Le contrôle des états quantiques de la lumière est une étape nécessaire pour la transmission et le traitement quantiques des informations. Un nuage d'atomes froids constitue un milieu optiquement non-linéaire très intéressant pour créer et manipuler des états photoniques. Le sujet de cette thèse est l'étude expérimentale de telles non-linéarités, induites entre des photons optiques par leur couplage avec des atomes de Rydberg. Les états de Rydberg sont des états atomiques très excités (n>30), qui permettent de créer des interactions photon-photon par l'intermédiaire de leurs interactions dipôle-dipôle à longue distance (>10µm). Nous utilisons une cavité de faible finesse pour transformer ces interactions en effets observables sur un faisceau de très faible intensité, ce qui peut permettre de produire des états non-classiques de lumière. / The control of quantum states of light is a necessary step for quantum information transportation and processing. Cold atomic memories are one of the prime candidates for storing and manipulating photonic states. This thesis is a study of optical non-linear effects created using Rydberg states. Rydberg states are highly excited states (n>30) of atoms, which are useful in realizing photon-photon interactions because of their long distance (>10µm) dipole-dipole interactions. We utilize a low finesse cavity to transform phase shifts into intensity correlations which would allow one to generate arbitrary non-classical states of light

<p>Cuprous oxide (Cu₂O) has recently been proposed as a promising solid-state host for ex-<br> citonic Rydberg states with large principal quantum numbers (n) whose exaggerated wave-<br> function sizes (∝ n²) facilitate gigantic, resonant dipole-dipole (∝ n⁴) and van der Waals<br> (∝ n¹¹) interactions, making them an ideal basis for solid-state Rydberg physics and quan-<br> tum technology. Synthetic, thin-film Cu₂O samples are of particular interest because they<br> can be made defect-free via carefully controlled fabrication and are, in principle, suitable<br> for the observation of extreme single-photon nonlinearities caused by Rydberg blockade. In<br> this work, we present the development of a spectroscopy experiment for characterizing the<br> behavior of Rydberg excitons and use it to study a synthetic thin film of Cu₂O grown on a<br> transparent substrate. We present evidence for the presence of states up to n = 8 and conduct<br> the first temperature-dependent study of Rydberg excitons in a thin film. We also propose<br> a technique for studying Rydberg-Rydberg interactions via the creation of high exciton den-<br> sities and establish a set of rate equations for modeling the processes by which excitons are<br> created, interact with each other, and decay. Finally, we conclude with a discussion of the<br> project’s outlook, as well as what future work will be undertaken to study the interactions<br> between Rydberg excitons and utilize them in scalable, integrable, Rydberg-based quantum<br> devices.<br> </p>

