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European Journal of Physics

Expressing complementarity and the x – p commutation relation through further quantum inequalities

R. F. Álvarez-Estrada

Published Feb 24, 2010• 2 citations
Publisher

Abstract

Complementarity and the commutation relation of position (x) and momentum (p) imply much more than the fundamental x-p uncertainty inequality. Here, we display some further consequences of the former that could have certain pedagogical interest and, so, contribute to the teaching of quantum mechanics. Inspired by an elementary derivation of the x-p uncertainty inequality, based upon a positive quadratic polynomial, we explore one possible extension, via quartic polynomials and simple algebra and integrations. Our analysis, aimed at providing some further pedagogic expression of genuine quantum behaviours, yields other quantum inequalities for expectation values, expressed through suitable discriminants associated with quartic algebraic equations, which differ from (and are not a strict consequence of) the x-p uncertainty inequality. Those quantum inequalities are confirmed, and genuine non-classical behaviours are exhibited, for simple cases: a harmonic oscillator, a hydrogenic atom and free Gaussian wave packets. The physical interest of the expectation values involved in the quantum inequalities and of the latter is discussed, in the framework of quantum optics and squeezing phenomena.

DOI
10.1088/0143-0807/31/2/019

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Complementarity and the commutation relation of position (x) and momentum (p) imply much more than the fundamental x-p uncertainty inequality. Here, we display some further consequences of the former that could have certain pedagogical interest and, so, contribute to the teaching of quantum mechanics. Inspired by an elementary derivation of the x-p uncertainty inequality, based upon a positive quadratic polynomial, we explore one possible extension, via quartic polynomials and simple algebra and integrations. Our analysis, aimed at providing some further pedagogic expression of genuine quantum behaviours, yields other quantum inequalities for expectation values, expressed through suitable discriminants associated with quartic algebraic equations, which differ from (and are not a strict consequence of) the x-p uncertainty inequality. Those quantum inequalities are confirmed, and genuine non-classical behaviours are exhibited, for simple cases: a harmonic oscillator, a hydrogenic atom and free Gaussian wave packets. The physical interest of the expectation values involved in the quantum inequalities and of the latter is discussed, in the framework of quantum optics and squeezing phenomena.
In a previous work, based upon complementarity and inspired by a standard derivation of the x?p uncertainty inequality (via a non-negative quadratic polynomial f2), we explored one possible extension, through a non-negative quartic polynomial f4, for non-commuting quantum variables. That work led to new quantum inequalities (expressed through the discriminant of the equation f4?=?0) and to connections with quantum optics. Here, we extend the study of quartic polynomials and report classes of genuine quantum states |?4,n? with interesting properties, which follow directly from f4?=?0 for x?p. We explore |?4,n? when they are generated from the coherent and the squeezed coherent states of the quantum harmonic oscillator and we display their relationships to states previously proposed by other authors through different constructions. In quantum optics, the |?4,n? associated to the coherent states are just displaced Fock states of light, which have already been generated experimentally.
Complementarity (expressed through the x–p commutation relationship) yields, by using suitable non-negative polynomials, quantum inequalities. The latter become strict equalities only for restricted sets of quantum states, which display genuine quantum features. We have shown in previous studies that: (i) a certain non-negative quartic polynomial f4 led to a new quantum inequality, expressed through the discriminant (D'r) of the equation f4 = 0 as D'r ≤ 0; (ii) D'r = 0 gave rise to an interesting class of non-Gaussian quantum states |ψ4〉 and (iii) in quantum optics, a subset of those |ψ4〉 were the displaced one-photon states, previously proposed by other authors through different motivations and already generated experimentally in 2002. The extension of our previous research studies will be reported here. We shall characterize the general class of non-Gaussian quantum states |ψ4,g〉 such that D'r = 0. The class of the |ψ4,g〉 is larger than and includes that of the former |ψ4〉. The Wigner function for the |ψ4,g〉 is obtained and is negative in some