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Atlantis studies in mathematical physics

Lecture 11: Semigroups and Dissipations. Markov Approximation. Quantum Dynamical Semigroups I

Gianfausto Dell’Antonio

Published Jan 1, 2015• 0 citations
Publisher

Abstract

In Lecture “Lecture 10: Derivations and Generators. K.M.S. Condition. Elements of Modular Structure. Standard Form” we have discussed some aspects of quantum dynamics when it is represented by a one-parameter group of automorphisms of a $$C^*$$ -algebra.

DOI
10.2991/978-94-6239-118-5_11

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In Lecture “Lecture 10: Derivations and Generators. K.M.S. Condition. Elements of Modular Structure. Standard Form” we have discussed some aspects of quantum dynamics when it is represented by a one-parameter group of automorphisms of a C∗C^*C∗ -algebra.
We investigate symmetric Gaussian Quantum Markov Semigroups (G-QMSs), a class of quantum dynamical semigroups acting on the algebra of bounded operators over a bosonic Fock space, whose generators are given in generalized GKLS form with linear and quadratic terms in canonical operators. Our main goal is to characterize the symmetry of such semigroups with respect to a faithful invariant Gaussian state, in terms of explicit algebraic conditions on the generator’s parameters. We provide necessary and sufficient conditions for GNS-symmetry using the action of the semigroup on Weyl operators, and show that under natural assumptions, every symmetric G-QMS is unitarily equivalent to a direct sum of Ornstein–Uhlenbeck-type semigroups. The results are further linked to modular theory by confirming the well-known fact that GNS-symmetry implies commutation with the modular automorphism group. Technical appendices discuss the role of Bogoliubov and metaplectic transformations in simplifying the structure of symmetric semigroups.
A KMS-symmetric quantum Markov semigroup is completely determined by the quadratic form in induces on the GNS Hilbert space. The quadratic forms that arise in this way are called quantum Dirichlet forms. In this article we give a complete description of bounded quantum Dirichlet forms in terms of derivations. In particular, we exhibit an explicit characterization of the bounded quantum Dirichlet forms on type I factors. We also show that any unbounded quantum Dirichlet form can be represented in terms of derivations. The key technical insight is that the modular group always restricts to a strongly continuous group on the domain of a quantum Dirichlet form such that the square root of the analytic generator is accretive with respect to the form norm.
The present work develops a classification of finite-dimensional irreducible invariant subspaces for quantum Markov semigroups (GKSL semigroups) whose generators are algebraically generated from two non-commuting orthogonal projections. The central mechanism is commutant collapse: a Hermitian rank-1 operator—the “bridge”—couples a two-dimensional Halmos fiber to a trivial direction, forces the commutant of the generated C^*-algebra to become trivial, and thereby saturates the algebra to a full matrix algebra by the double commutant theorem. We identify i-axis symmetry—the requirement that the noise operators generate a *-closed algebra—as the structural condition that guarantees bridges appear in Hermitian form. A single bridge connecting one Halmos fiber to one trivial direction forces the algebra M_3(\mathbb{C}); the resulting semigroup is primitive on that subspace. For k\ge 2 trivial directions coupled to the same fiber, we introduce the concept of center-breaking bridges. Whether the algebra saturates to M_{2+k}(\mathbb{C}) or stabilises at an intermediate barrier subalgebra depends on whether each successive bridge breaks the centre of the intermediate algebra. The dimension of the minimal non-trivial irreducible block is 2+k when all k bridges act in a centre-breaking manner. The Lie envelope of the three-dimensional sector is \mathfrak{su}(3) for generic parameters, with degeneracies at symmetric projection angles. We discuss dynamical closure rigidity—the property that a finitely generated operator system becomes irreducible under C^*-closure—and distinguish kinematic capacity from dynamical accessibility.