Session G1: Thesis Prize Session

Cavity QED with Trapped Atoms for Quantum Optics and Quantum Information

One requirement for the implementation of protocols in quantum information science is the ability to convert quantum information from stationary to travelling form. The strong coupling domain of optical cavity quantum electrodynamics (QED) provides a near-ideal setting for the pursuit of these goals. In our experiments, Cs atoms were trapped inside a cavity in the strong coupling regime. The atoms were confined using an intracavity dipole trap with lifetime of 3 s, in which the trapping field only weakly perturbs the cavity QED interactions. We have also experimentally realized the one-atom laser, whose properties included strongly nonclassical output (photon antibunching and sub-Poissonian photon statistics). Finally, we have achieved single-photon generation in a setting suitable for quantum networks. A strongly coupled trapped atom has been used to generate a stream of single photon pulses ``on demand,'' with intrinsic efficiency near unity. This system should enable the creation of atom-field entanglement and the distribution of quantum states among atoms in distantly separated cavities.   

Ultracold Bosonic Atoms in Optical Lattices

This thesis covers most of my work in the field of ultracold atoms loaded in optical lattices. It makes a route through the physics of cold atoms in periodic potentials starting from the simple noninteracting system and going into the many-body physics that describes the strongly correlated Mott insulator regime. Even though this thesis is a theoretical work all the chapters are linked either with experiments already done or with ongoing experimental efforts. In the first part I investigate the validity of mean field approximations based on the Discrete Nonlinear Schr\"{o}dinger equation and quadratic approximations of the Hamiltonian to describe the approach of the system from the superfluid to the Mott insulator regime. In the second part I adopt the closed time path (CTP) and two particle irreducible (2PI) effective action formalism to study the non-equilibrium dynamics of a condensate loaded every third lattice site of an optical lattice. I show this formalism to be a powerful tool to describe far-from-equilibrium situations, particularly through its ability to incorporate the non-local and non-Markovian aspects characteristic of the quantum dynamics. In the last part I investigate the properties of the system deep in the Mott insulator regime. By using perturbation theory I study the Mott insulator ground state and its excitation spectrum, the response of the system to Bragg spectroscopy, and propose a mechanism to correct for the residual number fluctuations inherent to the Mott insulator ground state.   

Quasi-Phase Matching of Soft X-Ray Light from High-Order Harmonic Generation Using Waveguide Structures

In this work, we experimentally demonstrate enhanced conversion efficiency for high harmonic generation in neon gas at the carbon edge (284 eV), and report the first observation of high harmonic generation from argon up to $\sim$ 250 eV. High-order harmonic generation (HHG) in gases is a useful source of coherent light in the extreme ultraviolet to soft x-ray regions of the spectrum. Phase matching of the HHG conversion process can be obtained in a gas filled hollow-core waveguide by adjusting the gas pressure to balance the effect on the phase velocity of the light due to the dispersion of the plasma, waveguide and neutral gas.\footnote{A. Rundquist, Science 280, 1412 (1998).} Unfortunately, at still relatively low ionization levels ($\sim 5 \%$), the plasma contribution to the phase velocity becomes much greater than the neutral gas contribution, making simple phase matching impossible for higher laser intensities, and therefore higher harmonic energies. In previous work,\footnote{I. P. Christov, Optics Express 7, 362 (2000).} \footnote{A. Paul, Nature 421, 51 (2003).} we demonstrated that by modulating the diameter of the hollow waveguide, we could quasi-phase match (QPM) the HHG conversion process. The effect of the modulations is to periodically modulate the driving laser intensity. Because the phase of the harmonic emission depends on the driving laser intensity, the modulations can both phase modulate the harmonic light, and suppress the harmonic generation in certain regions of the waveguide. Here,\footnote{E. A. Gibson, Science 302, 95 (2003).} \footnote{E. A. Gibson, Physical Review Letters 92, 033001 (2004).} we dramatically demonstrate the effect of quasi-phase matching of HHG in nearly fully-ionized gases. As a result of QPM, we observe harmonic emission at the carbon absorption edge (284 eV) in neon. Using argon gas, we observe HHG up to 250 eV - the highest harmonic energy previously observed in argon was 100 eV using 794 nm fundamental light. The use of the waveguide geometry makes it possible for us to observe such high harmonics because it counteracts the effect of plasma- induced defocusing.   

Tunable Interactions in Quantum Degenerate Lithium

Quantum degenerate gases provide an ideal environment for studying fundamental physics. In these systems, a Feshbach resonance can be utilized to tune the interactions between certain colliding pairs of atoms, yielding control over both the magnitude and sign of the interactions. This has opened the doorway to a new area in which the underlying physics of non-linear optical phenomena and many solid-state effects can be explored in the ideal environment of a quantum degenerate gas. We will first discuss the experimental realization of a quantum degenerate Bose-Fermi mixture via sympathetic cooling [truscott01]. By confining this quantum degenerate gas in an all optical potential, the atom-atom interactions of the bosons can be manipulated to produce bright matter-wave solitons [strecker02] which are individual Bose-Einstein condensates (BEC) that we have observed to propagate for over 3 seconds without dispersion. Further, a highly interacting Fermi gas can be produced near a Feshbach resonance, and through manipulation of the external magnetic field, long lived ultra-cold bosonic molecules can be formed from the Fermi gas [strecker03]. The unexpected long lifetime of these vibrationally excited (v$'$ = 38) molecules enables them to be evaporatively cooled to a molecular BEC. We use a pure molecular condensate as a probe of the BEC/BCS crossover region within the broad Feshbach resonance. Using an interrogation laser tuned to a bound-bound molecular resonance, the deeply bound molecular component of the gas is measured as a function of magnetic field, probing the fundamental many-body physics of a strongly interacting Fermi gas. \newline \newline [truscott01] A. G. Truscott, K. E. Strecker, W. I. McAlexander, G. B. Patridge, and R. G. Hulet, Science \textbf {291}, 2570 (2001). \newline [strecker02] K. E. Strecker, G. B. Partridge, A. G. Truscott, and R.G Hulet, Nature \textbf{417}, 150 (2002). \newline [strecker03] K. E. Strecker, G. B. Partridge and R. G. Hulet, Phys Rev. Lett. \textbf{91}, 080406 (2003).