domains. In quantum optics, a subclass of the |ψ4,g〉 is formed by the single-photon-added coherent states (non-Gaussian) generated experimentally, in turn, in 2004. Another subclass of the |ψ4,g〉 includes single-photon-added squeezed states and single-photon-added squeezed coherent states (both being non-Gaussian). Mandel's parameters are studied for those states: in particular, its negativity for the single-photon-added squeezed states is established for a certain interval of the squeeze factor. The (Hilbert–Schmidt) non-Gaussianities for both the displaced one-photon states and the single-photon-added squeezed states are obtained explicitly: both of them are numerically equal to 5/12. The possibility of generating the single-photon-added squeezed states experimentally is discussed briefly, by extending previous studies by other authors on producing related quantum states; in particular, we treat in outline one possible generation using optical parametric oscillators and parametric amplifiers.
Heisenberg guessed, after he established the matrix quantum mechanics, that the non-commutativity of the matrices of position and momentum implied that the position and momentum of a particle could not be precisely simultaneously determined. He consequently conjectured that time and energy should also have a similar relationship. Soon after, Robertson derived an inequality concerning the space coordinate and momentum, which was thought to be the mathematical expression of the uncertainty relation guessed by Heisenberg. Since then, people have tried various devices to prove the correctness of these two relations. However, no one conducted a careful analysis of Heisenberg’s primary paper. In this work, we point out some serious problems in Heisenberg’s paper and the literature talking about the uncertainty relationships: the physical concepts involved in the uncertainty relations are not clear; one physical concept had more than one explanation, i.e., switching concepts; there has never been measurement experiment to support the relations. The conclusions are that the so-called coordinate–momentum uncertainty relation has never been related to actual measurement and there does not exist a time–energy uncertainty relation.
The spread of the wave-function, or quantum uncertainty, is a key notion in quantum mechanics. At leading order, it is characterized by the quadratic moments of the position and momentum operators. These evolve and fluctuate independently from the position and momentum expectation values. They are extra degrees of quantum mechanics compared to classical mechanics, and encode the shape of wave-packets. Following the logic that quantum mechanics must be lifted to quantum field theory, we discuss the quantization of the quantum uncertainty as an operator acting on wave-functions over field space and derive its discrete spectrum, inherited from the sl2\textrm{sl}_{2}sl2​ Lie algebra formed by the operators x^2\hat{x}^{2}x^2, p^2\hat{p}^{2}p^​2 and xp^\widehat{xp}xp​. We further show how this spectrum appears in the value of the coupling of the effective conformal potential driving the evolution of extended Gaussian wave-packets according to Schrödinger equation, with the quantum uncertainty playing the same role as an effective intrinsic angular momentum. We conclude with an open question: is it possible to see experimental signatures of the quantization of the quantum uncertainty in non-relativistic physics, which would signal the departure from quantum mechanics to a QFT regime?
Rigorous and universal relationships among radial expectation values of any DDD-dimensional quantum-mechanical system are obtained, using R\'enyi-like position-momentum inequalities in an information-theoretical framework. Although the results are expressed in terms of four moments (two in position space and two in the momentum one), especially interesting are the cases that provide expressions of uncertainty in terms of products $\ensuremath{\langle}{r}^{a}\ensuremath{\rangle}{}^{1/a}\ensuremath{\langle}{p}^{b}\ensuremath{\rangle}{}^{1/b}$, widely considered in the literature, including the famous Heisenberg relationship $\ensuremath{\langle}{r}^{2}\ensuremath{\rangle}\ensuremath{\langle}{p}^{2}\ensuremath{\rangle}\ensuremath{\geqslant}{D}^{2}/4$. Improved bounds for these products have recently been provided, but are always restricted to positive orders a,b>0a,b>0a,b>0. The interesting part of this work are the inequalities for negative orders. A study of these relationships is carried out for atomic systems in their ground state. Some results are given in terms of relevant physical quantities, including the kinetic and electron-nucleus attraction energies, the diamagnetic susceptibility, and the height of the peak of the Compton profile, among others.