Coherent effects are studied in a dense atomic vapor driven by laser fields. With optical properties dramatically modified by these effects, the medium can be used to manipulate some of the properties of laser field. Our experiments demonstrate the coherent control over transmission, spatial distribution and noise feature of the laser field interacting with coherent media. The results have potential applications in the field such as precision metrology, precision spectroscopy, optical imaging and lithography. We develop an experiment to investigate the atomic excitation by few-cycle radio frequency (RF) pulses interacting with Zeeman sublevels. The system provides the flexibility to fully control all parameters of RF pulses. Such a flexibility can not be achieved in optical domain. Based on this system, experiments can be conducted to simulate processes in ultra-short laser physics. In particular, we study the carrier-envelope effect of few-cycle pulses and the strong off-resonant excitation by short pulses. We also discuss the selective reflection spectrum on a highly dense atomic vapor in which the dipole-dipole interaction can not be neglected. The spectrum broadening due to dipole-dipole interaction is much broader than the Doppler broadening. Our experiments show that the excitation by a pump laser can reduce the dipole-dipole interaction, thus reduce the broadening and improve the spectral resolution. The excitation dependence is studied at various atomic densities.

Laser-atom interactions create atomic coherence and large nonlinear atomic polarization. We investigate resonant laser-atom interactions to generate large nonlinearities and control them using magneto-optical fields. Coherent control of high-order susceptibilities and magneto-optical rotation are demonstrated. Experiments are supported by theoretical studies that effectively describe the observed phenomena. It is shown that a new coherent field, with polarization orthogonal to a weak signal field, can be parametrically generated via an all-resonant four-wave-mixing process. This is demonstrated in a double-ladder system having two intermediate states between a ground and an excited state. It is shown that the parametricgeneration process can be coherently controlled by coupling lasers and magnetic fields. It is theoretically established that the underlying physics is a resonant three-photon process with a wide domain of control parameters. Electromagnetically induced transparency (EIT), where absorption of a weak probe is suppressed via quantum interference, is demonstrated in a usual three-level ladder system. It is observed that in contrast with EIT in a usual ladder system, addition of a second channel helps to suppress the absorption of two weak probe fields in the double-ladder system. The resulting enhancement of transmission in two different channels is due to gain caused by three-photon processes. Coherent control is strongly limited by coherence lifetime, which is the inverse of the dephasing rate. A lambda-system, having two ground states coupled to a common excited state by lasers, can generate a new eigen (dark)-state that is transparent to incoming fields and hence suppresses fluorescence. However, ground-state dephasing perturbs the dark state. A new method for measuring the ground-state dephasing rate from fluorescence signals is proposed and a proof-of-principle experiment demonstrated. While two laser fields in a lambda-system are resonant with their respective transitions, the atomic polarizations are very sensitive to an applied magnetic field. This effect can be used for optical magnetometry. The degree of sensitivity of the magnetometer is determined by two competing parameters–atomic density and laser intensity. It is shown experimentally that the optimal sensitivity reaches saturation, which is contrary to the idea that sensitivity increases indefinitely with an increase in the above parameters.

