Physlib

PhyslibAlpha.ProbabilisticTheory.State.StateSpace

The state space

i. Overview

A state is a normalized positive functional: it gives the certain outcome `1` the value `1` and every positive observable a nonnegative value. So the states form a subset `stateSpace E` of the dual of `E`. This file studies it with the weak-star topology, the topology of `WeakDual ℝ E`.

Two states are close in the weak-star topology when they predict nearly the same expectation value for every observable, one observable at a time. In this topology the state space is compact, by the Banach–Alaoglu theorem, and metrizable when `E` is separable.

On `𝓢[ℝ, E]` itself the states carry the finer state metric.

ii. Key results

- `stateSpace E` is the set of states in the weak dual of `E`. - `StateSpace.toState` and `StateSpace.ofState` pass between points of `stateSpace E` and states. - `StateSpace.tendsto_iff_forall_apply_tendsto` proves that weak-star convergence is convergence of every expectation value. - `isCompact_stateSpace` proves that the state space is weak-star compact. - `convex_stateSpace` proves that the state space is convex. - `StateSpace.isPure_iff_mem_extremePoints` proves that the pure states are its extreme points.

iii. Table of contents

  • A. States as continuous functionals
  • B. The state space in the weak dual
  • C. Pointwise convergence
  • D. Compactness
  • E. Convexity and pure states

A. States as continuous functionals

B. The state space in the weak dual

C. Pointwise convergence

D. Compactness

E. Convexity and pure states

26 declarations

definition

State ω\omega as a continuous linear functional in E∗E^*

Given an Archimedean order-unit space EE and a state ω\omega (represented as a unital positive linear map in S(R,E)\mathcal{S}(\mathbb{R}, E)), this definition treats ω\omega as a continuous linear functional in the strong dual space E∗E^*. The continuity is established with respect to the order-unit norm ∥⋅∥1\|\cdot\|_1, where the functional satisfies ∣ω(A)∣≤∥A∥1|\omega(A)| \le \|A\|_1 for all A∈EA \in E, implying its operator norm is at most 11.

theorem

ω.toStrongDual(A)=ω(A)\omega.\text{toStrongDual}(A) = \omega(A)

Let EE be an Archimedean order-unit space. For any state ω∈S(R,E)\omega \in \mathcal{S}(\mathbb{R}, E) and any observable A∈EA \in E, the evaluation of ω\omega as a continuous linear functional in the strong dual space E∗E^* (denoted by ω.toStrongDual\omega.\text{toStrongDual}) at AA is equal to the value ω(A)\omega(A).

theorem

Injectivity of the Mapping from States to the Strong Dual

Let EE be an Archimedean order-unit space. The map toStrongDual:S(R,E)→E∗\text{toStrongDual} : \mathcal{S}(\mathbb{R}, E) \to E^* that associates each state (a unital positive linear map) ω∈S(R,E)\omega \in \mathcal{S}(\mathbb{R}, E) with a continuous linear functional in the strong dual space E∗E^* is injective.

theorem

The operator norm of a state ω\omega satisfies ∥ω∥≤1\|\omega\| \le 1

For any state ω∈S(R,E)\omega \in \mathcal{S}(\mathbb{R}, E) on an Archimedean order-unit space EE, the operator norm of ω\omega when considered as a continuous linear functional in the strong dual space E∗E^* satisfies ∥ω∥≤1\|\omega\| \le 1. Here, the norm is taken with respect to the order-unit norm ∥⋅∥1\|\cdot\|_1 on EE.

theorem

The norm of a state ω\omega is 11

Let EE be a nontrivial Archimedean order-unit space. For any state ω\omega (a unital positive linear map in S(R,E)\mathcal{S}(\mathbb{R}, E)), the operator norm of ω\omega viewed as a continuous linear functional in the strong dual space E∗E^* is equal to 11, i.e., ∥ω∥=1\|\omega\| = 1.

theorem

The map from states to the dual space is affine with respect to convex mixtures.

