Physlib

PhyslibAlpha.AlgebraicFramework.CStarAlgebra.Stinespring.Dilation

Stinespring's dilation theorem

Ported from `unbounded-alpha-public`'s `QuantumMechanics/Unbounded/OperatorAlgebra/Dynamics/ Stinespring/Core.lean`, restated against this repo's own bare Mathlib hypotheses (`[CStarAlgebra A] [PartialOrder A] [StarOrderedRing A]`) instead of the upstream-superseded `OperatorAlgebra` class — see `Kernel.lean`'s docstring for why this is a pure restatement, not new mathematics.

Stinespring's theorem is the general statement underlying every physical quantum operation: a completely positive map `J : A →CP (H →L[ℂ] H)` out of a C⋆-algebra always arises from coupling to a larger system and applying an ordinary `⋆`-representation. Concretely, it builds a Hilbert space `K`, a `⋆`-representation `π : A →⋆ₐ[ℂ] (K →L[ℂ] K)`, and an isometry-like embedding `V : H →L[ℂ] K` with

`J a = V⋆ π(a) V` (`canonical_stinespring_identity`).

This subsumes Naimark's dilation theorem as the commutative special case (a POVM is a CP map on the commutative algebra of bounded measurable functions; positivity there is automatically complete positivity, and indicator functions recover the projection-valued dilation from `π`).

Construction

The construction is a vector-valued (operator-valued) generalization of the GNS construction, built on the algebraic tensor product `A ⊗[ℂ] H`:

- `sesquiBilinear`/`tensorInner` : the (possibly degenerate) `B(H)`-valued-kernel-induced sesquilinear form `⟪a ⊗ h, b ⊗ k⟫ := ⟪h, J(a⋆b) k⟫` on `A ⊗[ℂ] H`. - `tensorCore`/`tensorSeminormed`/`tensorInnerProductSpace` : the pre-Hilbert-space structure this form induces (positivity is `Kernel.lean`'s CP-kernel positivity). - `leftMul`/`leftMulK` : left multiplication by `A` on the tensor product, shown contractive (`leftMul_norm_le`) and hence extending to the completion. - `Canonical.K J` : the completion of `A ⊗[ℂ] H` under the kernel seminorm — the canonical Stinespring dilation space. - `Canonical.canonicalRepresentation`, `Canonical.embedding` : the representation `π` and the embedding `V : H →L[ℂ] K J`. - `Canonical.canonical_stinespring_identity` : the Stinespring identity itself. - `Canonical.canonicalWitness` : the identity packaged as a `StinespringWitness`. - `exists_stinespringWitness` : the dilation theorem, stated as an existence theorem.

Main results

  • `Canonical.canonical_stinespring_identity`
  • `exists_stinespringWitness`

The seminorm and pre-Hilbert structures induced by the kernel

The canonical CP-dependent Hilbert space

The CP kernel changes the norm, so its pre-Hilbert space is made a type synonym. This is the same pattern used by Mathlib's own scalar GNS construction (`PositiveLinearMap.PreGNS`): the synonym lets the tensor product retain its original module structure while carrying a CP-dependent seminorm and inner product.

62 declarations

abbrev

Algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H

Given a C∗C^*-algebra AA and a complex Hilbert space HH, this definition represents the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H. It serves as the underlying carrier space for the Stinespring dilation construction.

definition

Linear functional b⊗k↦⟨h,J(a∗b)k⟩b \otimes k \mapsto \langle h, J(a^* b) k \rangle

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J: A \to \mathcal{B}(H), an element a∈Aa \in A, and a vector h∈Hh \in H, this definition is the linear functional fa,h:A⊗CH→Cf_{a,h} : A \otimes_{\mathbb{C}} H \to \mathbb{C} uniquely determined by its action on elementary tensors: fa,h(b⊗k)=⟨h,J(a∗b)k⟩f_{a,h}(b \otimes k) = \langle h, J(a^* b) k \rangle where b∈Ab \in A, k∈Hk \in H, and ⟨⋅,⋅⟩\langle \cdot, \cdot \rangle denotes the inner product on HH. This functional serves as the fundamental building block for constructing the inner product in Stinespring's dilation theorem.

theorem

Evaluation of the base functional on elementary tensors: fa,h(b⊗k)=⟨h,J(a∗b)k⟩f_{a,h}(b \otimes k) = \langle h, J(a^* b) k \rangle

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. For a completely positive map J:A→B(H)J: A \to \mathcal{B}(H), elements a,b∈Aa, b \in A, and vectors h,k∈Hh, k \in H, the linear functional fa,hf_{a,h} (the `baseFunctional`) evaluated at the elementary tensor b⊗k∈A⊗CHb \otimes k \in A \otimes_{\mathbb{C}} H satisfies: fa,h(b⊗k)=⟨h,J(a∗b)k⟩f_{a,h}(b \otimes k) = \langle h, J(a^* b) k \rangle where ⟨⋅,⋅⟩\langle \cdot, \cdot \rangle denotes the inner product on HH and a∗a^* is the adjoint of aa.

definition

Sesquilinear map (a,h)↦(b⊗k↦⟨h,J(a∗b)k⟩)(a, h) \mapsto (b \otimes k \mapsto \langle h, J(a^* b) k \rangle)

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J: A \to \mathcal{B}(H) be a completely positive map. This definition is the map S:A×H→(A⊗CH→C)S: A \times H \to (A \otimes_{\mathbb{C}} H \to \mathbb{C}) which is conjugate-linear in both a∈Aa \in A and h∈Hh \in H. For any fixed a∈Aa \in A and h∈Hh \in H, the resulting linear functional S(a,h)S(a, h) acts on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H such that for any elementary tensor b⊗k∈A⊗CHb \otimes k \in A \otimes_{\mathbb{C}} H: (S(a,h))(b⊗k)=⟨h,J(a∗b)k⟩(S(a, h))(b \otimes k) = \langle h, J(a^* b) k \rangle where ⟨⋅,⋅⟩\langle \cdot, \cdot \rangle is the inner product on HH (conventionally conjugate-linear in the first argument). This map packages the `baseFunctional` into a sesquilinear form over AA and HH.

definition

Sesquilinear form on A⊗CHA \otimes_{\mathbb{C}} H defined by ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), this definition constructs a (possibly degenerate) sesquilinear form on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H. This form is represented as a map that sends an element x∈A⊗CHx \in A \otimes_{\mathbb{C}} H to a linear functional, such that the mapping is conjugate-linear. For elementary tensors a⊗ha \otimes h and b⊗kb \otimes k in A⊗CHA \otimes_{\mathbb{C}} H, the form satisfies: ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H where ⟨⋅,⋅⟩H\langle \cdot, \cdot \rangle_H denotes the inner product on HH (conventionally conjugate-linear in the first argument) and a∗a^* is the adjoint of aa.

theorem

⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), the sesquilinear form ⟨⋅,⋅⟩\langle \cdot, \cdot \rangle defined on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H satisfies the following identity for any elementary tensors a⊗ha \otimes h and b⊗kb \otimes k (where a,b∈Aa, b \in A and h,k∈Hh, k \in H): ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H where a∗a^* is the adjoint of aa and ⟨⋅,⋅⟩H\langle \cdot, \cdot \rangle_H denotes the inner product on HH, which is conventionally conjugate-linear in the first argument.

theorem

Conjugate Symmetry of ⟨a⊗h,b⊗k⟩J\langle a \otimes h, b \otimes k \rangle_J

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let ⟨⋅,⋅⟩J\langle \cdot, \cdot \rangle_J be the sesquilinear form on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H defined by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For any a,b∈Aa, b \in A and h,k∈Hh, k \in H, the form satisfies conjugate symmetry on elementary tensors: ⟨a⊗h,b⊗k⟩J‾=⟨b⊗k,a⊗h⟩J\overline{\langle a \otimes h, b \otimes k \rangle_J} = \langle b \otimes k, a \otimes h \rangle_J where the overline denotes complex conjugation.

theorem

Conjugate Symmetry of the Stinespring Sesquilinear Form on A⊗HA \otimes H

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let ⟨⋅,⋅⟩J\langle \cdot, \cdot \rangle_J denote the sesquilinear form induced by JJ on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H (defined on elementary tensors by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H). For any elements x,y∈A⊗CHx, y \in A \otimes_{\mathbb{C}} H, the form satisfies conjugate symmetry: ⟨x,y⟩J‾=⟨y,x⟩J\overline{\langle x, y \rangle_J} = \langle y, x \rangle_J where the overline denotes complex conjugation.

theorem

Positivity of the induced sesquilinear form ⟨x,x⟩J≥0\langle x, x \rangle_J \ge 0 for a completely positive map JJ

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), the sesquilinear form ⟨⋅,⋅⟩J\langle \cdot, \cdot \rangle_J defined on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H (where ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H) is positive semi-definite. That is, for any x∈A⊗CHx \in A \otimes_{\mathbb{C}} H, it holds that 0≤⟨x,x⟩J0 \le \langle x, x \rangle_J.

definition

Pre-inner product structure on A⊗CHA \otimes_{\mathbb{C}} H induced by JJ

Given a completely positive map J:A→B(H)J: A \to \mathcal{B}(H) from a C∗C^*-algebra AA to the space of bounded linear operators on a complex Hilbert space HH, this definition constructs a pre-inner product space structure on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H. The structure is defined by the sesquilinear form ⟨⋅,⋅⟩J\langle \cdot, \cdot \rangle_J (specifically, the `tensorInner` map), which satisfies ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H for elementary tensors a⊗ha \otimes h and b⊗kb \otimes k. This structure includes the definitions for the inner product, conjugate symmetry, and non-negativity required to treat A⊗CHA \otimes_{\mathbb{C}} H as a pre-Hilbert space.

definition

Left multiplication by a∈Aa \in A on A⊗CHA \otimes_{\mathbb{C}} H

For an element aa of a C∗C^*-algebra AA, this is the C\mathbb{C}-linear map on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H (where HH is a complex Hilbert space) that acts as left multiplication by aa on the first factor. Specifically, it is defined on simple tensors by a⋅(b⊗h)=(ab)⊗ha \cdot (b \otimes h) = (ab) \otimes h for b∈Ab \in A and h∈Hh \in H.

