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High-Magnetic-Field Rydberg Atom Interactions.

Éric Paradis

Published Jan 1, 2013• 0 citations
OpenAlex

Abstract

In this thesis I examine high-magnetic-field Rydberg spectra, with the goal of discerning the viability of using such states as an interaction mechanism for quantum computation processes. In the presence of a strong magnetic field of B0 = 2.6 T, high-lying states of E ~ -55 cm(-1) are non-degenerate and show a wide range of magnetic dipole moments. Near-degenerate pairs of states of the same mJ manifold but of different axial parity Pi_z will strongly interact in the presence of an applied weak parallel electric field F, generating a pair of highly polar states. I show experimental results that, in agreement with theoretical work presented here, illustrate the presence of strong permanent dipole moments p ~ 1500 ea0 under these conditions. Such Rydberg states can also be tuned into resonance through careful control of the magnetic field, and are suitable for developing quantum phase gates and performing Rydberg-Rydberg interaction experiments. In order to perform such high-field Rydberg excitation experiments, a stable, dense and well-controlled cold atom cloud is needed within a high-magnetic-field environment. Toward this end, I present a full characterization of the utilized high-field trapping apparatus, including measurements of the trapped atom sample size, shape, and temperature. Analysis of the loss rate from the high-magnetic-field atom trap, both as a function of the applied magnetic field B0 and transverse confinement are presented. Experimental results for the high-field confinement of 85Rb are presented, as well as the first demonstration of a high-field 87Rb trap. Current trap limitations and possible improvements to the trapping apparatus are discussed. Other applications of the high-magnetic-field trap, such as the investigation of ultracold two-component plasmas and three-body recombination experiments, are also presented.

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Cold Rydberg atoms exposed to strong magnetic fields possess unique properties which open the pathway for an intriguing many-body dynamics taking place in Rydberg gases, consisting of either matter or anti-matter systems. We review both the foundations and recent developments of the field in the cold and ultracold regime where trapping and cooling of Rydberg atoms have become possible. Exotic states of moving Rydberg atoms, such as giant dipole states, are discussed in detail, including their formation mechanisms in a strongly magnetized cold plasma. Inhomogeneous field configurations influence the electronic structure of Rydberg atoms, and we describe the utility of corresponding effects for achieving tightly trapped ultracold Rydberg atoms. We review recent work on large, extended cold Rydberg gases in magnetic fields and their formation in strongly magnetized ultracold plasmas through collisional recombination. Implications of these results for current antihydrogen production experiments are pointed out, and techniques for the trapping and cooling of such atoms are investigated.

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Dipole-dipole interactions, the strongest, longest-range interactions possible between two neutral atoms, cannot be better manifested anywhere else than in a Rydberg atomic system. Rydberg atoms, having high principal quantum numbers n>>1 and dipole moments that scale as n^2, provide a powerful tool to examine dipole-dipole interactions. Therefore, we have studied the production and production rates of strontium Rydberg atoms created using two-photon excitation and have explored their properties in two distinct experiments. In the first experiment, very-high-n (n~300) Rydberg atoms are produced in a tightly collimated atomic beam allowing spectroscopic studies of their energy levels and their Stark effects. Simulations using a two-active-electron model, developed by our theoretical collaborators, allow detailed analysis of the results and are in remarkable agreement with the experimental results. The high density of Rydberg atoms achieved ~ 5*10^5 cm^(-3), in this experiment will allow studies of strongly interacting Rydberg-Rydberg systems. The second experiment, in which a cold strontium Rydberg gas is excited in a magneto-optic trap, features an imaging technique offering both spatial and temporal resolution. We use this technique to observe and study the evolution of an ultra-cold strontium Rydberg gas which reveals the importance of Rydberg-Rydberg interactions in the early stages of this evolution. Strongly interacting Rydberg gas provides an opportunity to realize a very strongly-correlated ultra-cold plasma.

