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Cutoff dipole relative bound

Statement

Assume the Axiom of Countable Choice and the Axiom of Choice (The Axiom of Countable Choice (ACω), The Axiom of Choice). Let α>0, let h be the transverse space and Hf=⨁nTn the free transverse Fock Hamiltonian of Transverse continuum free Fock Hamiltonian, and let A±(g), g∈h, be the maximal creation and annihilation operators on F(h) of Closed adjoint sector domains. Fix a cutoff Λ>0 and vectors g1,…,gJ∈h (J≥1) with supp⁡gj⊆{k: 0<ε(k)≤Λ}andgjε∈h,ε(k)=α∣k∣, and define the symmetric operator ϕ(g):=A−(g)+A+(g),D(ϕ(g)):=D(Hf). Let d≥1, let Ha be a Hermitian d×d matrix, and let D1,…,DJ be Hermitian d×d matrices with operator norms ∥Dj∥ (The operator norm as the least bound and as the unit-sphere or unit-ball supremum). Then:

(a) for every Ψ∈D(Hf), ∥A−(g)Ψ∥2≤∥gε∥2⟨HfΨ,Ψ⟩,∥A+(g)Ψ∥2=∥A−(g)Ψ∥2+∥g∥2∥Ψ∥2; in particular Ψ∈D(A+(g))∩D(A−(g)) and for every η>0 there is a finite Cη with ∥ϕ(g)Ψ∥≤η∥HfΨ∥+Cη∥Ψ∥;

(b) ϕ(g) is symmetric on D(Hf) and has Hf-bound zero (Relative boundedness with respect to an operator, Symmetric, self-adjoint and essentially self-adjoint operators);

(c) the free part H0:=Ha⊗I+I⊗Hf with domain Cd⊗D(Hf) is self-adjoint on Cd⊗F(h) (Hilbert space); for every η0∈R the operator H:=H0+η0∑j=1JDj⊗ϕ(gj),D(H):=Cd⊗D(Hf), is self-adjoint on Cd⊗F(h) and bounded below (Kato-Rellich theorem, Adjoint of a densely defined operator).

The product Dj⊗ϕ(gj) and the coupling η0 carry the dimension of energy in a physical normalisation; this normalisation is a model choice and no physical statement about radiation or spontaneous emission is made here.

Facts & Assumptions

Given: the cutoff Λ>0 and vectors gj with supp⁡gj⊆{0<ε≤Λ}, the maps A±(gj) and ϕ(gj) on F(h), the free Hamiltonian Hf with sector multipliers Tn and number projections Πn, Hermitian matrices Ha,Dj on Cd, and Ψ,Φ∈D(Hf).

[F1]

A±(g) are closed operators with A+(g)=A−(g)∗ and A−(g)=A+(g)∗; on the core F0 the sector formulas (A+(g)ψ)n=nP+(n)(g⊗ψn−1) and (A−(g)ψ)n=n+1Ψg(n)(ψn+1) hold, where Ψg(n) is the contraction in the first tensor factor with coefficient ⟨⋅,g⟩ (Closed adjoint sector domains, Creation contraction bounds).

[F2]

Hf=⨁nTn is self-adjoint and positive with Tn=εn∣P+(n), εn(k1,…,kn)=∑i≤nε(ki), and ⟨HfΨ,Ψ⟩=∑n⟨εnΨn,Ψn⟩<∞ for Ψ∈D(Hf) (Transverse continuum free Fock Hamiltonian).

[F3]

Under the iterated Euclidean product unitary, h⊗m is identified with a closed subspace of L2((R3)m;(C3)⊗m); elements of ran⁡P+(m) correspond to tensor-valued functions F invariant under simultaneous momentum and polarization-factor permutations, so ∥F∥ is invariant under momentum permutations, and the energy multiplier is εm(k1,…,km)=∑i=1mε(ki). For F in its operator domain, ⟨εmF,F⟩=∑i=1m∫ε(ki)∥F(k1,…,km)∥2, and symmetry makes each integral equal to m−1⟨εmF,F⟩ (Euclidean L² product unitary: L²(R^a)⊗̂L²(R^b) ≅ L²(R^{a+b}), Hilbert tensor construction: inner product, completion and product bases, Permutation sector projections on the n-fold tensor power).

