Generation of directional, coherent matter beams through dynamical instabilities in Bose-Einstein condensates

We present a theoretical analysis of a coupled, two-state Bose-Einstein condensate with non-equal scattering lengths, and show that dynamical instabilities can be excited. We demonstrate that these instabilities are exponentially amplified resulting in highly-directional, oppositely-propagating, coherent matter beams at specific momenta. To accomplish this we prove that the mean field of our system is periodic, and extend the standard Bogoliubov approach to consider a time-dependent, but cyclic, background. This allows us to use Floquet's theorem to gain analytic insight into such systems, rather than employing the usual Bogoliubov-de Gennes approach, which is usually limited to numerical solutions. We apply our theory to the metastable Helium atom laser experiment of Dall et al. [Phys. Rev. A 79, 011601(R) (2009)] and show it explains the anomalous beam profiles they observed. Finally we demonstrate the paired particle beams will be EPR-entangled on formation.Comment: Corrected reference

Resonant nonlinear magneto-optical effects in atoms

In this article, we review the history, current status, physical mechanisms, experimental methods, and applications of nonlinear magneto-optical effects in atomic vapors. We begin by describing the pioneering work of Macaluso and Corbino over a century ago on linear magneto-optical effects (in which the properties of the medium do not depend on the light power) in the vicinity of atomic resonances, and contrast these effects with various nonlinear magneto-optical phenomena that have been studied both theoretically and experimentally since the late 1960s. In recent years, the field of nonlinear magneto-optics has experienced a revival of interest that has led to a number of developments, including the observation of ultra-narrow (1-Hz) magneto-optical resonances, applications in sensitive magnetometry, nonlinear magneto-optical tomography, and the possibility of a search for parity- and time-reversal-invariance violation in atoms.Comment: 51 pages, 23 figures, to appear in Rev. Mod. Phys. in Oct. 2002, Figure added, typos corrected, text edited for clarit

Identification of natural product stereochemistry via calculation of ECD spectra

Most commercially available antibiotics are obtained from natural products, secondary metabolites of bacteria or other living organisms. Due to the importance of this class of compounds in medicinal chemistry and growing drug resistance, efforts to discover, characterize and isolate new or improved antibiotics are continually increasing. The assignment of the absolute configuration (AC) adopted by these compounds is a crucial aspect of the characterization step and knowledge of the stereochemistry is an important factor in deciphering the interaction of these compounds with the organism and thus, the mechanism of action. In order to assign the AC, several techniques, such as X-ray diffraction and NMR experiments as well as the standard electronic spectroscopy experiments (UV-Vis, ECD, etc.) or less widespread vibrational and rotational spectroscopy experiments (VCD, ROA, etc.) can be used, often in combination. However, sophisticated synthetic strategies or difficult isolation of the natural compound often leads to a small amount of product available, making some of the previous techniques unpractical; in addition to the potential structural complexity of the molecule, this can make the experimental assignment of the AC problematic. For this reason, a computational approach, aimed at calculating observable properties of the products, generating spectra and assigning the AC through comparison between the calculated and the experimental spectra, has proven useful in many cases. Formicamycin is a natural product, isolated from a new member of Streptomyces bacteria, which has shown great activity against pathogenic drug-resistant bacteria and fungi, without developing antimicrobial resistance. This dissertation shows that the chiral axis of Formicamycin can be assigned as R, through the calculation of electronic circular dichroism (ECD) spectra and comparison to the experimentally determined spectrum in methanol. ECD spectroscopy is very sensitive to the chiral environment of chromophores and can be used to distinguish between different isomers. The computational procedure has been broadly defined in previous studies and involves three general steps: 1) generation of an ensemble of structures, 2) optimization of the structures and calculation of the rotational strengths of each and 3) generation of the Boltzmannweighted spectrum. Here, two different force fields (OPLS3 and MMFFs) were used for generating the ensemble of conformers, followed by PBE0 DFT calculations to determine the optimal geometry and finally, TDDFT calculations to compute the rotational strengths of each conformer. Furthermore, the spectra were calculated in four different solvents, using the implicit SMD method, in order to inform future studies about “variable solvent circular dichroism”. Different conformations of a molecule can be controlled by the choice of solvent and it is hypothesised that a change in solvent will result in a “fingerprint” shift in the ECD spectra that could permit assignment of the stereochemistry. The entire process was automated using a module written in Python