L'interaction entre des champs électromagnétiques et des systèmes à plusieurs niveaux peut donner lieu à différents processus cohérents. La transparence électromagnétiquement induite (EIT) ou les oscillations cohérentes de populations (CPO) sont des phénomènes résonnants ayant pour conséquence d'annuler l'absorption du système pour un champ sonde. L'EIT se produit dans les systèmes à trois niveaux et met en jeu une cohérence entre niveaux non couplés optiquement, alors que les systèmes à deux niveaux suffisent pour faire apparaître le CPO qui ne met pas en jeu la dynamique des cohérences.Il est possible dans un milieu constitué d'atomes d'hélium métastable à température ambiante d'extraire un système à trois niveaux en Λ qui, lorsqu'il est excité par des polarisations circulaires, fait apparaître des fenêtres EIT de l'ordre de quelques dizaines de kHz de large. Lorsque ce système est excité par des polarisations linéaires et soumis à un champ magnétique de faible amplitude, c'est l'association des deux phénomènes de CPO et d'EIT qui détermine la réponse du système. Une fenêtre de transparence CPO de quelques dizaines de kHz de large a en particulier été observée.Ce type de phénomènes résonnants est associé à de grandes variations de l'indice de réfraction avec la fréquence du champ sonde, ayant pour conséquence de profondes modifications de la vitesse de groupe d'une impulsion se propageant dans le milieu. Notre système expérimental permet ainsi d'observer de la lumière lente, de la lumière rapide ainsi que des vitesses de groupe négatives. L'insertion de tels milieux dispersifs en cavité optique a été suggérée pour augmenter la précision de senseurs comme les gyroscopes lasers, mais leur bruit fondamental dépend de la durée de vie des photons dans la cavité. C’est pourquoi l’influence des milieux hautement dispersifs sur la durée de vie des photons dans une cavité est étudiée expérimentalement et théoriquement. / Many coherent processes can be observed when electromagnetic fields are applied to multi-level systems. Electromagnetically induced transparency (EIT) or coherent population oscillations (CPO) are resonant phenomena resulting in the cancellation of the absorption of the system for a probe field. EIT occurs in three-level systems and involves the coherence between two optically uncoupled levels, whereas two-level systems are sufficient to produce CPO which does not involve the dynamics of the coherences.A three-level Λ system can be isolated in a gas of metastable helium atoms at room temperature. When excited with circular polarizations, this system exhibits EIT resonances of a few tens of kHz. If this system is excited by linear polarizations and submitted to a weak magnetic field, the response of the system is given by the combination of these two phenomena, namely EIT and CPO. A narrow CPO transmission window of a few tens of kHz has then been observed.This kind of resonant phenomena is associated with very strong variations of the refractive index at the probe frequency, leading to major changes of the group velocity of a pulse propagating in the medium. Our experimental set-up allows us to observe slow light, fast light, and even negative group velocities. Insertion of such dispersive media inside an optical cavity has been suggested to increase the sensitivity of sensors such as laser gyroscopes, but their fundamental noise depends on the lifetime of photons inside the cavity. This is why the influence of highly dispersive media on the photon lifetime inside a cavity is studied experimentally and theoretically.

We develop a robust and realistic mechanism for the generation of indistinguishable single-photon (SP) pulses with identical frequency and polarization. They are produced on demand from a coupled double-Raman atom-cavity system driven by a sequence of laser pump pulses. This scheme features a high efficiency, the ability to produce a sequence of narrow-band SP pulses with a delay determined only by the pump repetition rate, and simplicity of the system free from complications such as repumping process and environmental dephasing. We propose and analyze a simple scheme of parametric frequency conversion for optical quantum information in cold atomic ensembles. Its remarkable properties are minimal losses and distortion of the pulse shape, and the persistence of quantum coherence and entanglement. Efficient conversion of frequency between different spectral regions is shown. A method for the generation of frequency-entangled single photon states is discussed. We suggest a robust and simple mechanism for the coherent excitation of molecules or atoms to a superposition of pre-selected states by a train of femtosecond laser pulses, combined with narrow-band coupling field. The theory of quantum beatings in the generation of ultra-violet radiation via a four wave mixing in pump-probe experiments is developed. The results are in good agreement with experimental data observed in Rb vapor when the laser phase fluctuations are significant.