[F4]

Cauchy-Schwarz for the weighted L2 integrals in step 1.1 follows from Cauchy-Schwarz inequality for L2; Cauchy-Schwarz in the Fock Hilbert space, used for ⟨HfΨ,Ψ⟩, follows from Cauchy–Schwarz: ∣⟨x,y⟩∣≤∥x∥ ∥y∥, with equality exactly for dependent pairs.

[F5]

For Ψ∈F0 the identity ∥A+(g)Ψ∥2=∥A−(g)Ψ∥2+∥g∥2∥Ψ∥2 holds (Closed adjoint sector domains (d)).

[F6]

Kato-Rellich: if A is self-adjoint and B is symmetric with D(A)⊆D(B) and A-bound less than 1, then A+B on D(A) is self-adjoint and bounded below when A is (Kato-Rellich theorem, Relative boundedness with respect to an operator).

[F7]

Tonelli applies to the nonnegative squared norms on the sigma-finite Lebesgue product spaces; Fubini applies to absolutely integrable scalar component pairings (Tonelli's theorem for nonnegative measurable functions on a sigma-finite product, Fubini's theorem for L^1 functions on a sigma-finite product).

Proof

1.1F1F2F3F4F7algebra

[Contraction estimate] Let Ψ∈D(Hf) and n≥0; represent Ψn+1∈ran⁡P+(n+1) by its symmetric function F under [F3]. For v∈C3 define the first-factor contraction Cv:(C3)⊗(n+1)→(C3)⊗n by Cv(a⊗b)=⟨a,v⟩b, extended linearly; finite orthonormal coordinates give ∥Cv∥≤∥v∥. The contraction is represented by the tensor-valued function G(k′)=∫Cg(k1)F(k1,k′) dk1: unweighted Cauchy–Schwarz gives integrable slices almost everywhere, Fubini identifies the pairing first for elementary tensors and then by density, and the bounded contraction formula [F1] extends the identity. Weighted Cauchy–Schwarz, tested against unit vectors of the finite-dimensional target tensor space, gives ∥G(k′)∥2≤(∫∥g(k1)∥2/ε(k1) dk1)(∫ε(k1)∥F(k1,k′)∥2 dk1); the null point k1=0 is omitted. Integrating over k′ and using the symmetry computation of [F3] gives ∥Ψg(n)Ψn+1∥2≤∥g/ε∥2⋅1n+1⟨εn+1F,F⟩=1n+1∥g/ε∥2⟨εn+1Ψn+1,Ψn+1⟩. Hence ∥(A−(g)Ψ)n∥2=(n+1)∥Ψg(n)Ψn+1∥2≤∥g/ε∥2⟨εn+1Ψn+1,Ψn+1⟩; summing over n and using [F2] gives ∥A−(g)Ψ∥2≤∥g/ε∥2⟨HfΨ,Ψ⟩.

1.2F2F3F4F6algebra

[The free part] Fix the standard orthonormal basis (ua)a=1d of Cd. The map (Ψa)a=1d↦∑aua⊗Ψa is an isometry from the finite Hilbert sum F(h)d onto Cd⊗F(h), since elementary tensors span a dense subspace and the finite sum is complete. Under this identification T:=I⊗Hf has dense domain D(Hf)d and acts componentwise. It is symmetric by [F2]. If Φ∈D(T∗) with T∗Φ=Z, testing the adjoint identity on a vector with only component a nonzero gives Φa∈D(Hf∗)=D(Hf) and Za=HfΦa for each a. Conversely symmetry gives T⊆T∗, so T=T∗ and T≥0. The matrix amplification R:=Ha⊗I is bounded and self-adjoint: its entries act by scalar multiples of the identity, its adjoint entries are the conjugate transposed entries, and finite-dimensional Cauchy–Schwarz bounds its norm. It has T-bound zero; hence [F6] makes H0=T+R self-adjoint on D(Hf)d=Cd⊗D(Hf) and bounded below by −∥R∥, also directly from T≥0 and ∣⟨RΨ,Ψ⟩∣≤∥R∥∥Ψ∥2. This componentwise adjoint argument establishes the general amplification here without applying the fixed-N kinetic-operator interface.

