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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 (), The Axiom of Choice). Let , let be the transverse space and the free transverse Fock Hamiltonian of Transverse continuum free Fock Hamiltonian, and let , , be the maximal creation and annihilation operators on of Closed adjoint sector domains. Fix a cutoff and vectors () with and define the symmetric operator Let , let be a Hermitian matrix, and let be Hermitian matrices with operator norms (The operator norm as the least bound and as the unit-sphere or unit-ball supremum). Then:
(a) for every , in particular and for every there is a finite with
(b) is symmetric on and has -bound zero (Relative boundedness with respect to an operator, Symmetric, self-adjoint and essentially self-adjoint operators);
(c) the free part with domain is self-adjoint on (Hilbert space); for every the operator is self-adjoint on and bounded below (Kato-Rellich theorem, Adjoint of a densely defined operator).
The product and the coupling 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 and vectors with , the maps and on , the free Hamiltonian with sector multipliers and number projections , Hermitian matrices on , and .
are closed operators with and ; on the core the sector formulas and hold, where is the contraction in the first tensor factor with coefficient (Closed adjoint sector domains, Creation contraction bounds).
is self-adjoint and positive with , , and for (Transverse continuum free Fock Hamiltonian).
Under the iterated Euclidean product unitary, is identified with a closed subspace of ; elements of correspond to tensor-valued functions invariant under simultaneous momentum and polarization-factor permutations, so is invariant under momentum permutations, and the energy multiplier is . For in its operator domain, , and symmetry makes each integral equal to (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).
Cauchy-Schwarz for the weighted integrals in step 1.1 follows from Cauchy-Schwarz inequality for ; Cauchy-Schwarz in the Fock Hilbert space, used for , follows from Cauchy–Schwarz: , with equality exactly for dependent pairs.
For the identity holds (Closed adjoint sector domains (d)).
Kato-Rellich: if is self-adjoint and is symmetric with and -bound less than , then on is self-adjoint and bounded below when is (Kato-Rellich theorem, Relative boundedness with respect to an operator).
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
[Contraction estimate] Let and ; represent by its symmetric function under [F3]. For define the first-factor contraction by , extended linearly; finite orthonormal coordinates give . The contraction is represented by the tensor-valued function : 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 ; the null point is omitted. Integrating over and using the symmetry computation of [F3] gives . Hence ; summing over and using [F2] gives .
[The free part] Fix the standard orthonormal basis of . The map is an isometry from the finite Hilbert sum onto , since elementary tensors span a dense subspace and the finite sum is complete. Under this identification has dense domain and acts componentwise. It is symmetric by [F2]. If with , testing the adjoint identity on a vector with only component nonzero gives and for each . Conversely symmetry gives , so and . The matrix amplification 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 -bound zero; hence [F6] makes self-adjoint on and bounded below by , also directly from and . This componentwise adjoint argument establishes the general amplification here without applying the fixed- kinetic-operator interface.
[The norm identity on ] Let and let be its truncations to sectors . Step 1.1 gives and . For , is in , so [F5] gives as . Thus is Cauchy; closedness of from [F1] implies and . Passing to the limit in [F5] for now proves .
[The relative bound] From steps 1.1 and 2.1, for ; since by [F4], the inequality with , gives, for every , with . This proves (a) and shows the -bound zero.
[Symmetry of ] For , steps 1.1 and 2.1 put both vectors in the domains of the two closed operators, and the mutual adjointness of [F1] gives . By step 3.1, is symmetric on the dense domain with -bound zero, which is (b).
[Kato-Rellich] Let with domain . Since the are bounded and each is symmetric with -bound zero (step 4.1, step 3.1), is symmetric and for every there are finite constants with ; as differs from by the bounded operator , the same estimate holds with replaced by up to constants, so the -bound of is zero and in particular less than . As is self-adjoint (step 1.2), [F6] gives that is self-adjoint on and bounded below. This proves (c).
[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.
Depends on
Closed adjoint sector domains
Transverse continuum free Fock Hamiltonian
Bounded fixed-N operator
Creation contraction bounds
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
Tonelli's theorem for nonnegative measurable functions on a sigma-finite product
Fubini's theorem for L^1 functions on a sigma-finite product
Cauchy-Schwarz inequality for
Cauchy–Schwarz: , with equality exactly for dependent pairs
Kato-Rellich theorem
Relative boundedness with respect to an operator
Symmetric, self-adjoint and essentially self-adjoint operators
Adjoint of a densely defined operator
Hilbert space
A bounded linear operator between normed spaces
The operator norm as the least bound and as the unit-sphere or unit-ball supremum
The Axiom of Countable Choice ()
The Axiom of Choice
Dependency tree · 2 levels
Sources
- S. Pratt, Lecture Notes on Quantum Mechanics (Michigan State University, PHY 851/852; retrieved via the Internet Archive snapshot of 2024-04-21), Chapter 9 Sections 9.5-9.7 pp. 139-143 (decay by emission, electromagnetic decays, dipole approximation)
- F. H. L. Essler, Advanced Quantum Theory (Oxford lecture notes, 2017/18), Sections 1.2-1.4 pp. 5-11 (creation and annihilation operators and their bounds)