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The one-body readout law: sector means of summed bounded one-particle quantities and their reduced forms
The conditional one-body readout law of the declared finite-mode many-body model: for a bounded self-adjoint one-particle quantity the summed sector operator is an assigned sharp observable, its mean in every declared sector preparation exists and is N times the pairing of the one-body reduced preparation with the quantity, and for declared Slater and orthonormal-mode symmetric preparations the reduced preparation is the equal-weight mixture of the occupied modes. The theorem is conditional on P1, P2 and P6, the adopted finite-mode idealization and the declared preparation; it asserts no measured mean, no individual-particle property, no separability of the sector state and no spatial (infinite-mode) reduction.
Statement
Slot convention. For , acts on as in Permutation sector projections on the n-fold tensor power. On one-based tuple positions use for . Conjugation by gives and ; thus the supplier's tensor action becomes . The parameters of and remain canonical zero-based permutations.
Adopt the state and composition postulate P1 (State and composition postulate P1: Hilbert spaces, preparations and composite systems), the observable and probability postulate P2 (Observable and probability postulate P2: self-adjoint observables, spectral probability and finite unsharp outcomes) and the identical-particle and spin postulate P6 (Identical-particle and spin assignments P6: permutation sectors and finite-spin representations) in the readings of Declared species sectors and their preparations: the framework's reading of P6 and Assigned sharp observables and the outcome law of the framework, and assume the Axiom of Countable Choice and the Axiom of Choice (The Axiom of Countable Choice (), The Axiom of Choice) as the choice hypotheses of the cited tensor, permutation and trace suppliers. Work in the finite-mode realization of The declared one-body reduced operator and the one-body readout of a sector preparation: one-particle space of finite dimension with modes , integer with nonzero sector space , and the slot operators , sum and one-body reduced preparation of that definition. Let be self-adjoint with declared unit in the sense of Dimensions, SI unit conventions and quantity types of the framework. Then:
(a) the readout is an assigned observable: the sector operator is bounded and self-adjoint, so the pair is an assigned sharp observable of the sector in the sense of Assigned sharp observables and the outcome law of the framework clause (a);
(b) the mean law: for every declared sector preparation the mean of the outcome law of in exists and equals
(c) product-type preparations: let be pairwise orthonormal. For and , let be the Slater vector of Slater direct and exchange calculation clause (a); for and , put and . For , put . In either case is a unit vector of the sector. For one has and
(d) scope: the theorem is conditional on the adopted sector of P6, on the declared finite-mode idealization and on the declared preparation; it asserts no measured mean, no property of an individual labelled particle, no separability of , no result for spatial infinite-mode models, and no sampling or detection model. The equality is exact within the model.
Facts & Assumptions
Given: the finite-mode sector data of the statement, the self-adjoint with unit , and a declared sector preparation .
Sector vocabulary. is a closed subspace of ; every permutation unitary preserves it and acts there as the scalar ; a declared sector preparation is a density operator on (Declared species sectors and their preparations: the framework's reading of P6 clauses (c)-(d), Permutation sector projections on the n-fold tensor power clause (c)).
Slot operators and reduction. For and the slot operator satisfies , is bounded with , and ; commutes with every and with , preserves the sectors, and for every sector preparation with zero extension one has , where is a density operator on (The one-body reduction trace law of the finite-mode permutation sectors clauses (a), (c), (d), (e)).
Observable reading and moments. An assigned sharp observable has outcome law ; for a bounded self-adjoint and every preparation the mean exists and equals , and for a pure preparation of a unit vector it equals (Assigned sharp observables and the outcome law of the framework clauses (a), (b), Mean, second moment and variance of an assigned observable in a preparation clauses (a), (d), Trace is a linear functional on , Cyclicity of the trace).