Quantum mechanical motion of electrons in atoms and molecules is at the heart of many photophysical and photochemical processes. As the natural timescale of electron dynamics is in the range of femtoseconds or shorter, ultrashort pulses are required to study such phenomena. The ultrashort pulse light-matter interaction at high intensity regime can however dramatically alter the atomic and molecular structures. Our current understanding of such transient electronic modification is far from complete, especially when complicated light-induced couplings are involved. In this dissertation, we investigated how a femtosecond strong-field pulse can control or modify the evolution of atomic or molecular polarization, representing electric dipole excitation in various systems. Extreme ultraviolet (XUV) attosecond pulse trains are used to coherently prepare superposition of excited states in various atomic and molecular systems. A subsequent phase-locked infrared (IR) femtosecond pulse is applied to perturb the dipoles, and transient changes in the transmitted XUV spectra are measured. This scheme is termed as XUV attosecond transient absorption spectroscopy. In the first study, we applied this technique to study the modification of Rydberg states in dilute helium gas. We observed several transient changes to the atomic structure, including the ac Stark shift, laser-induced quantum phase, laser-induced continuum structure, and quantum path interference. When the experiments were extended to the study of a dense helium gas sample, new spectral features in the absorption spectra emerged which cannot be explained by linear optical response models. We found that these absorption features arise from the interplay between the XUV resonant pulse propagation and the IR-imposed phase shift. A unified physical model was also developed to account for various scenarios. Extending our work to argon atoms, we studied how an external infrared field can be used to impulsively control different photo-excitation pathways and the transient absorption lineshape of an otherwise isolated autoionizing state. It is found that by controlling the field polarization of the IR pulse, we can modify the transient absorption line shape from Fano-like to Lorentzian-like profiles. Unlike atoms, in our study of autoionizing states of the oxygen molecule, we observed both positive and negative optical density changes for states with different electronic symmetries. The predictions of two distinct and simplified dipole perturbation models were compared against both the experimental results and a full theoretical calculation in order to understand the origin of the sign of absorption change. We relate this symmetry-dependent sign change to the Fano parameters of static photoabsorption. The same approach was applied to study molecular nitrogen, in which we observed the decay dynamics of IR perturbed doubly-excited Rydberg states with many vibrational progressions. In addition, we also conducted experiments to investigate Rydberg state dynamics of other molecular systems such as carbon dioxide. In summary, we experimentally explored the ephemeral light-induced phenomena associated with excited states of atoms and molecules. These studies provide real-time information on ultrafast electronic processes and provide strategies for direct time-domain control of the light-matter interaction.

Les atomes neutres sont des candidats prometteurs pour la réalisation et l’étude d’états intriqués à quelques dizaines de particules. Pour générer de tels états, une approche consiste à utiliser le mécanisme de blocage dipolaire résultant des fortes interactions dipôle-dipôle entre atomes de Rydberg.Suivant cette approche, cette thèse présente la conception et la caractérisation d’un dispositif expérimental permettant de manipuler des atomes de 87Rb individuels piégés dans des pinces op- tiques microscopiques, et à les exciter vers des états de Rydberg. Un environnement électrostatique stable et des électrodes de contrôle permettent une manipulation fine de ces états. Avec deux pinces optiques séparées de quelques microns, nous démontrons le blocage de Rydberg entre deux atomes, et nous observons leur excitation collective.Enfin, en opérant en régime de blocage partiel, nous développons une méthode permettant de mesurer l’interaction de van der Waals ∆E = C6 /R6 entre deux atomes séparés par une distance R contrôlée. Les coefficients C6 obtenus pour différents états de Rydberg sont en bon accord avec des calculs théoriques ab initio, et nous observons l’augmentation spectaculaire de l’interaction en fonction du nombre quantique principal n de l’état de Rydberg. / Neutral atoms are promising candidates for the realization of entangled states involving up to a few tens of particles. To generate such states, one approach consists in using the dipole blockade mechanism, which results from the strong dipole-dipole interactions between Rydberg atoms.Following this approach, this thesis describes the design and the characterization of an experimental apparatus allowing to manipulate single 87Rb atoms trapped in microscopic optical tweezers, and to excite them towards Rydberg states. A stable electrostatic environment and controlled electrodes enable the fine manipulation of these states. Using two optical tweezers separated by a few microns, we demonstrate the Rydberg blockade between two single atoms, and we observe their collective excitation.Finally, by operating in the partial blockade regime, we develop a method allowing to measure the van der Waals interaction ∆E = C6 /R6 between two atoms separated by a controlled distance R. The C6 coefficients obtained for various Rydberg states agree well with ab initio theoretical calculations, and we observe the dramatic increase of the interaction with the principal quantum number n of the Rydberg state.