Cavity Polarization Tuning for Enhanced Quantum Dot Interactions

Cavity enhanced interactions of light and matter have promising applications in quantum information technology. In particular, semiconductor quantum dots are extremely good candidates for single photon sources. Quantum dots have strong optical properties and can be directly embedded into photonic devices. Micropillar cavities with embedded quantum dots can be grown and fabricated, all with the use of mature semiconductor technology. Typically, microcavities are fabricated with a finite polarization mode splitting. Such splitting is detrimental in most applications, including the generation of single photons. The mechanisms that produce birefringence in semiconductor micropillar cavities are discussed in detail, and methods to eliminate such effects are discussed. A fine tuning method is presented, which is used to independently tune the cavity polarization splitting and quantum dot resonance. The fine tuning method makes use of the linear electro-optic effect to alter the birefringence of a single mirror. With high resolution reflection spectroscopy, the dynamics of a quantum dot in a polarization degenerate cavity are studied. Interactions are discussed, both in the context of classical, and quantum mechanical interactions

Pinning and Sliding of Driven Elastic Systems: from Domain Walls to Charge Density Waves

The review is devoted to the theory of collective and it local pinning effects in various disordered non-linear driven systems. Although the emphasis is put on charge and spin density waves and magnetic domain walls, the theory has also applications to flux lines and lattices thereof, dislocation lines, adsorbed mono-layers and related systems. In the first part we focus on the theory of the collective pinning which includes the equilibrium properties of elastic systems with frozen-in disorder as well as the features close to the dynamic depinning transition enforced by an external driving force and at finite temperatures. Thermal fluctuations smear out this transition and allow for a creep motion of the elastic objects even at small forces. An ac-driving force also destroys the sharp transition which is replaced by a velocity hysteresis. The second part is devoted to the local pinning picture and its applications. Inclusion of plastic deformations results in a rich cross-over behavior of the force-velocity relation as well as of the frequency dependence of the dynamic response. The local pinning recovers and exploits new elements of the energy landscape such as termination points of metastable branches or irreversibility of other ones related to generation of topological defects in the course of sliding. It also gives access to the quantum creep described as a tunneling between retarded and advanced configurations.Comment: 73 pages, 29 figures; Advances in Physics 2004 (in press

Dynamical Control of the Coherent Forward Scattering of XUV and X-Ray Radiation

This work aims at development of new dynamical methods to control a resonant light-matter interaction as well as at their application for producing new sources of coherent intense sub-fs radiation in x-ray range. The dynamical control is based on modulation in time and in space of the parameters of the atomic transition (coupled to the resonant high-frequency field) by means of sufficiently strong off-resonant low-frequency control field. In particular, it may result in efficient transfer of quasi-monochromatic VUV or XUV radiation into ultrashort pulses via its resonant interaction with atomic hydrogen gas or plasma of hydrogen-like ions respectively by means of two different techniques: time and space dependent linear Stark effect or interruption of resonant interaction by the tunneling ionization, as shown in previous works of our group. In this thesis, I further develop these ideas and demonstrate their important potential applications. First of all, both techniques are extended to arbitrary (non-hydrogen-like) atomic medium. Furthermore, advanced analytical and numerical solutions describing a process of pulses formation are found and shown to be in excellent agreement with each other. A deep physical analogy between the processes of coherent forward scattering of γ-ray radiation in the vibrated quasi-resonant nuclear absorber and the XUV field propagation in the quasi-resonant atomic medium in the presence of the moderately strong IR field is established. Finally, the application of the developed techniques for production of intense coherent attosecond sources of soft X-ray radiation, including so-called “water window” range (2.2-4.3nm) (especially promising for dynamical imaging of the proteins in a living cell) is proposed. Two different paths towards production of intense coherent attosecond pulses in a soft X-ray range are suggested: (i) via efficient transformation of the picosecond radiation of the high energy pulses of the existing x-ray plasma lasers into the trains of attosecond pulses in the resonant passive plasma, and (ii) via amplification of the low energy pulses of the existing high harmonic sources in the resonant active plasma of x-ray lasers