Sphère de Bloch | EPFL Graph Search

vignette|droite|L'état d'un système à deux niveaux, tel qu'un spin 1/2 ou plus généralement un qubit, peut être représenté par un point sur une sphère. La sphère de Bloch, du nom du physicien et mathématicien Félix Bloch, ou sphère de Poincaré (comme cas d'application de celle-ci), est une représentation géométrique d'un état pur d'un système quantique à deux niveaux ; c'est donc, entre autres, une représentation d'un qubit. Il est possible de généraliser la construction de cette sphère à un système à n niveaux. La mécanique quantique se formalise dans les espaces de Hilbert, ou plus exactement, dans les espaces de Hilbert projectifs. L'espace projectif des états purs d'un système à 2 niveaux est isomorphe à une sphère. La métrique naturelle de la sphère de Bloch est la métrique de Fubini-Study. Le qubit qubit vignette|État quantique représenté sur la sphère de Bloch, accompagné de ses coordonnées sphériques φ et θ. Considérons un état pur |\psi\rangle

Coherent oscillations between orbital angular momentum polariton states in an elliptic resonator

Benoît Marie Joseph Deveaud, François Morier-Genoud, Gaël Nardin, Barbara Pietka

We optically excited the eigenmodes of an elliptic resonator in a semiconductor microcavity. Using a pulsed excitation, we created a superposition of eigenmodes, and imaged the temporal evolution of the coherent emission pattern. A semiconductor quantum well was embedded in the microcavity structure. The system was operated in the strong light matter coupling regime, where the eigenmodes are hybrid half-photonic half-excitonic quasiparticles called exciton polaritons. Oscillations between orbital angular momentum states (or vortex states) were observed, and turned out to be remarkably well described within the Poincaré sphere representation.

2011

Mesoscopic magnetic systems: From fundamental properties to devices

Dirk Grundler

Research into mesoscopic magnetic systems, which incorporate magnetic elements with dimensions ranging from a few nm up to a few 10s of micrometer, has been spurred on by the developments in their fabrication, characterisation, and control. Electron beam and optical lithography are key methods for their fabrication because they can be used to create tailor- made planar arrangements of elements or more complex layered structures. These can be further transformed into high quality three-dimensional mesoscale architectures using strain engineering based origami approaches. Further methods include fabrication of magnetic nano- and micro-objects on curved templates prepared using ion beam erosion techniques or via self-assembly of non-magnetic particles, which does not require expensive equipment. In addition, direct write methods such as focused electron or ion beam induced deposition, along with two photon laser lithography, provide a means to create geometrically curved and three dimensional structures. The characterisation of the magnetic states can be performed with electrical measurements or using various magnetic microscopy, tomography, and scattering methods. These characterisation methods can then be used to observe the behaviour of complex magnetic textures including magnetic domain walls, vortices, Bloch points, and skyrmions. The control of the magnetic states, which is important for applications, can be achieved with a variety of external stimuli including magnetic or electric fields, spin currents, strain, photons and heat. This results in magnetization dynamics that can happen at timescales ranging from minutes down to less than a picosecond. Mesoscopic magnetic systems and their magnetic configurations are therefore not only of fundamental interest, but have the potential to be implemented in a wide range of device applications including ferro-, ferri- and antiferromagnetic spintronics, computation, magnonics, mechanically flexible and printable human-machine interfaces, electromobility, and medical applications including small-scale machinery. This Special Topic on mesoscopic magnetic systems includes, but is not restricted to, articles in the areas of current interest outlined below, which include experiments, theory and simulations in this fascinating field. We hope to have captured the variety of exciting areas in this Special Topic and that this will provide inspiration for the future, either in terms of fundamental science and discovery, or for more applied work to create novel devices.

2021

Mesoscopic magnetic systems: From fundamental properties to devices

Dirk Grundler

2021