theorem

La(b⊗h)=(ab)⊗hL_a(b \otimes h) = (ab) \otimes h

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. For any elements a,b∈Aa, b \in A and any vector h∈Hh \in H, the left multiplication operator leftMula\text{leftMul}_a acting on the simple tensor b⊗hb \otimes h in the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H satisfies: leftMula(b⊗h)=(ab)⊗h \text{leftMul}_a(b \otimes h) = (ab) \otimes h where abab denotes the product in AA.

theorem

Adjoint property of left multiplication: ⟨a⋅x,y⟩J=⟨x,a∗⋅y⟩J\langle a \cdot x, y \rangle_J = \langle x, a^* \cdot y \rangle_J

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let ⟨⋅,⋅⟩J\langle \cdot, \cdot \rangle_J be the sesquilinear form induced on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H defined on elementary tensors by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For any element a∈Aa \in A and any tensors x,y∈A⊗CHx, y \in A \otimes_{\mathbb{C}} H, the left multiplication operator satisfies the following adjoint relation: ⟨a⋅x,y⟩J=⟨x,a∗⋅y⟩J \langle a \cdot x, y \rangle_J = \langle x, a^* \cdot y \rangle_J where a⋅xa \cdot x denotes the action of aa on the first factor of the tensor product (i.e., a⋅(b⊗h)=(ab)⊗ha \cdot (b \otimes h) = (ab) \otimes h) and a∗a^* is the adjoint of aa in AA.

theorem

leftMul(ab)x=leftMul(a)(leftMul(b)x)\text{leftMul}(ab)x = \text{leftMul}(a)(\text{leftMul}(b)x)

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. For any elements a,b∈Aa, b \in A and any element xx in the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H, the left multiplication operator satisfies the identity leftMul(ab)x=leftMul(a)(leftMul(b)x)\text{leftMul}(ab)x = \text{leftMul}(a)(\text{leftMul}(b)x).

theorem

Left Multiplication on A⊗CHA \otimes_{\mathbb{C}} H is Additive in the Algebra Component AA

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. For any elements a,b∈Aa, b \in A and any element xx in the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H, the left multiplication of xx by the sum a+ba + b is equal to the sum of the left multiplication of xx by aa and the left multiplication of xx by bb, which can be written as (a+b)⋅x=a⋅x+b⋅x(a + b) \cdot x = a \cdot x + b \cdot x.

theorem

leftMul(r⋅a)=r⋅leftMul(a)\text{leftMul}(r \cdot a) = r \cdot \text{leftMul}(a)

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. For any scalar r∈Cr \in \mathbb{C}, any element a∈Aa \in A, and any element xx in the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H, the action of left multiplication by r⋅ar \cdot a on xx is equal to rr times the action of left multiplication by aa on xx. That is, leftMul(r⋅a)x=r⋅(leftMul(a)x)\text{leftMul}(r \cdot a)x = r \cdot (\text{leftMul}(a)x).

theorem

Left multiplication by 1∈A1 \in A is the identity map on A⊗CHA \otimes_{\mathbb{C}} H

Let AA be a C∗C^*-algebra and HH a complex Hilbert space. For any element xx in the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H, the left multiplication by the identity element 1∈A1 \in A acts as the identity map, i.e., 1⋅x=x1 \cdot x = x.

theorem

L∥a∥2⋅1−a∗a(x)=∥a∥2x−La∗a(x)L_{\|a\|^2 \cdot 1 - a^* a}(x) = \|a\|^2 x - L_{a^* a}(x) on A⊗CHA \otimes_{\mathbb{C}} H

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. For any element a∈Aa \in A and any element xx in the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H, let LbL_b denote the linear map of left multiplication by b∈Ab \in A on the first factor of the tensor product. Then the following identity holds: L∥a∥2⋅1−a∗a(x)=∥a∥2x−La∗a(x) L_{\|a\|^2 \cdot 1 - a^* a}(x) = \|a\|^2 x - L_{a^* a}(x) where ∥a∥\|a\| is the norm of aa, 11 is the identity element of AA, and a∗a^* is the adjoint of aa.

theorem

⟨qx,x⟩J=⟨x,qx⟩J\langle qx, x \rangle_J = \langle x, qx \rangle_J for self-adjoint q∈Aq \in A

Let AA be a C∗C^*-algebra and HH a complex Hilbert space. Let J:A→B(H)J: A \to \mathcal{B}(H) be a completely positive map, and let ⟨⋅,⋅⟩J\langle \cdot, \cdot \rangle_J be the sesquilinear form on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H defined by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For any self-adjoint element q∈Aq \in A (i.e., q∗=qq^* = q) and any x∈A⊗CHx \in A \otimes_{\mathbb{C}} H, the following identity holds: ⟨q⋅x,x⟩J=⟨x,q⋅x⟩J \langle q \cdot x, x \rangle_J = \langle x, q \cdot x \rangle_J where q⋅xq \cdot x denotes the left multiplication by qq on the first factor of the tensor product.

theorem

⟨Lqx,x⟩J≥0\langle L_q x, x \rangle_J \geq 0 for q≥0q \geq 0

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let ⟨⋅,⋅⟩J\langle \cdot, \cdot \rangle_J be the sesquilinear form on the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H defined by the relation ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For any positive element q∈Aq \in A (denoted q≥0q \geq 0) and any element xx in the tensor product A⊗CHA \otimes_{\mathbb{C}} H, the following inequality holds: 0≤⟨Lqx,x⟩J 0 \leq \langle L_q x, x \rangle_J where LqL_q is the linear map that performs left multiplication by qq on the first factor of the tensor product.