Slater and symmetric product vectors. For and pairwise orthonormal the Slater vector has norm one and equals (Slater direct and exchange calculation clauses (a), (b)). For , and give the same norm and projection identities directly. Setting , orthonormality gives , so is a unit vector of the symmetric sector with (Permutation sector projections on the n-fold tensor power clauses (b), (c), Hilbert tensor construction: inner product, completion and product bases).
Rank-one pairing. is bounded with and ; the map is linear in and conjugate-linear in (Rank-one operator calculus clauses (a), (b), (c)).
Proof
Given: the data of the statement and [F1]-[F5].
[The readout is a bounded self-adjoint sector observable] By [F2] each slot operator is bounded with , so is bounded with ; since , [F2] gives for every and . By [F2] commutes with , hence preserves and its orthogonal complement, and the restriction is a bounded self-adjoint operator on the sector space; with the unit the pair is therefore an assigned sharp observable in the sense of [F3].
[Reduced preparation of product-type states] Let be pairwise orthonormal, put and let be the unit vector of clause (c) for or ; both satisfy by [F4], and both equal . Since commutes with and is self-adjoint, , with and . For fixed , by [F1] and the elementary-tensor pairing, and orthonormality makes every factor vanish unless ; hence only the identity permutation contributes and .
[The mean law] Let be a declared sector preparation with zero extension and let be the readout of step 1.1. Since is bounded, [F3] gives , and by steps 1.1 and [F2], . This proves (a) and (b).
[Product-type means and the reduced preparation] In the situation of step 1.2, the mean is , while the mean law of step 2.1 gives . The identity extends from self-adjoint to every by writing and using linearity of both pairings. Testing it against and using [F5], and ; the identity for all therefore gives , since two endomorphisms of a finite-dimensional space with equal pairings against every rank-one operator are equal. This proves (c).
[Conclusion and scope] Steps 1.1-2.1 prove (a) and (b) and step 3.1 proves (c). Every quantity is a spectral datum of the declared finite-mode sector model and the argument used only P1, P2, P6 and the finite-dimensional tensor and trace calculus; no measured mean, individual-particle property, separability statement or spatial reduction entered, which is (d).
Remarks
- The factor and the normalization of . The reduced preparation is normalized as a probability state on the one-particle space, while sums slot terms; the product-type case shows the two conventions consistently: a Slater determinant of orthonormal modes has reduced preparation the equal-weight mixture of those modes.
- No separability claim. The mean law holds for arbitrary sector preparations, entangled or not; nothing in the argument factors , and the product-type clause is a special case.
Depends on
The declared one-body reduced operator and the one-body readout of a sector preparation
The one-body reduction trace law of the finite-mode permutation sectors
Declared species sectors and their preparations: the framework's reading of P6
The declared finite-mode model and its occupation readouts
Assigned sharp observables and the outcome law of the framework
Mean, second moment and variance of an assigned observable in a preparation
State and composition postulate P1: Hilbert spaces, preparations and composite systems
Observable and probability postulate P2: self-adjoint observables, spectral probability and finite unsharp outcomes
Identical-particle and spin assignments P6: permutation sectors and finite-spin representations
Density operators and scalar spectral probability
Rank-one operator calculus
Slater direct and exchange calculation
Permutation sector projections on the n-fold tensor power
Hilbert tensor construction: inner product, completion and product bases
Cyclicity of the trace
Trace is a linear functional on
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
Dimensions, SI unit conventions and quantity types of the framework
Dependency tree · 2 levels
Sources
- J. Binney and D. Skinner, The Physics of Quantum Mechanics, Sections 6.2-6.3 and 10.1 printed pages 116-126 and 226-230 (PDF pages 124-134 and 234-238): local operators on a composite system, the reduced density operator of one factor, and identical-particle exchange symmetry with the two-particle example
- F. H. L. Essler, Advanced Quantum Theory (Oxford lecture notes, 2017/18), Section 1.4.2 pp. 8-10: single-particle operators as sums over particle labels (46)-(48) and their second-quantised form (49)-(52)