411 research outputs found
On the relationship between a quantum Markov semigroup and its representation via linear stochastic Schroedinger equations
A quantum Markov semigroup can be represented via classical diffusion
processes solving a stochastic Schr\"odinger equation. In this paper we first
prove that a quantum Markov semigroup is irreducible if and only if classical
diffusion processes are total in the Hilbert space of the system. Then we study
the relationship between irreducibility of a quantum Markov semigroup and
properties of these diffusions such as accessibility, the Lie algebra rank
condition, and irreducibility. We prove that all these properties are, in
general, weaker than irreducibility of the quantum Markov semigroup,
nevertheless, they are equivalent for some important classes of semigroups.Comment: 16 page
Dynamical Semigroups for Unbounded Repeated Perturbation of Open System
We consider dynamical semigroups with unbounded Kossakowski-Lindblad-Davies
generators which are related to evolution of an open system with a tuned
repeated harmonic perturbation. Our main result is the proof of existence of
uniquely determined minimal trace-preserving strongly continuous dynamical
semigroups on the space of density matrices. The corresponding dual W
*-dynamical system is shown to be unital quasi-free and completely positive
automorphisms of the CCR-algebra. We also comment on the action of dynamical
semigroups on quasi-free states
Approximation and limit theorems for quantum stochastic models with unbounded coefficients
We prove a limit theorem for quantum stochastic differential equations with
unbounded coefficients which extends the Trotter-Kato theorem for contraction
semigroups. From this theorem, general results on the convergence of
approximations and singular perturbations are obtained. The results are
illustrated in several examples of physical interest.Comment: 23 page
Heisenberg Picture Approach to the Stability of Quantum Markov Systems
Quantum Markovian systems, modeled as unitary dilations in the quantum
stochastic calculus of Hudson and Parthasarathy, have become standard in
current quantum technological applications. This paper investigates the
stability theory of such systems. Lyapunov-type conditions in the Heisenberg
picture are derived in order to stabilize the evolution of system operators as
well as the underlying dynamics of the quantum states. In particular, using the
quantum Markov semigroup associated with this quantum stochastic differential
equation, we derive sufficient conditions for the existence and stability of a
unique and faithful invariant quantum state. Furthermore, this paper proves the
quantum invariance principle, which extends the LaSalle invariance principle to
quantum systems in the Heisenberg picture. These results are formulated in
terms of algebraic constraints suitable for engineering quantum systems that
are used in coherent feedback networks
Quantum Feynman-Kac perturbations
We develop fully noncommutative Feynman-Kac formulae by employing quantum
stochastic processes. To this end we establish some theory for perturbing
quantum stochastic flows on von Neumann algebras by multiplier cocycles.
Multiplier cocycles are constructed via quantum stochastic differential
equations whose coefficients are driven by the flow. The resulting class of
cocycles is characterised under alternative assumptions of separability or
Markov regularity. Our results generalise those obtained using classical
Brownian motion on the one hand, and results for unitarily implemented flows on
the other.Comment: 27 pages. Minor corrections to version 2. To appear in the Journal of
the London Mathematical Societ
- …