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arXiv:1203.5310

The Symplectic Camel and Quantum Universal Invariants: the Angel of Geometry vs. the Demon of Algebra

Maurice A. de Gosson

Published Mar 23, 2012• 2 citations
arXivHTMLPDF

Abstract

A positive definite symmetric matrix σ qualifies as a quantum mechanical covariance matrix if and only if σ+(1/2)i\hbarΩ\geq0 where Ω is the standard symplectic matrix. This well-known condition is a strong version of the uncertainty principle, which can be reinterpreted in terms of the topological notion of symplectic capacity, closely related to Gromov's non-squeezing theorem. We show that a recent refinement of the latter leads to a new class of geometric invariants. These are the volumes of the orthogonal projections of the covariance ellipsoid on symplectic subspaces of the phase space. We compare these geometric invariants to the algebraic "universal quantum invariants" of Dodonov and Serafini.

DOI
10.48550/arxiv.1203.5310

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A positive de…nite symmetric matrix quali…es as a quantum mechanical covariance matrix if and only if + 12 i~ 0 where is the standard symplectic matrix. This well-known condition is a strong version of the uncertainty principle, which can be reinterpreted in terms of the topological notion of symplectic capacity, closely related to Gromov’s non-squeezing theorem. We show that a recent re…nement of the latter leads to a new class of geometric invariants. These are the volumes of the orthogonal projections of the covariance ellipsoid on symplectic subspaces of the phase space. We compare these geometric invariants to the algebraic “universal quantum invariants”of Dodonov and Sera…ni.
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We define and study the notion of quantum polarity, which is a kind of geometric Fourier transform between sets of positions and sets of momenta. Extending previous work of ours, we show that the orthogonal projections of the covariance ellipsoid of a quantum state on the configuration and momentum spaces form what we call a dual quantum pair. We thereafter show that quantum polarity allows solving the Pauli reconstruction problem for Gaussian wavefunctions. The notion of quantum polarity exhibits a strong interplay between the uncertainty principle and symplectic and convex geometry and our approach could therefore pave the way for a geometric and topological version of quantum indeterminacy. We relate our results to the Blaschke-Santaló inequality and to the Mahler conjecture. We also discuss the Hardy uncertainty principle and the less-known Donoho-Stark principle from the point of view of quantum polarity.
This study develops a rigorous symplectic spectral framework for analyzing the relationship between the covariance matrix and an invertible antisymmetric commutator form in finite-dimensional quantum systems. The earlier positive-eigenvalue interpretation based directly on is replaced by the Hermitian operator H(\Sigma,\Omega) = i\Sigma^{1/2}\Omega^{-1}\Sigma^{1/2}. It is shown that the spectrum of this operator is real, symmetric about zero, and composed of pairs , where the positive quantities are the symplectic eigenvalues of relative to . The study defines the Özbil symplectic gap as g_{\mathrm{Ö}}(\Sigma,\Omega) = \min_j \nu_j, and proves that under the strict Robertson–Schrödinger condition, \Sigma+\frac{i}{2}\Omega>0 \quad\Longrightarrow\quad g_{\mathrm{Ö}}(\Sigma,\Omega)>\frac12. The paper also establishes the invariance of this quantity under simultaneous coordinate transformations, determinant and trace identities, continuity in the operator norm, and local Lipschitz stability on uniformly positive classes of covariance matrices. The scope of the result is stated explicitly. The derived quantity is not a Hamiltonian energy gap, a construction of a continuum quantum field theory, or a proof of the Yang–Mills mass gap. Instead, the work introduces a rigorous finite-dimensional geometric invariant for covariance–commutator pairs and outlines a research programme for extending the framework to finite-volume lattice systems, constructive renormalization-group analysis, and infinite-dimensional operator settings.
We apply the notion of polar duality from convex geometry to the study of quantum covariance ellipsoids in symplectic phase space. We consider in particular the case of “quantum blobs” introduced in previous work; quantum blobs are the smallest symplectic invariant regions of the phase space compatible with the uncertainty principle in its strong Robertson–Schrödinger form. We show that these phase space units can be characterized by a simple condition of reflexivity using polar duality, thus improving previous results. We apply these geometric constructions to the characterization of pure Gaussian states in terms of partial information on the covariance ellipsoid, which allows us to formulate statements related to symplectic tomography.
Abstract We investigate properties of the covariance matrix in the framework of non-commutative quantum mechanics for an one-parameter family of transformations between the familiar Heisenberg–Weyl algebra and a particular extension of it. Employing as a measure of the Robertson–Schrödinger uncertainty principle the linear symplectic capacity of the Weyl ellipsoid (and its dual), we determine its corresponding bounds. Inequalities between the capacities for non-commutative phase-spaces are established. We also present a constructive example based on a simple model to justify our theoretical predictions.