Fullerenes have highly excited electronic states with interesting properties for possible wide ranging applications including in electronics. These highly excited, Rydberg-like states, so-called superatom molecular orbitals (SAMOs), are diffuse low-angular momenta states with molecular orbitals centred on the hollow fullerene core. The SAMOs can be detected by femtosecond photoelectron spectroscopy (PES) and characterised by photoelectron angular distributions (PADs) combined with time-dependent density functional theory (TD-DFT) calculations. The photoelectron spectra of C60 and C70 show a peak structure below kinetic energies corresponding to the photon energy, superimposed on a thermal electron background. This peak structure was assigned to one-photon ionisation of the SAMO states based on PAD and TD-DFT. In this thesis, studies of the fullerene species C82 and Sc3N@C80 revealed PES and PAD with similar features to C60 and C70. The SAMO peaks became less prominent compared to the thermal electron background for increasing molecular size and decreasing symmetry, and were almost absent for the endohedral species. To provide more information about the influence of encapsulated atoms in the fullerene cage on the SAMO states, experiments on Li@C60 have been carried out. A lower thermal electron emission temperature and a splitting of the SAMO peaks has been observed for Li@C60 compared to C60. Nevertheless the binding energies are remarkably similar in all investigated fullerenes, which is important for any applications. Since the binding energies are about the same, but the ionisation potentials of the fullerenes are different, the excitation energy to the SAMOs scales with the ionisation energy. The reasons for the well-pronounced peak structure of the SAMO states in the PES of C60 could be explained by the similarity of the SAMOs to Rydberg states along with the higher photoionisation probabilities compared to valence states which were modelled by Benoît Mignolet and Françoise Remacle. As the SAMOs are highly excited electronic states, like Rydberg states, the potential energy surface of the neutral molecule and the ionised molecule are similar. Therefore the vibrational energy is conserved in the molecule during the photoionisation on the femtosecond time scale. The TD-DFT calculations on C60, carried out by Benoît Mignolet and Françoise Remacle, revealed the photoionisation probabilities of the SAMOs to be at least three orders of magnitude higher than for non-SAMOs for the applied experimental conditions. To test the prediction of the model, the relative photoionisation probabilities of the s-SAMO to p-SAMO and the s-SAMO to d-SAMO were obtained experimentally from the PES at various photon energies (2-3.5 eV) within this work. The analysis indicates remarkable agreement between the experiment and the theoretical values. Further quantum chemical calculations on a series of polycyclic aromatic hydrocarbons (PAHs) were carried out within this thesis, which revealed similar Rydberg-like molecular orbitals in analogy to the SAMOs in fullerenes. The first series included benzene, naphthalene, anthracene, tetracene, pentacene and hexacene. The second series consisted of phenanthrene, pyrene and coronene. Finally, the third series covered cubane, adamantane and dodecahedral C20. All modelled molecules showed diffuse, excited electronic states similar to the SAMOs. Within each series the binding energies of these states decrease with increasing molecular size as well as the ionisation energies, except for the 3rd series. A comparison between all series shows that the binding energies of the states for the 3rd series (the 3-D series) are slightly higher than for the 1st and 2nd series in relation to similar molecular size. The results of the coronene calculations are compared to experimental photoelectron spectra and are shown to be in good agreement with the experiments.

This thesis reports experimental observations of electric fields using Rydberg atoms, including dc field measurements near the surface of an atom chip, and demonstration of measurement techniques for ac fields far from the surface. Associated theoretical results are also presented, including Monte Carlo simulations of the decoherence of Rydberg states in electric field noise as well as an analytical calculation of the statistics of dc electric field inhomogeneity near polycrystalline metal surfaces. DC electric fields were measured near the heterogeneous metal and dielectric surface of an atom chip using optical spectroscopy on cold atoms released from the trapping potential. The fields were attributed to charges accumulating in the dielectric gaps between the wires on the chip surface. The field magnitude and direction depend on the details of the dc biasing of the chip wires, suggesting that fields may be minimized with appropriate biasing. Techniques to measure ac electric fields were demonstrated far from the chip surface, using the decay of a coherent superposition of two Rydberg states of cold atoms. We have used the decay of coherent Rabi oscillations to place some bounds on the magnitude and frequency dependence of ac field noise. The rate of decoherence of a superposition of two Rydberg states was calculated with Monte Carlo simulations. The states were assumed to have quadratic Stark shifts and the power spectrum of the electric field noise was assumed to have a power-law dependence of the form 1/f^κ. The decay is exponential at long times for both free evolution of the superposition and and Hahn spin-echo sequences with a π refocusing pulse applied to eliminate the effects of low-frequency field noise. This decay time may be used to calculate the magnitude of the field noise if κ is known. The dc field inhomogeneity near polycrystalline metal surfaces due to patch potentials on the surface has been calculated, and the rms field scales with distance to the surface as 1/z^2. For typical evaporated metal surfaces the magnitude of the rms field is comparable to the image field of an elementary charge near the surface.