definition

Seminormed group structure on A⊗CHA \otimes_{\mathbb{C}} H induced by JJ

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), this definition equips the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H with the structure of a seminormed additive commutative group. The seminorm is induced by the pre-inner product ⟨⋅,⋅⟩J\langle \cdot, \cdot \rangle_J defined on elementary tensors by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H.

definition

Inner product space structure on A⊗CHA \otimes_{\mathbb{C}} H induced by JJ

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), this definition equips the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H with an inner product space structure over C\mathbb{C}. The inner product is induced by the sesquilinear form ⟨⋅,⋅⟩J\langle \cdot, \cdot \rangle_J defined on elementary tensors by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H where a,b∈Aa, b \in A and h,k∈Hh, k \in H. This structure is defined in terms of the pre-inner product core `tensorCore` and is compatible with the seminormed group structure `tensorSeminormed`.

theorem

∥a⋅x∥≤∥a∥∥x∥\|a \cdot x\| \le \|a\| \|x\| for the JJ-induced seminorm on A⊗CHA \otimes_{\mathbb{C}} H

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Let J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map, where B(H)\mathcal{B}(H) denotes the space of bounded linear operators on HH. Let A⊗CHA \otimes_{\mathbb{C}} H be the algebraic tensor product equipped with the seminorm ∥⋅∥\|\cdot\| induced by the JJ-dependent inner product, which is defined on simple tensors by ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H. Then, for any a∈Aa \in A and any x∈A⊗CHx \in A \otimes_{\mathbb{C}} H, the left multiplication action a⋅xa \cdot x (defined by a⋅(b⊗h)=(ab)⊗ha \cdot (b \otimes h) = (ab) \otimes h) satisfies the inequality: ∥a⋅x∥≤∥a∥∥x∥\|a \cdot x\| \le \|a\| \|x\| where ∥a∥\|a\| is the norm in the C∗C^*-algebra AA.

theorem

Uniform Continuity of Addition on A⊗CHA \otimes_{\mathbb{C}} H under the JJ-induced Seminorm

Let AA be a C∗C^*-algebra and HH a complex Hilbert space. For any completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let A⊗CHA \otimes_{\mathbb{C}} H be the algebraic tensor product equipped with the seminorm ∥⋅∥J\|\cdot\|_J induced by the pre-inner product ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. Then the addition operation +:(A⊗CH)×(A⊗CH)→A⊗CH+ : (A \otimes_{\mathbb{C}} H) \times (A \otimes_{\mathbb{C}} H) \to A \otimes_{\mathbb{C}} H is uniformly continuous with respect to the pseudo-metric induced by this seminorm.

theorem

Uniform continuity of negation on the JJ-induced seminormed space A⊗CHA \otimes_{\mathbb{C}} H

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Let J:A→B(H)J: A \to \mathcal{B}(H) be a completely positive map. Consider the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the seminormed additive commutative group structure induced by JJ. Then the negation map x↦−xx \mapsto -x is uniformly continuous on A⊗CHA \otimes_{\mathbb{C}} H with respect to the induced pseudo-metric.

definition

Left multiplication by a∈Aa \in A as a continuous linear map on A⊗CHA \otimes_{\mathbb{C}} H with the JJ-induced seminorm

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map from AA to the space of bounded linear operators on HH. Let A⊗CHA \otimes_{\mathbb{C}} H be the algebraic tensor product equipped with the seminorm ∥⋅∥J\|\cdot\|_J induced by the JJ-dependent pre-inner product, which is defined on simple tensors by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For a given element a∈Aa \in A, this definition is the continuous linear map La:(A⊗CH)→(A⊗CH)L_a : (A \otimes_{\mathbb{C}} H) \to (A \otimes_{\mathbb{C}} H) that acts as left multiplication by aa on the first factor (i.e., La(b⊗h)=(ab)⊗hL_a(b \otimes h) = (ab) \otimes h). Its operator norm is bounded by the C∗C^*-norm of aa (∥La∥≤∥a∥\|L_a\| \le \|a\|).

theorem

Continuous Left Multiplication on A⊗CHA \otimes_{\mathbb{C}} H Equals Algebraic Left Multiplication

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Let J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map from AA to the space of bounded linear operators on HH. Let A⊗CHA \otimes_{\mathbb{C}} H be the algebraic tensor product equipped with the seminorm induced by the JJ-dependent pre-inner product, defined on simple tensors by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For any a∈Aa \in A, let LaL_a be the continuous linear map on this seminormed space that performs left multiplication by aa on the first factor. Then for any x∈A⊗CHx \in A \otimes_{\mathbb{C}} H, the action of LaL_a on xx coincides with the algebraic left multiplication of aa on xx.

definition

Left multiplication by a∈Aa \in A on the Stinespring completion KJK_J

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let KJK_J be the Hilbert space completion of the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H with respect to the JJ-induced semi-inner product defined by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For an element a∈Aa \in A, this definition is the unique continuous linear map on KJK_J that extends the left multiplication operator La(b⊗h)=(ab)⊗hL_a(b \otimes h) = (ab) \otimes h from the dense subspace A⊗CHA \otimes_{\mathbb{C}} H.

