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repo's own bare Mathlib hypotheses (`[CStarAlgebra A] [PartialOrder A] [StarOrderedRing A]`) instead of the upstream-superseded `OperatorAlgebra` class — see `Kernel.lean`'s docstring for why this is a pure restatement, not new mathematics.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
Algebraic tensor product
Given a -algebra and a complex Hilbert space , this definition represents the algebraic tensor product . It serves as the underlying carrier space for the Stinespring dilation construction.
Linear functional
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , an element , and a vector , this definition is the linear functional uniquely determined by its action on elementary tensors: where , , and denotes the inner product on . This functional serves as the fundamental building block for constructing the inner product in Stinespring's dilation theorem.
Evaluation of the base functional on elementary tensors:
Let be a -algebra and be a complex Hilbert space. For a completely positive map , elements , and vectors , the linear functional (the `baseFunctional`) evaluated at the elementary tensor satisfies: where denotes the inner product on and is the adjoint of .
Sesquilinear map
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. This definition is the map which is conjugate-linear in both and . For any fixed and , the resulting linear functional acts on the algebraic tensor product such that for any elementary tensor : where is the inner product on (conventionally conjugate-linear in the first argument). This map packages the `baseFunctional` into a sesquilinear form over and .
Sesquilinear form on defined by
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , this definition constructs a (possibly degenerate) sesquilinear form on the algebraic tensor product . This form is represented as a map that sends an element to a linear functional, such that the mapping is conjugate-linear. For elementary tensors and in , the form satisfies: where denotes the inner product on (conventionally conjugate-linear in the first argument) and is the adjoint of .
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , the sesquilinear form defined on the algebraic tensor product satisfies the following identity for any elementary tensors and (where and ): where is the adjoint of and denotes the inner product on , which is conventionally conjugate-linear in the first argument.
Conjugate Symmetry of
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. Let be the sesquilinear form on the algebraic tensor product defined by . For any and , the form satisfies conjugate symmetry on elementary tensors: where the overline denotes complex conjugation.
Conjugate Symmetry of the Stinespring Sesquilinear Form on
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , let denote the sesquilinear form induced by on the algebraic tensor product (defined on elementary tensors by ). For any elements , the form satisfies conjugate symmetry: where the overline denotes complex conjugation.
Positivity of the induced sesquilinear form for a completely positive map
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , the sesquilinear form defined on the algebraic tensor product (where ) is positive semi-definite. That is, for any , it holds that .
Pre-inner product structure on induced by
Given a completely positive map from a -algebra to the space of bounded linear operators on a complex Hilbert space , this definition constructs a pre-inner product space structure on the algebraic tensor product . The structure is defined by the sesquilinear form (specifically, the `tensorInner` map), which satisfies for elementary tensors and . This structure includes the definitions for the inner product, conjugate symmetry, and non-negativity required to treat as a pre-Hilbert space.
Left multiplication by on
For an element of a -algebra , this is the -linear map on the algebraic tensor product (where is a complex Hilbert space) that acts as left multiplication by on the first factor. Specifically, it is defined on simple tensors by for and .
Let be a -algebra and be a complex Hilbert space. For any elements and any vector , the left multiplication operator acting on the simple tensor in the algebraic tensor product satisfies: where denotes the product in .
Adjoint property of left multiplication:
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , let be the sesquilinear form induced on the algebraic tensor product defined on elementary tensors by . For any element and any tensors , the left multiplication operator satisfies the following adjoint relation: where denotes the action of on the first factor of the tensor product (i.e., ) and is the adjoint of in .
Let be a -algebra and be a complex Hilbert space. For any elements and any element in the algebraic tensor product , the left multiplication operator satisfies the identity .
Left Multiplication on is Additive in the Algebra Component
Let be a -algebra and be a complex Hilbert space. For any elements and any element in the algebraic tensor product , the left multiplication of by the sum is equal to the sum of the left multiplication of by and the left multiplication of by , which can be written as .
Let be a -algebra and be a complex Hilbert space. For any scalar , any element , and any element in the algebraic tensor product , the action of left multiplication by on is equal to times the action of left multiplication by on . That is, .