Ce travail se place dans le contexte des recherches sur les mémoires quantiques pour la lumière. L’information quantique est stockée dans un état de superposition atomique, dont la durée de vie détermine le temps maximum de stockage.On s’intéresse particulièrement aux matériaux capables de capturer la lumière par excitation résonnante d’une raie d’absorption, puis de conserver l’information quantique dans un état de superposition du fondamental électronique.Dans Tm3+:YAG, l’information est enregistrée dans un état de spin nucléaire. Cependant le champ magnétique qui lève la dégénérescence nucléaire entraîne les différents spins à des vitesses de précession différentes, ce qui tend à détruire l’aimantation initiale, porteuse de l’information.Une étude quantique du cristal est réalisée lors du premier chapitre de ce manuscrit. Les trois chapitres suivants traitent des différents mécanismes conduisant au déphasage des spins nucléaires. On y trouvera différente analyses théoriques qui seront confirmées par un ensemble de résultats expérimentaux, ainsi qu’une description détaillée du dispositif expérimental. Enfin le dernier chapitre, prospectif, exploite les outils développés au cours de la thèse pour préserver les cohérences optiques. Il présente quelques résultats expérimentaux prometteurs sur l’allongement du temps de vie de ces cohérences optiques. / This work takes place in the context of research about quantum memories for light. The quantum information is stored in an atomic superposition state whose lifetime sets the maximum storage time. We are particularly interested in materials which are able to hold the light by resonant excitation of an absorption line, preserving the quantum information in a superposition state of the electronicfundamental.n Tm3+:YAG the information is stored in a nuclear spin state. However, the magnetic field which lifts the nuclear degeneracy generates different precession speeds of the spins. This destroys theinitial magnetization carrier of the information.In the first chapter of this thesis, a quantum analysis of the crystal is done. The following three chapters are devoted to different mechanisms to control the nuclear spins dephasing. There it ispossible to find different theoretical analysis which will be confirmed by a series of experimental measurements, including an extended description of the set-up. Finally, the last chapter presentsthe different techniques used to preserve the optical coherence. Promising experimental measurements are presented to extend the life time of the optical coherences.

Nous développons un mécanisme robuste et réaliste pour la génération de photons uniques indiscernables avec des impulsions de fréquence et de polarisation identiques. Ils sont produits à la demande à partir d'un système couplé atome- cavité double-Raman en interaction avec une séquence d'impulsions laser de pompe. Ce processus combine un rendement élevé, la capacité de produire une séquence d'impulsions de photons uniques à bande étroite avec un retard déterminé seulement par le taux de répétition de la pompe, avec la simplicité du système libre de complications comme le repompage et le déphasage de l'environnement.Nous proposons et analysons un schéma simple de conversion paramétrique de fréquence pour l'information quantique optique dans des ensembles atomiques froids. Ses propriétés remarquables sont des pertes réduites, une distorsion de la forme des impulsions minimale, ainsi que la persistance de la cohérence quantique et de l’intrication. Une conversion efficace de fréquence entre les différentes régions spectrales est montrée. Une méthode de génération d’états caractérisant des photons uniques intriqués en fréquence est discutée.Nous proposons un mécanisme robuste et simple d'excitation cohérente de molécules et d’atomes en une superposition d'états pré-sélectionnés par un train d'impulsions laser femtoseconde, combinée avec un champ de couplage à largeur de bande étroite.La théorie des battements quantiques pour la génération du rayonnement ultra-violet par mélange à quatre ondes dans des expériences pompe-sonde est développée. Les résultats sont en bon accord avec les données expérimentales observées dans la vapeur de Rb lorsque les fluctuations de phase laser sont importantes. / We develop a robust and realistic mechanism for the generation of indistinguishable single-photon (SP) pulses with identical frequency and polarization. They are produced on demand from a coupled double-Raman atom-cavity system driven by a sequence of laser pump pulses. This scheme features a high efficiency, the ability to produce a sequence of narrow-band SP pulses with a delay determined only by the pump repetition rate, and simplicity of the system free from complications such as repumping process and environmental dephasing. We propose and analyze a simple scheme of parametric frequency conversion for optical quantum information in cold atomic ensembles. Its remarkable properties are minimal losses and distortion of the pulse shape, and the persistence of quantum coherence and entanglement. Efficient conversion of frequency between different spectral regions is shown. A method for the generation of frequency-entangled single photon states is discussed. We suggest a robust and simple mechanism for the coherent excitation of molecules or atoms to a superposition of pre-selected states by a train of femtosecond laser pulses, combined with narrow-band coupling field. The theory of quantum beatings in the generation of ultra-violet radiation via a four wave mixing in pump-probe experiments is developed. The results are in good agreement with experimental data observed in Rb vapor when the laser phase fluctuations are significant.