theorem

Extended left multiplication on KJK_J coincides with the original operator on A⊗CHA \otimes_{\mathbb{C}} H

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let A⊗CHA \otimes_{\mathbb{C}} H be the algebraic tensor product equipped with the semi-inner product induced by JJ, defined on elementary tensors by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. Let KJK_J be the Hilbert space completion of A⊗CHA \otimes_{\mathbb{C}} H. For any a∈Aa \in A, let LaL_a be the continuous linear map on A⊗CHA \otimes_{\mathbb{C}} H that acts as left multiplication by aa on the first factor, and let Lˉa\bar{L}_a be its unique continuous extension to KJK_J. Then for any x∈A⊗CHx \in A \otimes_{\mathbb{C}} H, we have Lˉa(x)=La(x)\bar{L}_a(x) = L_a(x), where xx on the left-hand side is viewed as an element of the completion KJK_J.

theorem

Multiplicativity of the Stinespring representation: π(ab)=π(a)∘π(b)\pi(ab) = \pi(a) \circ \pi(b)

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let KJK_J be the Hilbert space completion of the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H with respect to the semi-inner product defined by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For any a∈Aa \in A, let π(a)\pi(a) denote the unique continuous linear operator on KJK_J extending the left multiplication b⊗h↦(ab)⊗hb \otimes h \mapsto (ab) \otimes h. Then for any a,b∈Aa, b \in A, the operator corresponding to the product abab satisfies π(ab)=π(a)∘π(b)\pi(ab) = \pi(a) \circ \pi(b) where ∘\circ denotes the composition of operators.

definition

Pre-Hilbert space A⊗CHA \otimes_{\mathbb{C}} H induced by JJ

Given a C∗C^*-algebra AA, a complex Hilbert space HH, and a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), the type `Stinespring.Canonical.Pre J` is a type synonym for the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H. It is used to equip the tensor product with a specific semi-inner product induced by JJ, defined on elementary tensors as ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H.

instance

Additive commutative group structure on the pre-Hilbert space A⊗CHA \otimes_{\mathbb{C}} H induced by JJ

Given a C∗C^*-algebra AA, a complex Hilbert space HH, and a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), the type `Stinespring.Canonical.Pre J` (which is the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H) inherits the structure of an additive commutative group.

instance

Complex module structure on the pre-Hilbert space A⊗CHA \otimes_{\mathbb{C}} H induced by JJ

Given a C∗C^*-algebra AA, a complex Hilbert space HH, and a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), the type `Stinespring.Canonical.Pre J` (which is a type synonym for the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H) inherits the structure of a module (vector space) over the complex numbers C\mathbb{C}.

instance

Seminormed additive commutative group structure on `Pre J` induced by JJ

Given a C∗C^*-algebra AA, a complex Hilbert space HH, and a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), the type `Pre J` (a type synonym for the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H) is equipped with the structure of a seminormed additive commutative group. This seminorm is induced by the JJ-dependent inner product ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H.

instance

Inner product space structure on A⊗CHA \otimes_{\mathbb{C}} H induced by JJ

Given a C∗C^*-algebra AA, a complex Hilbert space HH, and a completely positive map J:A→B(H)J: A \to \mathcal{B}(H), the space `Pre J` (which is a type synonym for the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H) is equipped with a complex inner product space structure. The inner product is defined on elementary tensors by the relation ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H, where B(H)\mathcal{B}(H) denotes the space of bounded linear operators on HH.

instance

`Pre J` is a uniform additive group

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J: A \to \mathcal{B}(H) be a completely positive map. Let Pre J\text{Pre } J denote the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the seminorm induced by the semi-inner product ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. Then Pre J\text{Pre } J is a uniform additive group, meaning that its addition and subtraction operations are uniformly continuous with respect to the uniformity induced by its seminorm.

instance

Scalar Multiplication on Pre(J)\text{Pre}(J) is Bounded

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and B(H)\mathcal{B}(H) denote the algebra of bounded linear operators on HH. Given a completely positive map J:A→B(H)J: A \to \mathcal{B}(H), let Pre(J)\text{Pre}(J) be the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the seminormed structure induced by the JJ-dependent semi-inner product defined on elementary tensors by ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. Then the scalar multiplication by complex numbers C\mathbb{C} on Pre(J)\text{Pre}(J) is a bounded operation.

instance

Uniform Continuity of Constant Scalar Multiplication on Pre J\text{Pre } J

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let Pre J\text{Pre } J denote the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the seminormed structure induced by the semi-inner product ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H. For any constant scalar c∈Cc \in \mathbb{C}, the map x↦c⋅xx \mapsto c \cdot x on Pre J\text{Pre } J is uniformly continuous.

abbrev

Canonical Stinespring Hilbert space K(J)K(J)

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let Pre J\text{Pre } J denote the pre-Hilbert space formed by the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the semi-inner product ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H. The space K(J)K(J) is defined as the completion of Pre J\text{Pre } J with respect to the induced seminorm. This space serves as the canonical Hilbert space for the Stinespring dilation of JJ.