•May 28, 2013•0 citations
In this thesis I examine high-magnetic-field Rydberg spectra, with the goal of discerning the viability of using such states as an interaction mechanism for quantum computation processes. In the presence of a strong magnetic field of B0 = 2.6 T, high-lying states of E ~ -55 cm(-1) are non-degenerate and show a wide range of magnetic dipole moments. Near-degenerate pairs of states of the same mJ manifold but of different axial parity Pi_z will strongly interact in the presence of an applied weak parallel electric field F, generating a pair of highly polar states. I show experimental results that, in agreement with theoretical work presented here, illustrate the presence of strong permanent dipole moments p ~ 1500 ea0 under these conditions. Such Rydberg states can also be tuned into resonance through careful control of the magnetic field, and are suitable for developing quantum phase gates and performing Rydberg-Rydberg interaction experiments. In order to perform such high-field Rydberg excitation experiments, a stable, dense and well-controlled cold atom cloud is needed within a high-magnetic-field environment. Toward this end, I present a full characterization of the utilized high-field trapping apparatus, including measurements of the trapped atom sample size, shape, and temperature. Analysis of the loss rate from the high-magnetic-field atom trap, both as a function of the applied magnetic field B0 and transverse confinement are presented. Experimental results for the high-field confinement of 85Rb are presented, as well as the first demonstration of a high-field 87Rb trap. Current trap limitations and possible improvements to the trapping apparatus are discussed. Other applications of the high-magnetic-field trap, such as the investigation of ultracold two-component plasmas and three-body recombination experiments, are also presented.
Cold Rydberg atoms exposed to strong magnetic fields possess unique properties which open the pathway for an intriguing many-body dynamics taking place in Rydberg gases, consisting of either matter or anti-matter systems. We review both the foundations and recent developments of the field in the cold and ultracold regime where trapping and cooling of Rydberg atoms have become possible. Exotic states of moving Rydberg atoms, such as giant dipole states, are discussed in detail, including their formation mechanisms in a strongly magnetized cold plasma. Inhomogeneous field configurations influence the electronic structure of Rydberg atoms, and we describe the utility of corresponding effects for achieving tightly trapped ultracold Rydberg atoms. We review recent work on large, extended cold Rydberg gases in magnetic fields and their formation in strongly magnetized ultracold plasmas through collisional recombination. Implications of these results for current antihydrogen production experiments are pointed out, and techniques for the trapping and cooling of such atoms are investigated.
Interactions between single atoms are fundamental to physics and to control them is an ultimate goal. The exaggerated properties of Rydberg atoms offer to met the technical challenges to isolate and control single interaction channels in ultracold gases. Here, I present experiments on two subjects related to interactions of Rydberg atoms in dense ultracold clouds. One subject concerns coherence in strongly interacting ensembles of atoms, where the interaction between Rydberg atoms is induced via Stark-tuned Förster resonances. Pulsed experiments, following the idea of Ramsey experiments, are used for high resolution spectroscopy of the Förster defect and phase sensitive detection. Coherent oscillations between pair states and an interaction-induced phase shift of Rydberg atoms are measured. These experiments are accompanied by calculations of the interaction strength and by simulations using the concept of a pair state interferometer. The simulations nicely reproduce the experimental findings and support the observation that the ensemble of atoms in the presence of interactions can be described and controlled coherently. The second subject of this thesis is the measurement of a permanent dipole moment in a homonuclear diatomic molecule that arises by the interaction between a Rydberg atom and a ground state atom. Usually parity symmetry prohibits a permanent dipole moment in diatomic molecules, but here the strong asymmetry between the constituents of the ultralong-range Rydberg molecule allows breaking parity symmetry. These molecules consist of one ground state atom bound inside the Rydberg electron wavefunction of a highly excited atom. Calculations predict dipole moments on the order of 1 Debye. Experimental proof is reported on the measurement of a linear Stark effect of these molecules, in excellent agreement with the calculations.
