Two projects are studied in this thesis: (1) Atomic and Molecular Stabilization in Super-Intense High-Frequency Laser Fields. (2) Entanglement of Dipole Arrays in External Electric Fields. Chapter 1 presents a general introduction and the motivation for these two projects. Chapter 2 focuses on atomic stabilization of negative multiple charged onions in linearly polarized super-intense high-frequency laser field in 3-dimensional space using both frequency independent (Section 2.2) and dependent (Section 2.3) potentials. By theoretical calculation, it is predicted that He-, He2-, Li- and Li2- are stable in super-intense high-frequency laser field. Chapter 3 studies atomic stabilization in both linearly and circularly polarized super-intense high-frequency laser field in both 3 dimensional space and large-D limit. Chapter 4 studies stabilization of simple diatomic molecules in linearly polarized super-intense high-frequency laser field in both 3 dimensional space and large-D limit. It was proved, in chapter 3 and 4, that dimensional scaling, combined with the High-Frequency Floquet theory, provides a useful means to evaluate the stability of gas phase atomic anions in a superintense laser field. Chapter 5 focuses on He-He bonding in linearly polarized super-intense high-frequency laser field. The distribution of the vibronic transitions from electric ground state to [special characters omitted] excited state for He2 in superintense high frequency laser fields is obtained by numerical methods. Chapter 6 studies entanglement of multi-dimensional systems of dipoles in external electric fields. A new entanglement switch of qubits consisting of electric dipoles oriented along or against an external electric field and coupled by the electric dipole-dipole interaction is proposed in this chapter. The pairwise entanglement can be tuned and controlled by the ratio of the Rabi frequency and the dipole-dipole coupling strength. Tuning the entanglement can be achieved for one, two and three-dimensional arrangements of the qubits. The feasibility of building such an entanglement switch is also discussed in this chapter.
Two projects are studied in this thesis: (1) Atomic and Molecular Stabilization in Super-Intense High-Frequency Laser Fields. (2) Entanglement of Dipole Arrays in External Electric Fields. Chapter 1 presents a general introduction and the motivation for these two projects. Chapter 2 focuses on atomic stabilization of negative multiple charged onions in linearly polarized super-intense high-frequency laser field in 3-dimensional space using both frequency independent (Section 2.2) and dependent (Section 2.3) potentials. By theoretical calculation, it is predicted that He-, He2-, Li- and Li2- are stable in super-intense high-frequency laser field. Chapter 3 studies atomic stabilization in both linearly and circularly polarized super-intense high-frequency laser field in both 3 dimensional space and large-D limit. Chapter 4 studies stabilization of simple diatomic molecules in linearly polarized super-intense high-frequency laser field in both 3 dimensional space and large-D limit. It was proved, in chapter 3 and 4, that dimensional scaling, combined with the High-Frequency Floquet theory, provides a useful means to evaluate the stability of gas phase atomic anions in a superintense laser field. Chapter 5 focuses on He-He bonding in linearly polarized super-intense high-frequency laser field. The distribution of the vibronic transitions from electric ground state to [special characters omitted] excited state for He2 in superintense high frequency laser fields is obtained by numerical methods. Chapter 6 studies entanglement of multi-dimensional systems of dipoles in external electric fields. A new entanglement switch of qubits consisting of electric dipoles oriented along or against an external electric field and coupled by the electric dipole-dipole interaction is proposed in this chapter. The pairwise entanglement can be tuned and controlled by the ratio of the Rabi frequency and the dipole-dipole coupling strength. Tuning the entanglement can be achieved for one, two and three-dimensional arrangements of the qubits. The feasibility of building such an entanglement switch is also discussed in this chapter.
This thesis is a theoretical study of the behavior of electrons in atoms and molecules subjected to external electromagnetic fields. Three electronic systems have been central in this work, the atomic and quasimolecular Rydberg states and the carbon allotrope graphene. The exaggerated properties of the highly excited Rydberg atoms, such as long lifetime, large spatial extension and low binding energy make them excellent candidates for both theoretical and experimental considerations. The analytical solutions of the hydrogen atom in quantum mechanics provide a good description for most Rydberg atoms. This is exploited in the work presented in this thesis and the accompanying papers, where we show results of numerical computations on interacting Rydberg atoms, with and without the presence of radiating fields, as well as dynamic Rydberg wave packets influenced by a train of femtosecond laser pulses. In the final part of the thesis the optical process of high-order harmonic generation in an extended molecular structure, represented by graphene, is considered. Rydberg atoms have a large dipole moment due to the large average electronic radius in the atom, which leads to a very strong dipole-dipole coupling between interacting particles in a gas of cold Rydberg atoms. This interaction, which is dependent on the internuclear separation between the atoms, induces a shift in the energy levels, which again prohibits the excitation of more than one atom in the gas, an effect commonly known as the Rydberg blockade. A correlated behavior is observed for the intrashell transitions in mutually excited Rydberg atoms. The microwave field driven dynamics is significantly supressed for internuclear distances below a given conditional radius which scales linearly with the principal quantum number n. Such an entangled electronic behavior may open for the realization of quantum gates applied in quantum informatics. The long-range interaction of Rydberg atoms is dominated by the dipole-dipole terms. Nevertheless, the investigation of two interacting Rydberg atoms, without the influence of electromagnetic radiation, has shown that the lower order multipolar terms play a vital role in the energy shifts as well as for the electronic probability distributions. We found that the energy curves, when including all multipole orders, correspond to both repulsive and attractive states with a stronger repulsion than attraction. We also propose a laser assisted approach for stabilization of Rydberg gases by controlled transitions between such attractive and repulsive energy states. Single-electron ionization of a dynamic Rydberg wave packet confined within a single energy level has been investigated. The ionization probability of an intrashell Rydberg wave packet depends strongly on the degree of polarization. For a wave packet driven repeatedly between the circular (least polarized) state and the linear (most polarized) state by a rotating microwave field and subjected to a train of femtosecond laser pulses, the ionization probability will depend crucially on the laser repetition rate and number of pulses. The angular-resolved ionization probability and energy distribution in the continuum are also influenced by these parameters. The last part of the thesis is devoted to the study of high-order harmonic generation (HHG) in graphene. In the simple man’s model HHG can be described by an electron that is ionized and driven in an external field, after which it returns to the parent atom and recombines, emitting a high-energy photon with frequency that is a multiple of the incident laser frequency. We show that the harmonic spectra obtained from the interaction with linearly as well as circularly polarized femtosecond lasers yield harmonics up the classical limit for extended molecules. In contrast to diatomic molecules, the stable grid structure of graphene ensures that the harmonic generation cutoff remains constant with increasing signal power as the graphene diameter extends beyond the classical limit. Additionally, for circularly polarized lasers harmonics exceeding the classical predictions have been observed.
