Alberto Ibort

An introduction to the tomographic picture of quantum mechanics

Starting from the famous Pauli problem on the possibility of associating quantum states with probabilities, the formulation of quantum mechanics in which quantum states are described by fair probability distributions (tomograms, i.e. tomographic probabilities) is reviewed in a pedagogical style. The relation between the quantum state description and the classical state description is elucidated. The difference between those sets of tomograms is described by inequalities equivalent to a complete set of uncertainty relations for the quantum domain and to non-negativity of probability density on phase space in the classical domain. The intersection of such sets is studied. The mathematical mechanism that allows us to construct different kinds of tomographic probabilities like symplectic tomograms, spin tomograms, photon number tomograms, etc is clarified and a connection with abstract Hilbert space properties is established. The superposition rule and uncertainty relations in terms of probabilities as well as quantum basic equations like quantum evolution and energy spectra equations are given in an explicit form. A method to check experimentally the uncertainty relations is suggested using optical tomograms. Entanglement phenomena and the connection with semigroups acting on simplexes are studied in detail for spin states in the case of two-qubits. The star-product formalism is associated with the tomographic probability formulation of quantum mechanics.

On the geometry of multisymplectic manifolds.

A multisymplectic structure on a manifold is defined by a closed differential form with zero characteristic distribution. Starting from the linear case, some of the basic properties of multisymplectic structures are described. Various examples of multisymplectic manifolds are considered, and special attention is paid to the canonical multisymplectic structure living on a bundle of exterior k -forms on a manifold. For a class of multisymplectic manifolds admitting a ‘Lagrangian’ fibration, a general structure theorem is given which, in particular, leads to a classification of these manifolds in terms of a prescribed family of cohomology classes.

Bihamiltonian structures and Stäckel separability

Abstract It is shown that a class of Stackel separable systems is characterized in terms of a Gel’fand–Zakharevich bihamiltonian structure. This structure arises as an extension of a Poisson–Nijenhuis structure on phase space. It is also shown that the Casimir of the Gel’fand–Zakharevich bihamiltonian structure provides the family of commuting Killing tensors found by Benenti and that, because of Eisenhart’s theorem, characterize orthogonal separability. It is also shown that recently found properties of quasi-bihamiltonian systems are natural consequences of the geometry of the extension of the Poisson–Nijenhuis structure.

GLOBAL THEORY OF QUANTUM BOUNDARY CONDITIONS AND TOPOLOGY CHANGE

We analyze the global theory of boundary conditions for a constrained quantum system with classical configuration space a compact Riemannian manifold M with regular boundary Γ=∂M. The space ℳ of self-adjoint extensions of the covariant Laplacian on M is shown to have interesting geometrical and topological properties which are related to the different topological closures of M. In this sense, the change of topology of M is connected with the nontrivial structure of ℳ. The space ℳ itself can be identified with the unitary group of the Hilbert space of boundary data . This description, is shown to be equivalent to the classical von Neumanns description in terms of deficiency index subspaces, but it is more efficient and explicit because it is given only in terms of the boundary data, which are the natural external inputs of the system. A particularly interesting family of boundary conditions, identified as the set of unitary operators which are singular under the Cayley transform, (the Cayley manifold), turns out to play a relevant role in topology change phenomena. The singularity of the Cayley transform implies that some energy levels, usually associated with edge states, acquire an infinity energy when by an adiabatic change the boundary conditions reaches the Cayley submanifold 𝒞_. In this sense topological transitions require an infinite amount of quantum energy to occur, although the description of the topological transition in the space ℳ is smooth. This fact has relevant implications in string theory for possible scenarios with joint descriptions of open and closed strings. In the particular case of elliptic self-adjoint boundary conditions, the space 𝒞_ can be identified with a Lagrangian submanifold of the infinite dimensional Grassmannian. The corresponding Cayley manifold 𝒞_ is dual of the Maslov class of ℳ. The phenomena are illustrated with some simple low dimensional examples.