definition

Left multiplication by a∈Aa \in A on the pre-Hilbert space Pre J\text{Pre } J

Given a C∗C^*-algebra AA, a complex Hilbert space HH, and a completely positive map J:A→B(H)J: A \to \mathcal{B}(H), let Pre J\text{Pre } J be the pre-Hilbert space consisting of the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the semi-inner product ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H. For an element a∈Aa \in A, this definition is the C\mathbb{C}-linear operator on Pre J\text{Pre } J that acts as left multiplication on the first factor, mapping elementary tensors b⊗hb \otimes h to (ab)⊗h(ab) \otimes h.

theorem

preLeftMulJ(a)x=leftMulax\text{preLeftMul}_J(a)x = \text{leftMul}_a x

Let AA be a C∗C^*-algebra and HH a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let Pre J\text{Pre } J denote the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the semi-inner product ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle. For any a∈Aa \in A and x∈Pre Jx \in \text{Pre } J, the action of the left multiplication operator preLeftMulJ(a)\text{preLeftMul}_J(a) on xx is equal to the action of the generic algebraic left multiplication operator leftMula\text{leftMul}_a on xx.

definition

Continuous left multiplication operator on Pre J\text{Pre } J

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let Pre J\text{Pre } J denote the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the seminorm induced by the semi-inner product ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H. For any a∈Aa \in A, this definition is the continuous linear operator on Pre J\text{Pre } J that acts as left multiplication on the first factor, mapping elementary tensors b⊗hb \otimes h to (ab)⊗h(ab) \otimes h. Its continuity is established by the operator norm bound ∥a⋅x∥≤∥a∥∥x∥\|a \cdot x\| \le \|a\| \|x\|.

theorem

preLeftMulCLMJ(a)x=preLeftMulJ(a)x\text{preLeftMulCLM}_J(a) x = \text{preLeftMul}_J(a) x

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let Pre J\text{Pre } J denote the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the seminorm induced by the semi-inner product ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H. For any a∈Aa \in A and x∈Pre Jx \in \text{Pre } J, the action of the continuous linear operator preLeftMulCLMJ(a)\text{preLeftMulCLM}_J(a) on xx is equal to the action of the linear operator preLeftMulJ(a)\text{preLeftMul}_J(a) on xx.

theorem

The norm of the left multiplication operator on K(J)K(J) is bounded by ∥a∥\|a\|

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let K(J)K(J) be the canonical Stinespring Hilbert space, defined as the completion of the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H with respect to the seminorm induced by the semi-inner product ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For any element a∈Aa \in A, let LaL_a denote the completion of the continuous linear operator on K(J)K(J) that acts as left multiplication on the first tensor factor. Then, for any x∈K(J)x \in K(J), the following norm inequality holds: ∥Lax∥≤∥a∥∥x∥\|L_a x\| \le \|a\| \|x\|

definition

Left multiplication operator on the canonical Stinespring Hilbert space K(J)K(J)

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let K(J)K(J) be the canonical Stinespring Hilbert space, defined as the completion of the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H with respect to the seminorm induced by the semi-inner product ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For any element a∈Aa \in A, this definition is the continuous linear operator on K(J)K(J) obtained by extending the left multiplication action on the first factor of the tensor product (mapping b⊗hb \otimes h to (ab)⊗h(ab) \otimes h) to the entire completion. Its operator norm is bounded by ∥a∥\|a\|.

theorem

Left multiplication on the Stinespring completion K(J)K(J) is consistent with left multiplication on Pre J\text{Pre } J

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let Pre J\text{Pre } J denote the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the JJ-dependent semi-inner product ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H, and let K(J)K(J) be its completion (the canonical Stinespring Hilbert space). For any element a∈Aa \in A and any vector xx in the pre-Hilbert space Pre J\text{Pre } J, the left multiplication operator LaL_a on K(J)K(J) applied to xx (viewed as an element of the completion) is equal to the result of the left multiplication operator LapreL_a^{\text{pre}} on Pre J\text{Pre } J applied to xx.

theorem

`leftMulK J (a * b) = leftMulK J a ∘ leftMulK J b`

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J: A \to \mathcal{B}(H) be a completely positive map. Let K(J)K(J) be the canonical Stinespring Hilbert space, which is the completion of the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H with respect to the seminorm induced by the semi-inner product ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For any a∈Aa \in A, let LaL_a (represented by `leftMulK J a`) be the continuous linear operator on K(J)K(J) obtained by extending the left multiplication action b⊗h↦(ab)⊗hb \otimes h \mapsto (ab) \otimes h. Then, for any a,b∈Aa, b \in A, the operator induced by the product abab is equal to the composition of the operators induced by aa and bb respectively: Lab=La∘LbL_{ab} = L_a \circ L_b

theorem

Left multiplication by 1∈A1 \in A on K(J)K(J) is the identity operator

Let AA be a C∗C^*-algebra with identity 11, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let K(J)K(J) be the canonical Stinespring Hilbert space, defined as the completion of the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H under the JJ-dependent seminorm. The continuous linear operator on K(J)K(J) induced by left multiplication by the identity element 1∈A1 \in A is the identity operator on K(J)K(J).