This thesis reports the first observation of the resonant electric dipole-dipole interaction between cold Rydberg atoms using microwave spectroscopy, the observation of the magnetic field suppression of resonant interactions, and the development of a unique technique for precise magnetic field measurements. <br /><br /> A Rydberg state 46<em>d</em>_5/2 of laser cooled ⁸⁵Rb atoms has been optically excited. A fraction of these atoms has been transferred to another Rydberg state 47<em>p</em>_3/2 or 45<em>f</em>_5/2,7/2 to introduce resonant electric dipole-dipole interactions. The line broadening of the two-photon 46<em>d</em>_5/2-47<em>d</em>_5/2 microwave transition due to the interaction of 46<em>d</em>_5/2 with 47<em>p</em>_3/2 or 45<em>f</em>_5/2,7/2 atoms has been used as a probe of the interatomic interactions. This experiment has been repeated with a DC magnetic field applied. The application of a weak magnetic field (&le;0. 6G) has reduced the line broadening due to the resonant electric dipole-dipole interaction, indicating that the interactions are suppressed by the field. Theoretical models have been developed that predict the energy shifts due to the resonant electric dipole-dipole interaction, and the suppression of interactions by magnetic fields. A novel technique for sensitive measurement of magnetic fields using the 34<em>s</em>_1/2-34<em>p</em>_1/2 one-photon microwave transition has also been presented. Using this technique, it has been possible to calibrate magnetic fields in the magneto-optical trap (MOT) apparatus to less than 10mG, and put an upper bound of 17mG on any remaining field inhomogeneity.

This thesis reports the first observation of the resonant electric dipole-dipole interaction between cold Rydberg atoms using microwave spectroscopy, the observation of the magnetic field suppression of resonant interactions, and the development of a unique technique for precise magnetic field measurements. <br /><br /> A Rydberg state 46<em>d</em>_5/2 of laser cooled ⁸⁵Rb atoms has been optically excited. A fraction of these atoms has been transferred to another Rydberg state 47<em>p</em>_3/2 or 45<em>f</em>_5/2,7/2 to introduce resonant electric dipole-dipole interactions. The line broadening of the two-photon 46<em>d</em>_5/2-47<em>d</em>_5/2 microwave transition due to the interaction of 46<em>d</em>_5/2 with 47<em>p</em>_3/2 or 45<em>f</em>_5/2,7/2 atoms has been used as a probe of the interatomic interactions. This experiment has been repeated with a DC magnetic field applied. The application of a weak magnetic field (&le;0. 6G) has reduced the line broadening due to the resonant electric dipole-dipole interaction, indicating that the interactions are suppressed by the field. Theoretical models have been developed that predict the energy shifts due to the resonant electric dipole-dipole interaction, and the suppression of interactions by magnetic fields. A novel technique for sensitive measurement of magnetic fields using the 34<em>s</em>_1/2-34<em>p</em>_1/2 one-photon microwave transition has also been presented. Using this technique, it has been possible to calibrate magnetic fields in the magneto-optical trap (MOT) apparatus to less than 10mG, and put an upper bound of 17mG on any remaining field inhomogeneity.