Ultracold molecules are expected to find applications in cold chemistry, quantum phases, precision measurements and quantum information. In this thesis three novel applications of cold molecules are studied. First the thesis presents a general method for coherent control of collisions between non-identical particles. It is shown that by preparing two alkali-metal atoms in a superposition of hyperfine states, the elastic-to-inelastic cross section ratio can be manipulated at ultracold temperatures by tuning laser parameters in the presence of a magnetic field. The static field is needed to induce quantum interference between scattering states. Extensions of this scheme for ultracold molecular reactive scattering are discussed. Second, the thesis describes rotational excitons and polarons in molecular ensembles trapped in optical lattices. Rotational excitons can be manipulated using static electric and magnetic fields. For a one-dimensional molecular array with substitutional impurities any localized exciton state can be delocalized by applying a suitable electric field. The electric field induces correlations between diagonal and off-diagonal disorder. It is also shown that the translational motion of polar molecules in an optical lattice can lead to phonons. The lattice dynamics and the phonon spectrum depend on the strength and orientation of a static electric field. An array of polar molecules in an optical lattice can be described by generalized polaron model with tunable parameters including diagonal and off-diagonal exciton-phonon interactions. It is shown that in a strong electric field the system is described by a generalized Holstein model, and at weak electric fields by the Su-Schrieffer-Heeger (SSH) model. The possibility of observing a sharp polaron transition in the SSH model using polar alkali-metal dimers is discussed. Finally, the thesis presents a method to generate entanglement of polar molecules using strong off-resonant laser pulses. Bipartite entanglement between alkali-metal dimers separated by hundreds of nanometers can be generated. Maximally entangled states can be prepared by tuning the pulse intensity and duration. A scheme is proposed to observe the violation of Bell’s inequality based on molecular orientation correlation measurements. It is shown that using a combination of microwave and off-resonant optical pulses, arbitrary tripartite and many-particle states can be prepared.
The goal of the present article is to review the major developments that have led to the current understanding of molecule–field interactions and experimental methods for manipulating molecules with electromagnetic fields. Molecule–field interactions are at the core of several, seemingly distinct areas of molecular physics. This is reflected in the organisation of this article, which includes sections on field control of molecular beams, external field traps for cold molecules, control of molecular orientation and molecular alignment, manipulation of molecules by non-conservative forces, ultracold molecules and ultracold chemistry, controlled many-body phenomena, entanglement of molecules and dipole arrays, and stability of molecular systems in high-frequency super-intense laser fields. The article contains 852 references.
Quantitative results are presented for stabilization of atoms against ionization by strong laser fields over a wide range of frequencies for circular and linear polarization. With circular polarization, stabilization occurs for all frequencies. At high frequency (\ensuremath{\omega}>1 a.u.), the stabilization intensity is predicted by the occurrence of photon thresholds. This mechanism gives excellent numerical agreement both with present calculations and the results of Pont and Gavrila [Phys. Rev. Lett. 65, 2362 (1990)]. A different mechanism operates for \ensuremath{\omega}1 a.u. Linear polarization differs strongly from circular polarization in many respects.
Ultracold dipolar molecules present many new and novel properties for studying the quantum dynamics of many-body physics. In particular, large electric dipole moments found in polar molecules can be tuned with external electric fields which provide for precise control over dipole-dipole interactions. JILA's second generation ultracold KRb molecule experiment provides new capabilities for applying external electric fields. Generating these fields requires several stable high-voltage sources in order to precisely control the transitions in ultracold dipolar molecules. This thesis will discuss my work towards creating stable high-voltage sources to be used in the second generation KRb apparatus.
Stabilization, the exotic process by which an atom in a superintense laser field, of the order of the atomic unit, reacts to the increase in intensity or decrease in frequency by increasing its lifetime, has been recognized as elusive for atomic ground states. We now show that the stabilization regime can be effectively attained at present laser performances (intensities, frequencies, pulse shapes) when starting with atoms prepared initially in Rydberg states of high magnetic quantum number with respect to the axis of linear polarization.