Left multiplication by is the identity map on
Let be a -algebra and a complex Hilbert space. For any element in the algebraic tensor product , the left multiplication by the identity element acts as the identity map, i.e., .
on
Let be a -algebra and be a complex Hilbert space. For any element and any element in the algebraic tensor product , let denote the linear map of left multiplication by on the first factor of the tensor product. Then the following identity holds: where is the norm of , is the identity element of , and is the adjoint of .
for self-adjoint
Let be a -algebra and a complex Hilbert space. Let be a completely positive map, and let be the sesquilinear form on the algebraic tensor product defined by . For any self-adjoint element (i.e., ) and any , the following identity holds: where denotes the left multiplication by on the first factor of the tensor product.
for
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , let be the sesquilinear form on the algebraic tensor product defined by the relation . For any positive element (denoted ) and any element in the tensor product , the following inequality holds: where is the linear map that performs left multiplication by on the first factor of the tensor product.
Seminormed group structure on induced by
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , this definition equips the algebraic tensor product with the structure of a seminormed additive commutative group. The seminorm is induced by the pre-inner product defined on elementary tensors by .
Inner product space structure on induced by
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , this definition equips the algebraic tensor product with an inner product space structure over . The inner product is induced by the sesquilinear form defined on elementary tensors by where and . This structure is defined in terms of the pre-inner product core `tensorCore` and is compatible with the seminormed group structure `tensorSeminormed`.
for the -induced seminorm on
Let be a -algebra and be a complex Hilbert space. Let be a completely positive map, where denotes the space of bounded linear operators on . Let be the algebraic tensor product equipped with the seminorm induced by the -dependent inner product, which is defined on simple tensors by . Then, for any and any , the left multiplication action (defined by ) satisfies the inequality: where is the norm in the -algebra .
Uniform Continuity of Addition on under the -induced Seminorm
Let be a -algebra and a complex Hilbert space. For any completely positive map , let be the algebraic tensor product equipped with the seminorm induced by the pre-inner product . Then the addition operation is uniformly continuous with respect to the pseudo-metric induced by this seminorm.
Uniform continuity of negation on the -induced seminormed space
Let be a -algebra and be a complex Hilbert space. Let be a completely positive map. Consider the algebraic tensor product equipped with the seminormed additive commutative group structure induced by . Then the negation map is uniformly continuous on with respect to the induced pseudo-metric.
Left multiplication by as a continuous linear map on with the -induced seminorm
Let be a -algebra, be a complex Hilbert space, and be a completely positive map from to the space of bounded linear operators on . Let be the algebraic tensor product equipped with the seminorm induced by the -dependent pre-inner product, which is defined on simple tensors by . For a given element , this definition is the continuous linear map that acts as left multiplication by on the first factor (i.e., ). Its operator norm is bounded by the -norm of ().
Continuous Left Multiplication on Equals Algebraic Left Multiplication
Let be a -algebra and be a complex Hilbert space. Let be a completely positive map from to the space of bounded linear operators on . Let be the algebraic tensor product equipped with the seminorm induced by the -dependent pre-inner product, defined on simple tensors by . For any , let be the continuous linear map on this seminormed space that performs left multiplication by on the first factor. Then for any , the action of on coincides with the algebraic left multiplication of on .
Left multiplication by on the Stinespring completion
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , let be the Hilbert space completion of the algebraic tensor product with respect to the -induced semi-inner product defined by . For an element , this definition is the unique continuous linear map on that extends the left multiplication operator from the dense subspace .
Extended left multiplication on coincides with the original operator on
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. Let be the algebraic tensor product equipped with the semi-inner product induced by , defined on elementary tensors by . Let be the Hilbert space completion of . For any , let be the continuous linear map on that acts as left multiplication by on the first factor, and let be its unique continuous extension to . Then for any , we have , where on the left-hand side is viewed as an element of the completion .
Multiplicativity of the Stinespring representation:
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , let be the Hilbert space completion of the algebraic tensor product with respect to the semi-inner product defined by . For any , let denote the unique continuous linear operator on extending the left multiplication . Then for any , the operator corresponding to the product satisfies where denotes the composition of operators.
Pre-Hilbert space induced by
Given a -algebra , a complex Hilbert space , and a completely positive map , the type `Stinespring.Canonical.Pre J` is a type synonym for the algebraic tensor product . It is used to equip the tensor product with a specific semi-inner product induced by , defined on elementary tensors as .
Additive commutative group structure on the pre-Hilbert space induced by
Given a -algebra , a complex Hilbert space , and a completely positive map , the type `Stinespring.Canonical.Pre J` (which is the algebraic tensor product ) inherits the structure of an additive commutative group.
Complex module structure on the pre-Hilbert space induced by
Given a -algebra , a complex Hilbert space , and a completely positive map , the type `Stinespring.Canonical.Pre J` (which is a type synonym for the algebraic tensor product ) inherits the structure of a module (vector space) over the complex numbers .
