inherits a unital subalgebra from $\mathcal{L}(\mathcal{H})$, and according to the first condition in its definition $\mathcal{A}$, it does indeed inherit a $*$-subalgebra structure as further explained in the next section on C* -algebras. Furthermore, one also has available a notable bicommutant theorem which states that: “$\mathcal{A}$ is a von Neumann algebra if and only if $\mathcal{A}$ is a $*$-subalgebra of $\mathcal{L}(\mathcal{H})$, closed for the smallest topology defined by continuous maps $(\xi,\eta)\longmapsto(A\xi,\eta)$ for all $$ where $<.,.>$ denotes the inner product defined on $\mathcal{H}$ ”.

For a well-presented treatment of the geometry of the state spaces of quantum operator algebras, the reader is referred to Aflsen and Schultz (2003; [AS2k3]).

### 0.3.2 Hopf algebra

First, a unital associative algebra consists of a linear space $A$ together with two linear maps:

 $\displaystyle m$ $\displaystyle:A\otimes A{\longrightarrow}A~{},~{}(multiplication)$ (0.2) $\displaystyle\eta$ $\displaystyle:\mathbb{C}{\longrightarrow}A~{},~{}(unity)$

satisfying the conditions

 $\displaystyle m(m\otimes\mathbf{1})$ $\displaystyle=m(\mathbf{1}\otimes m)$ (0.3) $\displaystyle m(\mathbf{1}\otimes\eta)$ $\displaystyle=m(\eta\otimes\mathbf{1})={\rm id}~{}.$

This first condition can be seen in terms of a commuting diagram :

 $\begin{CD}A\otimes A\otimes A@>{m\otimes{\rm id}}>{}>A\otimes A\\ @V{{\rm id}\otimes m}V{}V@V{}V{m}V\\ A\otimes A@ >m>>A\end{CD}$ (0.4)

Next suppose we consider ‘reversing the arrows’, and take an algebra $A$ equipped with a linear homorphisms $\Delta:A{\longrightarrow}A\otimes A$, satisfying, for $a,b\in A$ :

 $\displaystyle\Delta(ab)$ $\displaystyle=\Delta(a)\Delta(b)$ (0.5) $\displaystyle(\Delta\otimes{\rm id})\Delta$ $\displaystyle=({\rm id}\otimes\Delta)\Delta~{}.$

We call $\Delta$ a comultiplication, which is said to be coasociative in so far that the following diagram commutes

 $\begin{CD}A\otimes A\otimes A@<{\Delta\otimes{\rm id}}<{} (0.6)

There is also a counterpart to $\eta$, the counity map $\varepsilon:A{\longrightarrow}\mathbb{C}$ satisfying

 $({\rm id}\otimes\varepsilon)\circ\Delta=(\varepsilon\otimes{\rm id})\circ% \Delta={\rm id}~{}.$ (0.7)

A bialgebra $(A,m,\Delta,\eta,\varepsilon)$ is a linear space $A$ with maps $m,\Delta,\eta,\varepsilon$ satisfying the above properties.

Now to recover anything resembling a group structure, we must append such a bialgebra with an antihomomorphism $S:A{\longrightarrow}A$, satisfying $S(ab)=S(b)S(a)$, for $a,b\in A$ . This map is defined implicitly via the property :

 $m(S\otimes{\rm id})\circ\Delta=m({\rm id}\otimes S)\circ\Delta=\eta\circ% \varepsilon~{}~{}.$ (0.8)

We call $S$ the antipode map.

A Hopf algebra is then a bialgebra $(A,m,\eta,\Delta,\varepsilon)$ equipped with an antipode map $S$ .

Commutative and non-commutative Hopf algebras form the backbone of quantum ‘groups’ and are essential to the generalizations of symmetry. Indeed, in most respects a quantum ‘group’ is closely related to its dual Hopf algebra; in the case of a finite, commutative quantum group its dual Hopf algebra is obtained via Fourier transformation of the group elements. When Hopf algebras are actually associated with their dual, proper groups of matrices, there is considerable scope for their representations on both finite and infinite dimensional Hilbert spaces.

### 0.3.3 Groupoids

Recall that a groupoid ${\mathsf{G}}$ is, loosely speaking, a small category with inverses over its set of objects $X=Ob({\mathsf{G}})$ . One often writes ${\mathsf{G}}^{y}_{x}$ for the set of morphisms in ${\mathsf{G}}$ from $x$ to $y$ . A topological groupoid consists of a space ${\mathsf{G}}$, a distinguished subspace ${\mathsf{G}}^{(0)}={\rm Ob(\mathsf{G)}}\subset{\mathsf{G}}$, called the space of objects of ${\mathsf{G}}$, together with maps

 $r,s~{}:~{}\hbox{}$ (0.9)