40 research outputs found
Feedback control of spin systems
The feedback stabilization problem for ensembles of coupled spin 1/2 systems
is discussed from a control theoretic perspective. The noninvasive nature of
the bulk measurement allows for a fully unitary and deterministic closed loop.
The Lyapunov-based feedback design presented does not require spins that are
selectively addressable. With this method, it is possible to obtain control
inputs also for difficult tasks, like suppressing undesired couplings in
identical spin systems.Comment: 16 pages, 15 figure
Diffusive spin transport
Information to be stored and transported requires physical carriers. The
quantum bit of information (qubit) can for instance be realised as the spin 1/2
degree of freedom of a massive particle like an electron or as the spin 1
polarisation of a massless photon. In this lecture, I first use irreducible
representations of the rotation group to characterise the spin dynamics in a
least redundant manner. Specifically, I describe the decoherence dynamics of an
arbitrary spin S coupled to a randomly fluctuating magnetic field in the
Liouville space formalism. Secondly, I discuss the diffusive dynamics of the
particle's position in space due to the presence of randomly placed impurities.
Combining these two dynamics yields a coherent, unified picture of diffusive
spin transport, as applicable to mesoscopic electronic devices or photons
propagating in cold atomic clouds.Comment: Lecture notes, published in A. Buchleitner, C. Viviescas, and M.
Tiersch (Eds.), "Entanglement and Decoherence. Foundations and Modern
Trends", Lecture Notes in Physics 768, Springer, Berlin (2009
Quantifying Entanglement Production of Quantum Operations
The problem of entanglement produced by an arbitrary operator is formulated
and a related measure of entanglement production is introduced. This measure of
entanglement production satisfies all properties natural for such a
characteristic. A particular case is the entanglement produced by a density
operator or a density matrix. The suggested measure is valid for operations
over pure states as well as over mixed states, for equilibrium as well as
nonequilibrium processes. Systems of arbitrary nature can be treated, described
either by field operators, spin operators, or any other kind of operators,
which is realized by constructing generalized density matrices. The interplay
between entanglement production and phase transitions in statistical systems is
analysed by the examples of Bose-Einstein condensation, superconducting
transition, and magnetic transitions. The relation between the measure of
entanglement production and order indices is analysed.Comment: 20 pages, Revte
Leptodora kindtii survival in the laboratory
Leptodora kindtii , a pelagic predatory cladoceran, suffers high mortality on transfer to laboratory, which makes the experimental work difficult. We investigated the causes of high mortality, using four variables: water volume, animal density, light intensity, and origin of water for culturing, i.e., water from native or a non-native lake. For the experiments we used Leptodora and water from Lake Loosdrecht and Lake Maarsseveen (The Netherlands). Water was found to be the most important factor; the animals did not necessarily do better in lake water from which they were collected. Water volume and animal density were of limited importance, and light intensity did not affect survival.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41752/1/10452_2004_Article_DO00000328.pd
Bound smoothing under chirality constraints
SIGLETIB Hannover: RO 8278(90-017) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman
#DELTA#-matroids and metroids
SIGLETIB Hannover: RO 8278(90-042) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman