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Fluctuations of the oscillator coherent states and the position-momentum relation of the model
The preparation spreads of position and momentum in the coherent states of the one-dimensional oscillator model, the exact value hbar/2 of their product, the one-dimensional position-momentum relation Delta Q Delta P >= hbar/2 for every unit preparation vector in the common domain of the two model operators, and the saturation of that relation by every coherent preparation. The statement is a conditional consequence of the postulates P1, P2 and P5, of the adopted oscillator model, of the cited uncertainty and moment results and of the cited mathematics; it concerns the two separately assigned outcome laws in one preparation and asserts no joint outcome law, no apparatus resolution, no disturbance and no empirical result.
Statement
Adopt the state and composition postulate P1, the observable and probability postulate P2 and the kinematics and massive-particle model postulate P5, read through the one-dimensional oscillator model of The quadratic-potential oscillator model of the framework clauses (a), (b), (c), and the coherent-state family of The coherent states of the oscillator model; assume the Axiom of Countable Choice and the Axiom of Choice (The Axiom of Countable Choice (), The Axiom of Choice). Let and be the model's position and momentum observables with their declared units ( and ), let , let be the preparation spread of an assigned sharp observable in the reading of Preparation variance and spread of an assigned sharp observable, and write for the model's coherent preparation of label . Then:
(a) Coherent spreads. For every the vector lies in , the preparation spreads of and in the pure preparation of exist, and
(b) Position-momentum relation of the model. Every unit vector satisfies read in the product unit ; consequently the value attained in (a) is the minimum over the common domain, and it is attained by every coherent preparation;
(c) Saturation. For every , with the centered vectors one has ; in particular and are linearly dependent, so ;
(d) Status. Clauses (a)-(c) are conditional consequences of P1, P2, P5, the adopted oscillator model, the cited preparation-moment and uncertainty results and the cited mathematics. They concern the spreads of the two separately assigned outcome laws of and in one preparation; they introduce no joint outcome law of position and momentum, no measurement error, no disturbance of the preparation, no dynamics and no empirical value. Whether a laboratory preparation and apparatus realize the pair of observables and produce this spread is a tested identification under the preparation and apparatus postulate P7.
Facts & Assumptions
Given: the adopted postulates P1, P2, P5; the Axiom of Countable Choice and the Axiom of Choice; the one-dimensional oscillator model with parameters and length ; the coherent states ; a unit vector ; a complex number .
The model's observables. and are assigned sharp observables of the model with , , , , where is the weak derivative, and are the , instances of the componentwise operators of Componentwise position and momentum operators on the component-valued wave-function space; the flow of the isolated model is generated by the assigned energy observable (The quadratic-potential oscillator model of the framework clauses (b), (c); Kinematics and massive-particle model postulate P5: Newtonian frames and one-particle operators; Assigned sharp observables and the outcome law of the framework clause (a)).
Coherent data. For every : is a unit vector with and for every ; ; and (The coherent states of the oscillator model clauses (b), (c); Coherent-state convergence clauses (a), (b), (c), (d), (e)).
Exact coherent quantities. With and the position and momentum operators on and , the transported ladder operators, and the quadratic-form means and variances of the coherent vectors are and , ; also the adjoint relation holds, the transport of along the unitary (Coherent evolution and variance clauses (d), (e); Coherent-state convergence clause (e); Closed ladder domains clauses (b), (d), (e)).
Means, variances and spreads. For an assigned sharp observable and a unit vector the mean and second moment of the outcome law exist, , , and the preparation variance and spread are read in the unit and its square (Mean, second moment and variance of an assigned observable in a preparation clauses (a), (b); Preparation variance and spread of an assigned sharp observable clauses (b), (c), (d)).
Product domain and commutator in one dimension. For , the product domain is and for every in it (The position-momentum commutator identity on the product domain clauses (b), (c)).
Robertson relation. For two assigned sharp observables , and a unit vector : ; and if in addition , then (The Robertson relation for preparation spreads of assigned sharp observables clauses (a), (b)).
Approximation by product-domain vectors. For every nonzero there is a sequence of unit vectors with , , , and with , , and the analogous momentum limits (Approximation of Q-P graph-domain vectors by product-domain vectors).