theorem

leftMulKJ(a+b)=leftMulKJ(a)+leftMulKJ(b)\text{leftMulK}_J(a + b) = \text{leftMulK}_J(a) + \text{leftMulK}_J(b)

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let K(J)K(J) be the canonical Stinespring Hilbert space (the completion of the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H under the JJ-dependent inner product). For any a∈Aa \in A, let leftMulKJ(a)\text{leftMulK}_J(a) be the continuous linear operator on K(J)K(J) induced by left multiplication by aa on the first factor of the tensor product. Then for any a,b∈Aa, b \in A, the left multiplication operator is additive: leftMulKJ(a+b)=leftMulKJ(a)+leftMulKJ(b)\text{leftMulK}_J(a + b) = \text{leftMulK}_J(a) + \text{leftMulK}_J(b)

theorem

π(r⋅a)=r⋅π(a)\pi(r \cdot a) = r \cdot \pi(a) for the Stinespring Left Multiplication Operator

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let K(J)K(J) be the canonical Stinespring Hilbert space (the completion of the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H under the JJ-dependent inner product), and let π(a)\pi(a) denote the continuous linear operator on K(J)K(J) induced by left multiplication by a∈Aa \in A. Then for any scalar r∈Cr \in \mathbb{C} and any element a∈Aa \in A, the operator π(r⋅a)\pi(r \cdot a) is equal to r⋅π(a)r \cdot \pi(a).

theorem

The Left Multiplication Operator on the Stinespring Space K(J)K(J) Satisfies π(a∗)=π(a)∗\pi(a^*) = \pi(a)^*

Let AA be a C∗C^*-algebra, HH a complex Hilbert space, and J:A→B(H)J: A \to \mathcal{B}(H) a completely positive map. Let K(J)K(J) be the canonical Stinespring Hilbert space, which is the completion of the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H with respect to the seminorm induced by the semi-inner product ⟨a⊗h,b⊗k⟩J=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle_J = \langle h, J(a^* b) k \rangle_H. For any a∈Aa \in A, let π(a)\pi(a) be the continuous linear operator on K(J)K(J) (represented as `leftMulK J a`) obtained by extending the left multiplication action b⊗h↦(ab)⊗hb \otimes h \mapsto (ab) \otimes h. Then for all a∈Aa \in A, it holds that π(a∗)=π(a)∗,\pi(a^*) = \pi(a)^*, where a∗a^* is the adjoint of aa in AA, and π(a)∗\pi(a)^* is the adjoint operator of π(a)\pi(a) on the Hilbert space K(J)K(J).

definition

Canonical embedding V:H→A⊗CHV: H \to A \otimes_{\mathbb{C}} H mapping hh to 1⊗h1 \otimes h

Given a completely positive map J:A→B(H)J : A \to \mathcal{B}(H) from a C∗C^*-algebra AA to the algebra of bounded linear operators on a complex Hilbert space HH, the canonical embedding is the complex linear map V:H→A⊗CHV: H \to A \otimes_{\mathbb{C}} H (where A⊗CHA \otimes_{\mathbb{C}} H is the pre-Hilbert space `Pre J` induced by JJ) that maps each vector h∈Hh \in H to the elementary tensor 1⊗h1 \otimes h, where 11 is the identity of AA.

theorem

The canonical pre-embedding VV satisfies V(h)=1⊗hV(h) = 1 \otimes h

Let AA be a unital C∗C^*-algebra and HH be a complex Hilbert space. For a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), the canonical pre-embedding V:H→A⊗CHV : H \to A \otimes_{\mathbb{C}} H (into the pre-Hilbert space induced by JJ) maps any vector h∈Hh \in H to the elementary tensor of the identity 1∈A1 \in A and hh. That is, V(h)=1⊗h.V(h) = 1 \otimes h.

theorem

The squared norm of the Stinespring canonical pre-embedding VV is ∥V(h)∥2=Re⁡⟨h,J(1)h⟩\|V(h)\|^2 = \operatorname{Re} \langle h, J(1) h \rangle

Let AA be a unital C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let V:H→A⊗CHV : H \to A \otimes_{\mathbb{C}} H be the canonical pre-embedding that maps h↦1⊗hh \mapsto 1 \otimes h, where A⊗CHA \otimes_{\mathbb{C}} H is the pre-Hilbert space equipped with the JJ-dependent semi-inner product ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H. For any vector h∈Hh \in H, the square of the norm of its image under VV is given by ∥V(h)∥2=Re⁡(⟨h,J(1)h⟩H),\|V(h)\|^2 = \operatorname{Re}(\langle h, J(1) h \rangle_H), where 11 is the identity element of AA.

theorem

Norm bound for the canonical Stinespring pre-embedding: ∥V(h)∥≤∥J(1)∥∥h∥\|V(h)\| \leq \sqrt{\|J(1)\|} \|h\|

Let AA be a unital C∗C^*-algebra and HH be a complex Hilbert space. For a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), let V:H→A⊗CHV : H \to A \otimes_{\mathbb{C}} H be the canonical pre-embedding defined by V(h)=1⊗hV(h) = 1 \otimes h. The algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H is equipped with a seminorm induced by the JJ-dependent inner product ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H. Then for any vector h∈Hh \in H, the norm of the image under the pre-embedding satisfies: ∥V(h)∥≤∥J(1)∥⋅∥h∥,\|V(h)\| \leq \sqrt{\|J(1)\|} \cdot \|h\|, where ∥J(1)∥\|J(1)\| is the operator norm of J(1)J(1) in B(H)\mathcal{B}(H).