Geometry from dynamics, classical and quantum

Foreword: The birth and the long gestation of a project.- Some examples of linear and nonlinear physical systems and their dynamical equations.- Equations of the motion for evolution systems.- Linear systems with infinite degrees of freedom.- Constructing nonlinear systems out of linear ones.- The language of geometry and dynamical systems: the linearity paradigm.- Linear dynamical systems: The algebraic viewpoint.- From linear dynamical systems to vector fields.- Exterior differential calculus on linear spaces.- Exterior differential calculus on submanifolds.- A tensorial characterization of linear structures.- Partial linear structures: Vector bundles.- Covariant calculus.- Riemannian and Pseudo-Riemannian metrics on linear vector spaces.- Invariant geometric structures and the classical formulations of dynamics of Poisson, Jacobi, Hamilton and Lagrange.- Linear vector fields.- Additional invariant structures for linear vector fields.- Poisson structures.- The inverse problem for Poisson structures.- Symplectic structures.- Lagrangian structures.- Invariant Hermitean structures and the geometry of quantum systems.- Invariant Hermitean inner products.- Complex structures and complex exterior calculus.- Algebras associated with Hermitean structures.- The geometry of quantum dynamical evolution.- The Geometry of Quantum Mechanics and the GNS construction.- Alternative Hermitean structures for quantum systems.- Folding and unfolding Classical and Quantum systems.- Introduction: separable dynamics.- The geometrical description of reduction.- The algebraic description.- Reduction in Quantum Mechanics.- Integrable and superintegrable systems.- The geometrization of the notion of integrability.- The normal form of an integrable system.- Lax representation.- The Calogero system: inverse scattering.- Lie-Scheffers systems.- The inhomogeneous linear equation revisited.- Inhomogeneous linear systems.- Non-linear superposition rule.- Related maps.- Lie systems on Lie groups and homogeneous spaces.- Some examples of Lie systems.- Hamiltonian systems of Lie type.

Dirac brackets in constrained dynamics

This work has been partially supported through grants DGICYT (Spain) (Projects PB94-0106, PB97- 1487 and PB95-0401), ConsejeroÂa de EducacioÂn y Cultura de la Comunidad AutoÂnoma de Madrid, and UNED (Spain)

A generalization of Chetaev’s principle for a class of higher order nonholonomic constraints

The constraint distribution in nonholonomic mechanics has a double role. On the one hand, it is a kinematic constraint, that is, it is a restriction on the motion itself. On the other hand, it is also a restriction on the allowed variations when using D’Alembert’s principle to derive the equations of motion. We will show that many systems of physical interest where D’Alembert’s principle does not apply can be conveniently modeled within the general idea of the principle of virtual work by the introduction of both kinematic constraints and variational constraints as being independent entities. This includes, for example, elastic rolling bodies and pneumatic tires. Also, D’Alembert’s principle and Chetaev’s principle fall into this scheme. We emphasize the geometric point of view, avoiding the use of local coordinates, which is the appropriate setting for dealing with questions of global nature, like reduction.

On the geometry of Lie algebras and Poisson tensors

A geometric programme to analyse the structure of Lie algebras is presented with special emphasis on the geometry of linear Poisson tensors. The notion of decomposable Poisson tensors is introduced and an algorithm to construct all solvable Lie algebras is presented. Poisson-Liouville structures are also introduced to discuss a new class of Lie algebras which include, as a subclass, semi-simple Lie algebras. A decomposition theorem for Poisson tensors is proved for a class of Poisson manifolds including linear ones. Simple Lie algebras are also discussed from this viewpoint and lower-dimensional real Lie algebras are analysed.

Inequivalence of quantum field theories on noncommutative spacetimes: Moyal versus Wick-Voros planes

In this paper, we further develop the analysis started in an earlier paper on the inequivalence of certain quantum field theories on noncommutative spacetimes constructed using twisted fields. The issue is of physical importance. Thus it is well known that the commutation relations among spacetime coordinates, which define a noncommutative spacetime, do not constrain the deformation induced on the algebra of functions uniquely. Such deformations are all mathematically equivalent in a very precise sense. Here we show how this freedom at the level of deformations of the algebra of functions can fail on the quantum field theory side. In particular, quantum field theory on the Wick-Voros and Moyal planes are shown to be inequivalent in a few different ways. Thus quantum field theory calculations on these planes will lead to different physics even though the classical theories are equivalent. This result is reminiscent of chiral anomaly in gauge theories and has obvious physical consequences. The construction of quantum field theories on the Wick-Voros plane has new features not encountered for quantum field theories on the Moyal plane. In fact it seems impossible to construct a quantum field theory on the Wick-Voros plane which satisfies all the properties needed of field theories on noncommutative spaces. The Moyal twist seems to have unique features which make it a preferred choice for the construction of a quantum field theory on a noncommutative spacetime.

Explicit solutions of supersymmetric KP hierarchies: Supersolitons and solitinos

Wide classes of explicit solutions of the Manin‐Radul and Jacobian supersymmetric KP hierarchies are constructed by using line bundles over complex supercurves based on the Riemann sphere. Their construction extends several ideas of the standard KP theory, such as wave functions, ∂‐equations and τ‐functions. Thus, supersymmetric generalizations of N‐soliton solutions, including a new purely odd ‘‘solitino’’ solution, as well as rational solutions, are found and characterized.