Inner-product calculus and Cauchy-Schwarz. The pairing is linear in the first argument and conjugate-linear in the second, , with equality if and only if are linearly dependent, and inner products and norms are continuous for norm convergence (Real and complex inner product spaces, with the inner product linear in the first argument, Hilbert space, Cauchy–Schwarz: , with equality exactly for linearly dependent vectors).
Proof
Given: the data of the Statement and [F1]-[F8].
[The model's operators and the coherent formulas] By [F1] the model's , are the , componentwise operators with the displayed dense domains, and both are assigned sharp observables; by [F2] each is a unit vector with , and . By [F3] the vectors and lie in , so , and the displayed coherent means and central norms hold.
[Spreads as centered norms] Let be a unit vector. By [F4] applied to the assigned observables and , the outcome-law means and variances exist and , and likewise .
[Bound on the product domain] Let be a unit vector. By [F5], , so by linearity of the pairing in its first argument. By the Robertson form of [F6] applied to the assigned sharp observables and , .
[Approximating a common-domain vector] Let be a unit vector. By [F7] there is a sequence of unit vectors with , , , , and the analogous momentum limits.
[Clause (a)] By step 1.1 and step 1.2 applied to the unit vector , the preparation variances are the central squared norms of [F3]: and ; taking square roots gives and , whose product is in the unit .
[Clause (b), relation] Let be a unit vector and let be the approximating sequence of step 1.4. For every , step 1.3 gives ; by step 1.2, , and the limits of step 1.4 together with the continuity of the inner product [F8] give , and likewise . Taking square roots and passing to the limit in the inequality gives .
[Clause (c), saturation] Fix and put , , both in by step 1.1. Using the conjugate-linearity in the second argument, the coherent data and the adjoint relation of [F2] and [F3], so that , and , together with and , the four pairings are , , and , so ; hence, using the first formula of [F3] and , , because . By [F3], , so . Therefore and , while step 2.1 gives , so equality holds in the Cauchy-Schwarz inequality of [F8] and are linearly dependent.
[Clause (b), minimality] By step 2.2 every unit vector in has spread product at least , and by step 2.1 every coherent preparation attains the value ; hence the minimum of the spread product over the unit vectors of the common domain is , attained by each .
[Clause (d)] The spread values and the relation were obtained from the model's assigned observables, the preparation-moment theorem, the product-domain commutator, the cutoff approximation and the Robertson relation; the saturation computation used the same operators and the exact coherent-state quantities. No joint outcome law, apparatus resolution, disturbance, dynamics or empirical value entered, and the identification of a laboratory pair with is a tested identification under P7. This is clause (d).
Depends on
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
Kinematics and massive-particle model postulate P5: Newtonian frames and one-particle operators
The quadratic-potential oscillator model of the framework
The coherent states of the oscillator model
Assigned sharp observables and the outcome law of the framework
Preparation variance and spread of an assigned sharp observable
Dimensions, SI unit conventions and quantity types of the framework
Mean, second moment and variance of an assigned observable in a preparation
The Robertson relation for preparation spreads of assigned sharp observables
The position-momentum commutator identity on the product domain
Approximation of Q-P graph-domain vectors by product-domain vectors
Componentwise position and momentum operators on the component-valued wave-function space
Coherent evolution and variance
Coherent-state convergence
Closed ladder domains
Hilbert space
Real and complex inner product spaces, with the inner product linear in the first argument
Cauchy–Schwarz: , with equality exactly for linearly dependent vectors
The Axiom of Countable Choice ()
The Axiom of Choice
Dependency tree · 2 levels
Sources
- B. Zwiebach, MIT 8.05 Quantum Physics II (Fall 2013), Chapter 6 'Quantum Dynamics', §4.3 printed pages 30-33: the coherent-state uncertainties (4.31)-(4.33) and the time-independence of Delta x
- J. Binney and D. Skinner, The Physics of Quantum Mechanics, §3.1 printed pages 37-41 (PDF pages 45-49): the Gaussian ground state, its position and momentum dispersions and the saturated uncertainty product
- D. Tong, Quantum Mechanics (author-hosted lecture set), §2.2 printed pages 34-41 (PDF pages 37-44): the oscillator eigenstates and the normalizable Gaussian ground state