Seminormed additive commutative group structure on `Pre J` induced by
Given a -algebra , a complex Hilbert space , and a completely positive map , the type `Pre J` (a type synonym for the algebraic tensor product ) is equipped with the structure of a seminormed additive commutative group. This seminorm is induced by the -dependent inner product .
Inner product space structure on induced by
Given a -algebra , a complex Hilbert space , and a completely positive map , the space `Pre J` (which is a type synonym for the algebraic tensor product ) is equipped with a complex inner product space structure. The inner product is defined on elementary tensors by the relation , where denotes the space of bounded linear operators on .
`Pre J` is a uniform additive group
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. Let denote the algebraic tensor product equipped with the seminorm induced by the semi-inner product . Then is a uniform additive group, meaning that its addition and subtraction operations are uniformly continuous with respect to the uniformity induced by its seminorm.
Scalar Multiplication on is Bounded
Let be a -algebra, be a complex Hilbert space, and denote the algebra of bounded linear operators on . Given a completely positive map , let be the algebraic tensor product equipped with the seminormed structure induced by the -dependent semi-inner product defined on elementary tensors by . Then the scalar multiplication by complex numbers on is a bounded operation.
Uniform Continuity of Constant Scalar Multiplication on
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , let denote the algebraic tensor product equipped with the seminormed structure induced by the semi-inner product . For any constant scalar , the map on is uniformly continuous.
Canonical Stinespring Hilbert space
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. Let denote the pre-Hilbert space formed by the algebraic tensor product equipped with the semi-inner product . The space is defined as the completion of with respect to the induced seminorm. This space serves as the canonical Hilbert space for the Stinespring dilation of .
Left multiplication by on the pre-Hilbert space
Given a -algebra , a complex Hilbert space , and a completely positive map , let be the pre-Hilbert space consisting of the algebraic tensor product equipped with the semi-inner product . For an element , this definition is the -linear operator on that acts as left multiplication on the first factor, mapping elementary tensors to .
Let be a -algebra and a complex Hilbert space. Given a completely positive map , let denote the algebraic tensor product equipped with the semi-inner product . For any and , the action of the left multiplication operator on is equal to the action of the generic algebraic left multiplication operator on .
Continuous left multiplication operator on
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , let denote the algebraic tensor product equipped with the seminorm induced by the semi-inner product . For any , this definition is the continuous linear operator on that acts as left multiplication on the first factor, mapping elementary tensors to . Its continuity is established by the operator norm bound .
Let be a -algebra and be a complex Hilbert space. Given a completely positive map , let denote the algebraic tensor product equipped with the seminorm induced by the semi-inner product . For any and , the action of the continuous linear operator on is equal to the action of the linear operator on .
The norm of the left multiplication operator on is bounded by
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. Let be the canonical Stinespring Hilbert space, defined as the completion of the algebraic tensor product with respect to the seminorm induced by the semi-inner product . For any element , let denote the completion of the continuous linear operator on that acts as left multiplication on the first tensor factor. Then, for any , the following norm inequality holds:
Left multiplication operator on the canonical Stinespring Hilbert space
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. Let be the canonical Stinespring Hilbert space, defined as the completion of the algebraic tensor product with respect to the seminorm induced by the semi-inner product . For any element , this definition is the continuous linear operator on obtained by extending the left multiplication action on the first factor of the tensor product (mapping to ) to the entire completion. Its operator norm is bounded by .
Left multiplication on the Stinespring completion is consistent with left multiplication on
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. Let denote the algebraic tensor product equipped with the -dependent semi-inner product , and let be its completion (the canonical Stinespring Hilbert space). For any element and any vector in the pre-Hilbert space , the left multiplication operator on applied to (viewed as an element of the completion) is equal to the result of the left multiplication operator on applied to .
`leftMulK J (a * b) = leftMulK J a ∘ leftMulK J b`
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. Let be the canonical Stinespring Hilbert space, which is the completion of the algebraic tensor product with respect to the seminorm induced by the semi-inner product . For any , let (represented by `leftMulK J a`) be the continuous linear operator on obtained by extending the left multiplication action . Then, for any , the operator induced by the product is equal to the composition of the operators induced by and respectively:
Left multiplication by on is the identity operator
Let be a -algebra with identity , be a complex Hilbert space, and be a completely positive map. Let be the canonical Stinespring Hilbert space, defined as the completion of the algebraic tensor product under the -dependent seminorm. The continuous linear operator on induced by left multiplication by the identity element is the identity operator on .