Interaction-induced stabilization of ultracold Rydberg atoms in a Ioffe-Pritchard trap — Subject of this thesis is a theoretical investigation of the quantum properties of highly excited atoms in an inhomogeneous magnetic field configuration. It is demonstrated how the large size of the Rydberg atoms alters the coupling to strong magnetic gradients. We find that Rydberg atoms can be tightly trapped in long-lived electronic states. This confinement permits the creation of a one-dimensional Rydberg gas stabilized against auto-ionization by a dipolar repulsion between the atoms, which is generated imposing a homogeneous electric field. This strongly anisotropic interaction is also responsible for a well controllable equilibrium configuration of two Rydberg atoms in the trap whose distance can be changed by tuning the electric field.
On one side, atom-chip experiments have demonstrated to be a versatile tool to study quantum physics in cold atoms systems. On the other side, Rydberg atoms have exaggerated properties that makes them good candidates to study quantum information and quantum simulations protocols. In this thesis both system are put together with the aim to study Rydberg physics (in particular atom-atom interaction) in an atom-chip experiment. Due to the exaggerated properties of Rydberg atoms they are very sensitive to electric fields. Thus, the combination of Rydberg atoms with surfaces can be challenging. Therefore a detailed characterization of the mentioned stray electric fields via the analysis of Stark-Zeeman maps is crucial. We employ two-photon Rydberg Stark spectroscopy of a micron-sized cloud of magnetically trapped ultracold rubidium atoms to characterize the electric fields near our atom-chip. Once we characterized the local electric fields we were able to study in more detail the effect of Rydberg excitation in the ultracold cloud of atoms. For instance we observed the formation of ultralong-range Rydberg molecules in our system. In these molecules a ground state atom is weakly bound to the Rydberg electron. We also attempted to observe the effect of Rydberg blockade in the system by looking at the characteristics of Rydberg spectra. Finally we studied the possibility of inducing rf transitions within a Rydberg state manifold in order to enhance our detection method and populate states that are not accessible directly with a two-photon laser transition.
We study the spectra of diamagnetic rubidium Rydberg atoms in strong magnetic and weak electric parallel fields, in the nnn-mixing regime. Our emphasis is on isolated pairs of near-degenerate, opposite-parity, diamagnetic states that become mixed by the weak electric field. Such level pairs allow for the generation of nondegenerate states with large, tunable permanent electric dipole moments and large optical excitation cross sections from the atomic ground state. We investigate how the dipole moments and the zero-electric-field energy defects of these level pairs can be tuned using small variations of the electric and magnetic fields. Using calculations, we explore the abundance of such level pairs over wide spectral regions for several magnetic quantum numbers. Applications of polar, diamagnetic Rydberg states in Rydberg-atom interaction experiments are briefly discussed.
Dipole-dipole interactions, the strongest, longest-range interactions possible between two neutral atoms, cannot be better manifested anywhere else than in a Rydberg atomic system. Rydberg atoms, having high principal quantum numbers n>>1 and dipole moments that scale as n^2, provide a powerful tool to examine dipole-dipole interactions. Therefore, we have studied the production and production rates of strontium Rydberg atoms created using two-photon excitation and have explored their properties in two distinct experiments. In the first experiment, very-high-n (n~300) Rydberg atoms are produced in a tightly collimated atomic beam allowing spectroscopic studies of their energy levels and their Stark effects. Simulations using a two-active-electron model, developed by our theoretical collaborators, allow detailed analysis of the results and are in remarkable agreement with the experimental results. The high density of Rydberg atoms achieved ~ 5*10^5 cm^(-3), in this experiment will allow studies of strongly interacting Rydberg-Rydberg systems. The second experiment, in which a cold strontium Rydberg gas is excited in a magneto-optic trap, features an imaging technique offering both spatial and temporal resolution. We use this technique to observe and study the evolution of an ultra-cold strontium Rydberg gas which reveals the importance of Rydberg-Rydberg interactions in the early stages of this evolution. Strongly interacting Rydberg gas provides an opportunity to realize a very strongly-correlated ultra-cold plasma.