2.1F1F5step 1.1algebra

[The norm identity on D(Hf)] Let Ψ∈D(Hf) and let Ψ(N) be its truncations to sectors ≤N. Step 1.1 gives A−(g)Ψ(N)→A−(g)Ψ and Ψ(N)→Ψ. For N>M, χ:=Ψ(N)−Ψ(M) is in F0, so [F5] gives ∥A+(g)χ∥2=∥A−(g)χ∥2+∥g∥2∥χ∥2→0 as M,N→∞. Thus A+(g)Ψ(N) is Cauchy; closedness of A+(g) from [F1] implies Ψ∈D(A+(g)) and A+(g)Ψ(N)→A+(g)Ψ. Passing to the limit in [F5] for Ψ(N) now proves ∥A+(g)Ψ∥2=∥A−(g)Ψ∥2+∥g∥2∥Ψ∥2.

3.1F4step 1.1step 2.1algebra

[The relative bound] From steps 1.1 and 2.1, ∥ϕ(g)Ψ∥≤∥A−(g)Ψ∥+∥A+(g)Ψ∥≤2∥g/ε∥⟨HfΨ,Ψ⟩+∥g∥∥Ψ∥ for Ψ∈D(Hf); since ⟨HfΨ,Ψ⟩≤∥HfΨ∥ ∥Ψ∥ by [F4], the inequality 2ab≤ηa+η−1b with a=∥HfΨ∥, b=∥g/ε∥2∥Ψ∥ gives, for every η>0, ∥ϕ(g)Ψ∥≤η∥HfΨ∥+Cη∥Ψ∥ with Cη=η−1∥g/ε∥2+∥g∥. This proves (a) and shows the Hf-bound zero.

4.1F1step 1.1step 2.1step 3.1algebra

[Symmetry of ϕ(g)] For Ψ,Φ∈D(Hf), steps 1.1 and 2.1 put both vectors in the domains of the two closed operators, and the mutual adjointness A+(g)=A−(g)∗ of [F1] gives ⟨ϕ(g)Ψ,Φ⟩=⟨A−(g)Ψ,Φ⟩+⟨A+(g)Ψ,Φ⟩=⟨Ψ,A+(g)Φ⟩+⟨Ψ,A−(g)Φ⟩=⟨Ψ,ϕ(g)Φ⟩. By step 3.1, ϕ(g) is symmetric on the dense domain D(Hf) with Hf-bound zero, which is (b).

5.1F6step 3.1step 4.1step 1.2algebra

[Kato-Rellich] Let B:=η0∑jDj⊗ϕ(gj) with domain Cd⊗D(Hf). Since the Dj are bounded and each ϕ(gj) is symmetric with Hf-bound zero (step 4.1, step 3.1), B is symmetric and for every η>0 there are finite constants with ∥BΨ∥≤η∥HfΨ∥+Cη∥Ψ∥; as H0 differs from I⊗Hf by the bounded operator Ha⊗I, the same estimate holds with Hf replaced by H0 up to constants, so the H0-bound of B is zero and in particular less than 1. As H0 is self-adjoint (step 1.2), [F6] gives that H=H0+B is self-adjoint on Cd⊗D(Hf) and bounded below. This proves (c).

6.1step 1.1step 2.1step 3.1step 4.1step 1.2step 5.1algebra∎

[Conclusion and scope] Steps 1.1 and 2.1 prove (a), step 4.1 proves (b), and steps 1.2 and 5.1 prove (c). The final paragraph of the statement records that the normalisation of the coupling and of the dipole matrices is a model choice, and no physical radiation or emission statement is asserted. This completes the proof.

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