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Two-site Pauli observables commute at disjoint sites
Statement
Take , spin dimension , and distinct sites . On define the Pauli matrices
Let and be their one-site observables embedded in the two-site algebra by the factor inclusions of Local spin matrix algebras and their factor inclusions. Then and are self-adjoint unitaries of norm one and commute. This is a finite example of the locality clause in Quasi-local algebra completion and locality.
Facts & Assumptions
Given: The two-site qubit algebra and its isometric factor inclusions.
A finite region has algebra , and inclusion into a larger region tensors with the identity (Local spin matrix algebras and their factor inclusions).
Those factor inclusions preserve products, adjoints and operator norm (Local spin factor inclusions preserve the operator norm).
Observables supported on disjoint finite regions commute in their common finite tensor algebra (Quasi-local algebra completion and locality).
Proof
Given: , , and facts [F1]–[F3].
Direct matrix multiplication gives , , , and because each preserves the Euclidean norm and is nonzero. By [F2], the embedded and have the same adjoint, square and norm properties.
If , lexicographic factor order gives and , so . If , the factors are interchanged and both products are . Thus in either ordering .
The supports and are disjoint; the computed commutation is therefore the stated instance of the general disjoint-locality clause.