This thesis reports the use of both dc and ac electric fi eld induced resonant energy transfer, RET, between cold Rydberg atoms as a useful tool for enhancement of interatomic interactions. A general technique for laser frequency stabilization and its suitability for Rydberg atom excitation is also demonstrated. RET between cold Rydberg atoms was used to determine Rydberg atom energy levels. The ⁸⁵Rb atoms are laser cooled and trapped in a magneto-optical trap. For energy level determination experiment, atoms were optically excited to 32d₅/₂ Rydberg states. The two-atom process 32d₅/₂ + 32d₅/₂ → 34p₃/₂+30g is resonant at an electric fi eld of approximately 0.3 V/cm through dipole dipole interaction. The experimentally observed resonant fi eld, together with the Stark map calculation is used to make a determination of the ⁸⁵Rb ng-series quantum defect to be ⵒg(n = 30) = 0.00405(6). The ac Stark eff ect was also used to induce RET between cold Rydberg atoms. When a 28.5 GHz dressing field was set at speci fic fi eld strengths, the two-atom dipole-dipole process 43d₅/₂ + 43d₅/₂ → 45p₃/₂ + 41f was dramatically enhanced, due to induced degeneracy of the initial and final states. This method for enhancing interactions is complementary to dc electric- field-induced RET, but has more flexibility due to the possibility of varying the applied frequency. At a dressing field of 28.5 GHz all of the participating levels (43d₅/₂, 45p₃/₂ and 41f) show signi cant shifts and these give a complicated series of resonances. An oscillating electric fi eld at 1.356 GHz was also used to promote the above RET process where the atoms are initially excited to the 43d₅/₂ Rydberg states. The ac fi eld strength was scanned to collect RET spectra. Di fferent resonances were observed for diff erent magnetic sublevels involved in the process. Compared to the higher dressing field frequency of 28.5 GHz, the choice of dressing frequency of 1.356 GHz, which is slightly blue detuned from the 41f - 41g transition, and structure of the spectra may be understood, by analogy with the dc field case.

This thesis reports the use of both dc and ac electric fi eld induced resonant energy transfer, RET, between cold Rydberg atoms as a useful tool for enhancement of interatomic interactions. A general technique for laser frequency stabilization and its suitability for Rydberg atom excitation is also demonstrated. RET between cold Rydberg atoms was used to determine Rydberg atom energy levels. The ⁸⁵Rb atoms are laser cooled and trapped in a magneto-optical trap. For energy level determination experiment, atoms were optically excited to 32d₅/₂ Rydberg states. The two-atom process 32d₅/₂ + 32d₅/₂ → 34p₃/₂+30g is resonant at an electric fi eld of approximately 0.3 V/cm through dipole dipole interaction. The experimentally observed resonant fi eld, together with the Stark map calculation is used to make a determination of the ⁸⁵Rb ng-series quantum defect to be ⵒg(n = 30) = 0.00405(6). The ac Stark eff ect was also used to induce RET between cold Rydberg atoms. When a 28.5 GHz dressing field was set at speci fic fi eld strengths, the two-atom dipole-dipole process 43d₅/₂ + 43d₅/₂ → 45p₃/₂ + 41f was dramatically enhanced, due to induced degeneracy of the initial and final states. This method for enhancing interactions is complementary to dc electric- field-induced RET, but has more flexibility due to the possibility of varying the applied frequency. At a dressing field of 28.5 GHz all of the participating levels (43d₅/₂, 45p₃/₂ and 41f) show signi cant shifts and these give a complicated series of resonances. An oscillating electric fi eld at 1.356 GHz was also used to promote the above RET process where the atoms are initially excited to the 43d₅/₂ Rydberg states. The ac fi eld strength was scanned to collect RET spectra. Di fferent resonances were observed for diff erent magnetic sublevels involved in the process. Compared to the higher dressing field frequency of 28.5 GHz, the choice of dressing frequency of 1.356 GHz, which is slightly blue detuned from the 41f - 41g transition, and structure of the spectra may be understood, by analogy with the dc field case.