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
State as a continuous linear functional in
Given an Archimedean order-unit space and a state (represented as a unital positive linear map in ), this definition treats as a continuous linear functional in the strong dual space . The continuity is established with respect to the order-unit norm , where the functional satisfies for all , implying its operator norm is at most .
Let be an Archimedean order-unit space. For any state and any observable , the evaluation of as a continuous linear functional in the strong dual space (denoted by ) at is equal to the value .
Injectivity of the Mapping from States to the Strong Dual
Let be an Archimedean order-unit space. The map that associates each state (a unital positive linear map) with a continuous linear functional in the strong dual space is injective.
The operator norm of a state satisfies
For any state on an Archimedean order-unit space , the operator norm of when considered as a continuous linear functional in the strong dual space satisfies . Here, the norm is taken with respect to the order-unit norm on .
The norm of a state is
Let be a nontrivial Archimedean order-unit space. For any state (a unital positive linear map in ), the operator norm of viewed as a continuous linear functional in the strong dual space is equal to , i.e., .
The map from states to the dual space is affine with respect to convex mixtures.
Let be an Archimedean order-unit space. For any two states and any , the continuous linear functional in the strong dual space corresponding to the convex mixture is equal to the convex combination of their corresponding functionals, given by , where and denote the representations of and as continuous linear functionals.
State space of an Archimedean order-unit space
Given an Archimedean order-unit space with order unit , the state space is defined as the set of continuous linear functionals in the weak dual space that are normalized, such that , and positive, such that for all with .
State from a point in the state space
For an Archimedean order-unit space , this function maps an element of the state space (defined as the set of normalized positive continuous linear functionals in the weak dual ) to its representation as a unital positive linear map .
State as a point in the state space
For an Archimedean order-unit space , this function maps a state (modeled as a unital positive linear map) to its corresponding element in the state space , which is the set of normalized positive continuous linear functionals in the weak dual space .
Evaluation of at equals
For an Archimedean order-unit space , let be a state in the state space (the set of normalized positive continuous linear functionals in the weak dual ). For any element , the evaluation of (viewed as a functional in the weak dual) at is equal to the evaluation of its representation as a unital positive linear map, , at . That is, .
Let be an Archimedean order-unit space. For any state , where denotes the space of states modeled as unital positive linear maps from to , applying the conversion map followed by its inverse returns the original state . That is, .
for
Let be an Archimedean order-unit space. For any element in the state space (the set of normalized positive continuous linear functionals in the weak dual ), converting 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 . That is, .
For an Archimedean order-unit space , there is an equivalence between the state space (the set of normalized positive continuous linear functionals in the weak dual ) and the space of states (modeled as unital positive linear maps from to ). This bijection is defined by the inverse mappings `toState` and `ofState`.
The mapping is injective
For an Archimedean order-unit space , the mapping , which identifies an element of the state space (defined as the set of normalized positive continuous linear functionals in the weak dual ) with its representation as a state (a unital positive linear map ), is injective.
Borel measurable space on
For an Archimedean order-unit space , the state space is equipped with the measurable space structure defined by the Borel -algebra of the weak-star topology.
The state space of is a Borel space
Let be an Archimedean order-unit space. The state space , considered as a subspace of the weak-star dual , is a Borel space. This means that its predefined measurable space structure is exactly the Borel -algebra generated by the weak-star topology.
The evaluation map is continuous on
Let be an Archimedean order-unit space and let be its state space equipped with the weak-star topology. For any element , the evaluation map from the state space to is continuous. In physical terms, this means that each expectation value depends continuously on the state.
Weak-star convergence of states is equivalent to pointwise convergence of expectation values
In the state space of an Archimedean order-unit space , a net of states converges to a state in the weak-star topology if and only if for every observable , the corresponding expectation values converge, i.e., .
The state space is closed
For an Archimedean order-unit space , the state space is a closed subset of the weak dual space (the dual of equipped with the weak-star topology).
The norm of any state is at most (or )
In an Archimedean order-unit space with order unit and the associated order-unit norm , every functional in the state space satisfies , where denotes the operator norm on the dual space .
The state space is compact
Let be an Archimedean order-unit space. The state space of , denoted as , which consists of the normalized positive linear functionals in the weak dual space , is compact with respect to the weak-star topology.
The state space of an Archimedean order-unit space is compact
Let be an Archimedean order-unit space. The state space of , denoted by and consisting of all normalized positive linear functionals in the weak dual space , is a compact topological space with respect to the weak-star topology.
Separability of implies metrizability of the state space
Let be an Archimedean order-unit space. If is a separable topological space, then the state space (the set of normalized positive linear functionals in the weak dual of equipped with the weak-* topology) is a metrizable space.
The state space is convex
Let be an Archimedean order-unit space. The state space of , denoted as , is the set of normalized positive linear functionals in the weak dual space . This set is convex, meaning that any mixture (convex combination) of states is also a state.
The range of `ofState` is
Let be an Archimedean order-unit space. The range of the mapping `ofState`, which embeds states (unital positive linear maps) into the weak dual space , is exactly the set of normalized positive continuous linear functionals.
A state is pure it is an extreme point of the state space
Let be an Archimedean order-unit space. A state in the state space (the set of normalized positive continuous linear functionals in the weak dual ) is a pure state if and only if is an extreme point of the state space .