Let EE be an Archimedean order-unit space. For any two states ω,φ∈S[R,E]\omega, \varphi \in \mathcal{S}[\mathbb{R}, E] and any t∈[0,1]t \in [0, 1], the continuous linear functional in the strong dual space E∗E^* corresponding to the convex mixture tω+(1−t)φt\omega + (1-t)\varphi is equal to the convex combination of their corresponding functionals, given by t⋅ω∗+(1−t)⋅φ∗t \cdot \omega^* + (1 - t) \cdot \varphi^*, where ω∗\omega^* and φ∗\varphi^* denote the representations of ω\omega and φ\varphi as continuous linear functionals.

definition

State space of an Archimedean order-unit space EE

Given an Archimedean order-unit space EE with order unit 1\mathbf{1}, the state space is defined as the set of continuous linear functionals ff in the weak dual space E∗E^* that are normalized, such that f(1)=1f(\mathbf{1}) = 1, and positive, such that f(A)≥0f(A) \ge 0 for all A∈EA \in E with A≥0A \ge 0.

definition

State from a point in the state space

For an Archimedean order-unit space EE, this function maps an element ω\omega of the state space (defined as the set of normalized positive continuous linear functionals in the weak dual E∗E^*) to its representation as a unital positive linear map E→RE \to \mathbb{R}.

definition

State ω\omega as a point in the state space E∗E^*

For an Archimedean order-unit space EE, this function maps a state ω∈S(R,E)\omega \in \mathcal{S}(\mathbb{R}, E) (modeled as a unital positive linear map) to its corresponding element in the state space stateSpace E\text{stateSpace } E, which is the set of normalized positive continuous linear functionals in the weak dual space E∗E^*.

theorem

Evaluation of ω∈stateSpace E\omega \in \text{stateSpace } E at AA equals toState(ω)(A)\text{toState}(\omega)(A)

For an Archimedean order-unit space EE, let ω\omega be a state in the state space stateSpace E\text{stateSpace } E (the set of normalized positive continuous linear functionals in the weak dual E∗E^*). For any element A∈EA \in E, the evaluation of ω\omega (viewed as a functional in the weak dual) at AA is equal to the evaluation of its representation as a unital positive linear map, toState(ω)\text{toState}(\omega), at AA. That is, ω(A)=toState(ω)(A)\omega(A) = \text{toState}(\omega)(A).

theorem

toState(ofState(ω))=ω\text{toState}(\text{ofState}(\omega)) = \omega

Let EE be an Archimedean order-unit space. For any state ω∈S(R,E)\omega \in \mathcal{S}(\mathbb{R}, E), where S(R,E)\mathcal{S}(\mathbb{R}, E) denotes the space of states modeled as unital positive linear maps from EE to R\mathbb{R}, applying the conversion map ofState\text{ofState} followed by its inverse toState\text{toState} returns the original state ω\omega. That is, toState(ofState(ω))=ω\text{toState}(\text{ofState}(\omega)) = \omega.

theorem

ofState(toState(ω))=ω\text{ofState}(\text{toState}(\omega)) = \omega for ω∈stateSpace E\omega \in \text{stateSpace } E

Let EE be an Archimedean order-unit space. For any element ω\omega in the state space stateSpace E\text{stateSpace } E (the set of normalized positive continuous linear functionals in the weak dual E∗E^*), converting ω\omega to its representation as a state (a unital positive linear map) and then back to an element of the state space results in the original element ω\omega. That is, ofState(toState(ω))=ω\text{ofState}(\text{toState}(\omega)) = \omega.

definition

stateSpace E≃S(R,E)\text{stateSpace } E \simeq \mathcal{S}(\mathbb{R}, E)

For an Archimedean order-unit space EE, there is an equivalence between the state space stateSpace E\text{stateSpace } E (the set of normalized positive continuous linear functionals in the weak dual E∗E^*) and the space S(R,E)\mathcal{S}(\mathbb{R}, E) of states (modeled as unital positive linear maps from EE to R\mathbb{R}). This bijection is defined by the inverse mappings `toState` and `ofState`.

theorem

The mapping toState\text{toState} is injective

For an Archimedean order-unit space EE, the mapping toState:stateSpace E→S[R,E]\text{toState} : \text{stateSpace } E \to \mathcal{S}[\mathbb{R}, E], which identifies an element ω\omega of the state space (defined as the set of normalized positive continuous linear functionals in the weak dual E∗E^*) with its representation as a state (a unital positive linear map E→RE \to \mathbb{R}), is injective.

instance

Borel measurable space on stateSpace E\text{stateSpace } E

For an Archimedean order-unit space EE, the state space stateSpace E\text{stateSpace } E is equipped with the measurable space structure defined by the Borel σ\sigma-algebra of the weak-star topology.

instance

The state space of EE is a Borel space

Let EE be an Archimedean order-unit space. The state space stateSpace E\text{stateSpace } E, considered as a subspace of the weak-star dual E∗E^*, is a Borel space. This means that its predefined measurable space structure is exactly the Borel σ\sigma-algebra generated by the weak-star topology.

theorem

The evaluation map ω↦ω(A)\omega \mapsto \omega(A) is continuous on stateSpace E\text{stateSpace } E

Let EE be an Archimedean order-unit space and let stateSpace E\text{stateSpace } E be its state space equipped with the weak-star topology. For any element A∈EA \in E, the evaluation map ω↦ω(A)\omega \mapsto \omega(A) from the state space to R\mathbb{R} is continuous. In physical terms, this means that each expectation value depends continuously on the state.

theorem

Weak-star convergence of states is equivalent to pointwise convergence of expectation values

In the state space of an Archimedean order-unit space EE, a net of states fif_i converges to a state ω\omega in the weak-star topology if and only if for every observable A∈EA \in E, the corresponding expectation values converge, i.e., fi(A)→ω(A)f_i(A) \to \omega(A).

theorem

The state space is closed

For an Archimedean order-unit space EE, the state space stateSpace E\text{stateSpace } E is a closed subset of the weak dual space E∗E^* (the dual of EE equipped with the weak-star topology).

theorem

The norm of any state is at most 11 (or stateSpace E⊆Bˉ(0,1)\text{stateSpace } E \subseteq \bar{B}(0, 1))

In an Archimedean order-unit space EE with order unit 1\mathbf{1} and the associated order-unit norm ∥⋅∥1\|\cdot\|_1, every functional ff in the state space stateSpace E\text{stateSpace } E satisfies ∥f∥≤1\|f\| \le 1, where ∥⋅∥\|\cdot\| denotes the operator norm on the dual space E∗E^*.

theorem

The state space stateSpace E\text{stateSpace } E is compact

Let EE be an Archimedean order-unit space. The state space of EE, denoted as stateSpace E\text{stateSpace } E, which consists of the normalized positive linear functionals in the weak dual space E∗E^*, is compact with respect to the weak-star topology.

instance

The state space of an Archimedean order-unit space is compact

Let EE be an Archimedean order-unit space. The state space of EE, denoted by stateSpace E\text{stateSpace } E and consisting of all normalized positive linear functionals in the weak dual space E∗E^*, is a compact topological space with respect to the weak-star topology.

instance

Separability of EE implies metrizability of the state space

Let EE be an Archimedean order-unit space. If EE is a separable topological space, then the state space stateSpace E\text{stateSpace } E (the set of normalized positive linear functionals in the weak dual of EE equipped with the weak-* topology) is a metrizable space.

theorem

The state space is convex

Let EE be an Archimedean order-unit space. The state space of EE, denoted as stateSpace E\text{stateSpace } E, is the set of normalized positive linear functionals in the weak dual space E∗E^*. This set is convex, meaning that any mixture (convex combination) of states is also a state.

theorem

The range of `ofState` is stateSpace E\text{stateSpace } E

Let EE be an Archimedean order-unit space. The range of the mapping `ofState`, which embeds states ω∈S(R,E)\omega \in \mathcal{S}(\mathbb{R}, E) (unital positive linear maps) into the weak dual space E∗E^*, is exactly the set stateSpace E\text{stateSpace } E of normalized positive continuous linear functionals.

theorem

A state is pure   ⟺  \iff it is an extreme point of the state space

Let EE be an Archimedean order-unit space. A state ω\omega in the state space stateSpace E\text{stateSpace } E (the set of normalized positive continuous linear functionals in the weak dual E∗E^*) is a pure state if and only if ω\omega is an extreme point of the state space stateSpace E\text{stateSpace } E.