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. Let be the canonical Stinespring Hilbert space (the completion of the algebraic tensor product under the -dependent inner product). For any , let be the continuous linear operator on induced by left multiplication by on the first factor of the tensor product. Then for any , the left multiplication operator is additive:
for the Stinespring Left Multiplication Operator
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. Let be the canonical Stinespring Hilbert space (the completion of the algebraic tensor product under the -dependent inner product), and let denote the continuous linear operator on induced by left multiplication by . Then for any scalar and any element , the operator is equal to .
The Left Multiplication Operator on the Stinespring Space Satisfies
Let be a -algebra, a complex Hilbert space, and a completely positive map. Let be the canonical Stinespring Hilbert space, which is the completion of the algebraic tensor product with respect to the seminorm induced by the semi-inner product . For any , let be the continuous linear operator on (represented as `leftMulK J a`) obtained by extending the left multiplication action . Then for all , it holds that where is the adjoint of in , and is the adjoint operator of on the Hilbert space .
Canonical embedding mapping to
Given a completely positive map from a -algebra to the algebra of bounded linear operators on a complex Hilbert space , the canonical embedding is the complex linear map (where is the pre-Hilbert space `Pre J` induced by ) that maps each vector to the elementary tensor , where is the identity of .
The canonical pre-embedding satisfies
Let be a unital -algebra and be a complex Hilbert space. For a completely positive map , the canonical pre-embedding (into the pre-Hilbert space induced by ) maps any vector to the elementary tensor of the identity and . That is,
The squared norm of the Stinespring canonical pre-embedding is
Let be a unital -algebra, be a complex Hilbert space, and be a completely positive map. Let be the canonical pre-embedding that maps , where is the pre-Hilbert space equipped with the -dependent semi-inner product . For any vector , the square of the norm of its image under is given by where is the identity element of .
Norm bound for the canonical Stinespring pre-embedding:
Let be a unital -algebra and be a complex Hilbert space. For a completely positive map , let be the canonical pre-embedding defined by . The algebraic tensor product is equipped with a seminorm induced by the -dependent inner product . Then for any vector , the norm of the image under the pre-embedding satisfies: where is the operator norm of in .
Canonical continuous linear pre-embedding
Let be a unital -algebra, a complex Hilbert space, and a completely positive map. The canonical pre-embedding is the continuous linear map that sends each vector to the elementary tensor in the pre-Hilbert space (the algebraic tensor product equipped with the -dependent seminorm). The continuity is established by the bound .
Canonical Stinespring embedding
Let be a unital -algebra and be a complex Hilbert space. For a completely positive map , the canonical embedding is the continuous linear map that sends each vector to the element in the completed Stinespring Hilbert space corresponding to the elementary tensor in the pre-Hilbert space (the algebraic tensor product equipped with the -dependent seminorm).
The canonical Stinespring embedding equals in
Let be a unital -algebra, a complex Hilbert space, and a completely positive map. For any vector , the canonical Stinespring embedding maps to the element in the Stinespring Hilbert space that corresponds to the elementary tensor in the pre-Hilbert space (the algebraic tensor product equipped with the -dependent seminorm).
Canonical Stinespring -representation
Let be a -algebra, be a complex Hilbert space, and be a completely positive map. The canonical Stinespring representation is the -algebra homomorphism that maps each element to the bounded linear operator on the Stinespring Hilbert space induced by left multiplication on the first factor of the tensor product . Specifically, for any , is the continuous extension of the map to the completion .
Canonical Stinespring Identity
Let be a unital -algebra, be a complex Hilbert space, and be a completely positive map. Let be the canonical Stinespring Hilbert space, be the canonical embedding, and be the canonical -representation. Then for any , the Stinespring dilation identity holds: where denotes the adjoint of the operator .
Canonical Stinespring Witness for
Let be a -algebra and be a complex Hilbert space. For a completely positive map , the canonical Stinespring witness is the structure that bundles together the canonical Hilbert space , the canonical -representation , and the canonical embedding . These components satisfy the Stinespring dilation identity: for all , where denotes the adjoint of the operator .
Stinespring's Dilation Theorem for Completely Positive Maps
Let be a unital -algebra and be a complex Hilbert space. For every completely positive map , there exists a Stinespring dilation. Specifically, there exists a Hilbert space (constructed as the completion of under the semi-inner product ), a -representation , and a bounded linear operator such that for all , the identity holds.