definition

Canonical continuous linear pre-embedding V:H→Pre JV: H \to \text{Pre } J

Let AA be a unital C∗C^*-algebra, HH a complex Hilbert space, and J:A→B(H)J: A \to \mathcal{B}(H) a completely positive map. The canonical pre-embedding V:H→Pre JV: H \to \text{Pre } J is the continuous linear map that sends each vector h∈Hh \in H to the elementary tensor 1⊗h1 \otimes h in the pre-Hilbert space Pre J\text{Pre } J (the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the JJ-dependent seminorm). The continuity is established by the bound ∥V(h)∥≤∥J(1)∥∥h∥\|V(h)\| \leq \sqrt{\|J(1)\|} \|h\|.

definition

Canonical Stinespring embedding V:H→K(J)V: H \to K(J)

Let AA be a unital C∗C^*-algebra and HH be a complex Hilbert space. For a completely positive map J:A→B(H)J: A \to \mathcal{B}(H), the canonical embedding V:H→K(J)V: H \to K(J) is the continuous linear map that sends each vector h∈Hh \in H to the element in the completed Stinespring Hilbert space K(J)K(J) corresponding to the elementary tensor 1⊗h1 \otimes h in the pre-Hilbert space Pre J\text{Pre } J (the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the JJ-dependent seminorm).

theorem

The canonical Stinespring embedding V(h)V(h) equals 1⊗h1 \otimes h in K(J)K(J)

Let AA be a unital C∗C^*-algebra, HH a complex Hilbert space, and J:A→B(H)J: A \to \mathcal{B}(H) a completely positive map. For any vector h∈Hh \in H, the canonical Stinespring embedding V:H→K(J)V: H \to K(J) maps hh to the element in the Stinespring Hilbert space K(J)K(J) that corresponds to the elementary tensor 1⊗h1 \otimes h in the pre-Hilbert space Pre J\text{Pre } J (the algebraic tensor product A⊗CHA \otimes_{\mathbb{C}} H equipped with the JJ-dependent seminorm).

definition

Canonical Stinespring ⋆\star-representation π:A→B(K(J))\pi : A \to \mathcal{B}(K(J))

Let AA be a C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. The canonical Stinespring representation is the ⋆\star-algebra homomorphism π:A→B(K(J))\pi : A \to \mathcal{B}(K(J)) that maps each element a∈Aa \in A to the bounded linear operator on the Stinespring Hilbert space K(J)K(J) induced by left multiplication on the first factor of the tensor product A⊗CHA \otimes_{\mathbb{C}} H. Specifically, for any a∈Aa \in A, π(a)\pi(a) is the continuous extension of the map b⊗h↦(ab)⊗hb \otimes h \mapsto (ab) \otimes h to the completion K(J)K(J).

theorem

Canonical Stinespring Identity J(a)=V∗π(a)VJ(a) = V^* \pi(a) V

Let AA be a unital C∗C^*-algebra, HH be a complex Hilbert space, and J:A→B(H)J : A \to \mathcal{B}(H) be a completely positive map. Let K(J)K(J) be the canonical Stinespring Hilbert space, V:H→K(J)V : H \to K(J) be the canonical embedding, and π:A→B(K(J))\pi : A \to \mathcal{B}(K(J)) be the canonical ⋆\star-representation. Then for any a∈Aa \in A, the Stinespring dilation identity holds: J(a)=V∗π(a)V,J(a) = V^* \pi(a) V, where V∗V^* denotes the adjoint of the operator VV.

definition

Canonical Stinespring Witness for JJ

Let AA be a C∗C^*-algebra and HH be a complex Hilbert space. For a completely positive map J:A→B(H)J : A \to \mathcal{B}(H), the canonical Stinespring witness is the structure that bundles together the canonical Hilbert space K(J)K(J), the canonical ⋆\star-representation π:A→B(K(J))\pi : A \to \mathcal{B}(K(J)), and the canonical embedding V:H→K(J)V : H \to K(J). These components satisfy the Stinespring dilation identity: J(a)=V∗π(a)VJ(a) = V^* \pi(a) V for all a∈Aa \in A, where V∗V^* denotes the adjoint of the operator VV.

theorem

Stinespring's Dilation Theorem for Completely Positive Maps

Let AA be a unital C∗C^*-algebra and HH be a complex Hilbert space. For every completely positive map J:A→B(H)J : A \to \mathcal{B}(H), there exists a Stinespring dilation. Specifically, there exists a Hilbert space KK (constructed as the completion of A⊗CHA \otimes_{\mathbb{C}} H under the semi-inner product ⟨a⊗h,b⊗k⟩=⟨h,J(a∗b)k⟩H\langle a \otimes h, b \otimes k \rangle = \langle h, J(a^* b) k \rangle_H), a ∗*-representation π:A→B(K)\pi : A \to \mathcal{B}(K), and a bounded linear operator V:H→KV : H \to K such that for all a∈Aa \in A, the identity J(a)=V∗π(a)VJ(a) = V^* \pi(a) V holds.