Showing papers by "Stéphane Guérin published in 2002"

Optimal control in laser-induced population transfer for two- and three-level quantum systems

Abstract: We apply the techniques of control theory and of sub-Riemannian geometry to laser-induced population transfer in two- and three-level quantum systems. The aim is to induce complete population transfer by one or two laser pulses minimizing the pulse fluences. Sub-Riemannian geometry and singular-Riemannian geometry provide a natural framework for this minimization, where the optimal control is expressed in terms of geodesics. We first show that in two-level systems the well-known technique of “π-pulse transfer” in the rotating wave approximation emerges naturally from this minimization. In three-level systems driven by two resonant fields, we also find the counterpart of the “π-pulse transfer.” This geometrical picture also allows one to analyze the population transfer by adiabatic passage.

Optimization of population transfer by adiabatic passage

Abstract: We examine the adiabatic limit of population transfer in two-level models driven by a chirped laser field. We show that the nonadiabatic correction is minimized when the adiabatic eigenenergies associated to the dynamics are parallel. In the diagram of the difference of the eigenenergy surfaces as a function of the parameters, this corresponds to an adiabatic passage along a level line. The analytical arguments are based on the Dykhne-Davis-Pechukas treatment. We illustrate this behavior with various examples.

Orientation of polar molecules by laser induced adiabatic passage.

Abstract: We show that two overlapping linearly polarized laser pulses of frequencies ω and its second harmonic 2ω can strongly orient linear polar molecules, by adiabatic passage along dressed states. The resulting robust orientation can be interpreted as a laser-induced localization in the effective double well potential created by the fields, which induces a preliminary molecular alignment. The direction of the orientation can be selected by the relative phase of the fields.

Topology of adiabatic passage

Abstract: We examine the topology of eigenenergy surfaces characterizing the population transfer processes based on adiabatic passage. We show that this topology is the essential feature for the analysis of the population transfers and the prediction of its final result. We reinterpret diverse known processes, such as stimulated Raman adiabatic passage (STIRAP), frequency-chirped adiabatic passage and Stark-chirped rapid adiabatic passage. Moreover, using this picture, we display new related possibilities of transfer. In particular, we show that we can selectively control the level that will be populated in STIRAP process in $\ensuremath{\Lambda}$ or V systems by the choice of the peak amplitudes or the pulse sequence.

Adiabatic creation of entangled states by a bichromatic field designed from the topology of the dressed eigenenergies

Abstract: Preparation of entangled pairs of coupled two-state systems driven by a bichromatic external field is studied. We use a system of two coupled spin-1/2 that can be translated into a three-state ladder model whose intermediate state represents the entangled state. We show that this entangled state can be prepared in a robust way with appropriate fields. Their frequencies and envelopes are derived from the topological properties of the model.

Resistance to corrosion an embrittlement of T91 steel in stagnant Pb-Bi of eutectic composition

Abstract: On etudie la resistance a la corrosion et a la fragilisation de l'acier T91 en milieu Pb-Bi stagnant de composition eutectique dans la gamme de temperatures 150-650°C. La degradation de la resistance a la corrosion se manifeste au-dessus de 350°C, avec l'adherence du Pb-Bi visible a 600°C sur les inclusions affleurantes en surface de l'acier, ainsi que l'apparition d'effets de corrosion localisee a 650°C. Neanmoins, les effets du vieillissement dans Pb-Bi ne se refletent pas sur les proprietes mecaniques ex situ a temperature ambiante dans l'air. Au contraire, lorsque les essais de traction sont realises in situ a la temperature de vieillissement dans Pb-Bi, on peut observer un effet fragilisant. L'effet fragilisant depend de la temperature, de la vitesse de deformation, de l'existence de concentrateurs de contraintes, et plus specialement de la composition chimique de l'atmosphere de couverture. Cet effet est visible dans Pb-Bi sous atmosphere d'He-4%H 2 alors qu'il est quasiment indetectable dans Pb-Bi sous vide. On fait decroitre au maximum l'energie de rupture (environ de 30%) sur eprouvettes entaillees sollicitees a vitesse de deplacement de traverse 6x10 -4 mm/s dans Pb-Bi a 350°C sous atmosphere He-4%H 2 .

Influence of long term exposure to molten lead on the microstructure and mechanical behaviour of T91 steel

Abstract: Des echantillons d'acier a 9%Cr martensitique T91 ont subi des recuits de 6 heures a un mois en bain de plomb liquide a 525°C sous controle d'activite de l'oxygene. A faible activite d'oxygene, une fine couche d'oxyde de chrome se forme; sous une pression plus elevee, la couche devient plus epaisse et complexe. De nombreuses porosites apparaissent pres de la surface dans les echantillons apres seulement quelques jours en bain de plomb. D'apres la fleche residuelle d'echantillons recuits en flexion, la relaxation de la contrainte est nettement plus rapide en bain de plomb que sous atmosphere gazeuse. Cependant, le comportement en traction de l'acier demeure inchange apres un vieillissement d'un mois dans le bain de plomb.