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Distinct preparations are separated by the statistics of a bounded sharp observable
A conditional separation statement for the framework's preparations: two distinct preparations of a declared system assign different probabilities to some outcome of some bounded sharp observable of the framework, namely to the outcome 1 of a rank-one orthogonal projection; the difference is bounded by the trace-norm distance of the two states. The item asserts no finite-measurement procedure that identifies a preparation, no apparatus realization and no empirical result; it concerns the model's state space under the state and observable postulates.
Statement
Adopt the state and composition postulate State and composition postulate P1: Hilbert spaces, preparations and composite systems (P1) and the observable and probability postulate Observable and probability postulate P2: self-adjoint observables, spectral probability and finite unsharp outcomes (P2), and assume the Axiom of Countable Choice and the Axiom of Choice (The Axiom of Countable Choice (), The Axiom of Choice). Let be the state space assigned to a declared system and let be two preparations of the system in the sense of The state space of the framework: preparations, pure rays and density operators clauses (a)-(c). Then there is a bounded sharp observable of the framework, namely an orthogonal projection of rank one, such that the outcome laws of Assigned sharp observables and the outcome law of the framework differ at the outcome set : Moreover for every rank-one projection one has ; consequently, if for every bounded sharp observable (or merely for every rank-one projection), then .
The statement is conditional on P1 and P2 and asserts no procedure that determines a preparation from finitely many measurements, no apparatus realization, no error rate and no empirical result.
Facts & Assumptions
Given: The adopted postulates P1 and P2 (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); the Axiom of Countable Choice and the Axiom of Choice (The Axiom of Countable Choice (), The Axiom of Choice); the assigned state space with preparation class of The state space of the framework: preparations, pure rays and density operators; preparations with .
Mathematical separation. For distinct density operators there is a unit vector with ; for the rank-one projection the spectral law of satisfies and likewise for , and (Distinct density operators are separated by the law of a bounded rank-one projection clauses (a)-(c)).
The framework's outcome law and its reading. An assigned sharp observable has spectral PVM and outcome law , equal to for a pure preparation; the law is a probability measure on the Borel sets of , and the outcome-set reading of the model is that of Assigned sharp observables and the outcome law of the framework (The Born outcome law: probability measure, mixtures, atoms and spectral support clauses (a)-(c), clause (ii)).
Projections are assigned bounded observables. An orthogonal projection is a bounded self-adjoint operator with on (The Hilbert orthogonal projection onto a closed subspace, Hilbert projections are linear, self-adjoint and contractive), hence qualifies as a sharp observable of the framework when assigned with the dimensionless unit of its spectral coordinate.
Proof
[Existence of a separating bounded observable] Let and be as in [F1]: such a unit vector exists because and is a nonzero self-adjoint trace-class operator. By [F3] the operator is a bounded self-adjoint projection, hence an assigned sharp observable of the framework once its unit (the dimensionless spectral coordinate, which here takes the values and ) is declared; its spectral law at is and by [F2], and [F1] gives .
[Uniform bound and separation] The bound is [F1] applied to the rank-one projection . If all rank-one projections have equal laws for and , then for every unit by the same identification, contradicting the choice of in [F1] unless ; hence statistical identity for all such observables forces the preparations to coincide. This proves the statement, and the assertion involves no measurement procedure beyond the model's outcome law.
Depends on
The Axiom of Choice
The Axiom of Countable Choice ()
The Hilbert orthogonal projection onto a closed subspace
Assigned sharp observables and the outcome law of the framework
The state space of the framework: preparations, pure rays and density operators
Distinct density operators are separated by the law of a bounded rank-one projection
Hilbert projections are linear, self-adjoint and contractive
Observable and probability postulate P2: self-adjoint observables, spectral probability and finite unsharp outcomes
State and composition postulate P1: Hilbert spaces, preparations and composite systems
The Born outcome law: probability measure, mixtures, atoms and spectral support
Dependency tree · 2 levels
Sources
- J. Preskill, Lecture Notes for Ph219/CS219: Quantum Information, Chapter 2 (updated July 2015), Sections 2.2.1, 2.3.2 and 2.6, printed pages 8-9, 21-23 and 36-39 (Bloch-vector determination of qubit states; statistics distinguishing states and the trace-distance notion)
- J. Binney and D. Skinner, The Physics of Quantum Mechanics, Section 6.3, printed pages 121-126 (the density operator and the trace rule, with distinct states having different statistics)