Femtosecond filamentation in transparent media

There is growing interest in states of matter with topological order. These are characterized by highly stable ground states robust to perturbations that preserve the topology, and which support excitations with so-called anyonic statistics. Topologically ordered states can arise in two-dimensional lattice-spin models, which were proposed as the basis for a new class of quantum computation. Here, we show that the relevant hamiltonians for such spin lattice models can be systematically engineered with polar molecules stored in optical lattices, where the spin is represented by a single-valence electron of a heteronuclear molecule. The combination of microwave excitation with dipole–dipole interactions and spin–rotation couplings enables building a complete toolbox for effective two-spin interactions with designable range, spatial anisotropy and coupling strengths significantly larger than relevant decoherence rates. Finally, we illustrate two models: one with an energy gap providing for error-resilient qubit encoding, and another leading to topologically protected quantum memory.

https://arxiv.org/pdf/quant-ph/0512222.pdf

A toolbox for lattice-spin models with polar molecules

The authors review progress in understanding the nature of atomic collisions occurring at temperatures ranging from the millidegrees Kelvin to the nanodegrees Kelvin regime. The review includes advances in experiments with atom beams, light traps, and purely magnetic traps. Semiclassical and fully quantal theories are described and their appropriate applicability assessed. The review divides the subject into two principal categories: collisions in the presence of one or more light fields and ground-state collisions in the dark.

Experiments and theory in cold and ultracold collisions

Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold !!1 mK" temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the “complications” of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.

/pdf/ultracold-photoassociation-spectroscopy-long-range-molecules-31d6x6vqxx.pdf

Ultracold photoassociation spectroscopy: Long-range molecules and atomic scattering

Summary: Carbazole-based oligomeric and polymeric materials have been studied for almost 25 years for their unique electrical, electrochemical and optical properties. Interestingly, carbazole units can be linked in two different ways leading to either poly(3,6-carbazole) or poly(2,7-carbazole) derivatives. While the former class seems to be very interesting for electrochemical and phosphorescence applications, the latter shows very promising optical properties in the visible range for light emitting diodes (LED). The major intrinsic difference between these two classes is the effective conjugation length: poly(2,7-carbazole) materials having the longer one, due to their poly(p-phenylene)-like structure. Using different synthetic strategies and substitution patterns, the physico-chemical properties of both classes can be fine-tuned, leading to high performance materials for a large number electronic applications.

Chemical structures for poly(3,6-carbazole) and poly(2,7-carbazole) and the materials used as the starting points for their respective syntheses.

Polycarbazoles: 25 Years of Progress

16 lines of the principal series of Cs(62s1/2 to 7-162p3/2 and 62s1/2 to 7-122p1/2) were investigated in absorption. Cesium vapour density was varied from 1013 up to 1017 cm-3. Red asymmetries of all lines (extended red wings) were detected. This can be understood if electron transitions in the long-range interaction region follow the selection rules of molecules. It was established that the red and the blue wings of the lines each can be fitted to lorentzian profiles in small wavelength intervals. With the help of quasistatic theory the interaction constants from the long-range interaction potential Delta omega =C3/r3 multiplied by the oscillator strength were determined from measurements. For all lines the ratio of C3(red)/C3(blue) was found to equal 2 in accordance with earlier theoretical calculations. In the far line wing, deviation from a lorentzian profile occurs. This was interpreted as a consequence of electron transition in the short-range region of interatomic potentials.

Asymmetric self-broadening of Cs principal lines

Laser induced fluorescence studies of the lithium diffuse band at 458 nm were performed using single‐and double‐photon excitations. Single‐photon excitation was performed using UV lines of Ar++ and Kr++ lasers, and the diffuse band fluorescence was induced by collisions in 7Li2 and 6Li2. The double‐photon excitation process populated relevant upper states through several accidental coincidences for two‐step excitation in 7Li2. In two cases, in addition to structured continuum emission, an adjacent line spectrum was observed. In five other cases, two‐photon accidental coincidences produced collision induced fluorescence of the 7Li2 diffuse band. We also found (in a few cases) that the (two‐photon) excitation energy of the 2 3Πg state (with respect to the 1 3Σ+u state) was collisionally transferred to simultaneously excited lithium 2p atoms, thus creating a lithium atom in the 4d state.

The 458 nm diffuse band of the lithium dimer

Photochemical production of the electronically excited NaCd excimer

We report measurements and a theoretical explanation of the cusp-shaped satellite bands in the blue wing of the cesium D2 resonance line which have been observed for the first time. The bands are identified as transitions where the upper state dissociates into the 6 2P
 3/2
 + 6 2
 S
 1/2
 atomic asymptote. The experiment has been performed using a standard absorption setup, computer controlled data acquisition and computer data processing. We have shown that the peculiar shape of the difference-potential curve is solely responsible for the spectrum containing the cusp-shaped satellite bands. The appearance of these satellite bands has been discussed and explained relating the theory of satellite bands to the catastrophe theory. The shape of the line wing and of the satellite bands have been calculated using the Fourier transform technique. To ensure a more stringent comparison between the experimental and the theoretical spectrum, we have analyzed and compared the derivatives of the measured and the calculated satellite band shape. On the contrary to the customary direct comparison between the measured and the calculated absorption coefficient, the derivative clearly shows all differences and resemblances between satellite band profiles. The degree of coincidence of the experimentally observed and the theoretically calculated satellite band shape can be used as an ultimate check on the assessment of the quality of potential-energy curves involved in the formation of satellite bands.

Cusp satellite bands in the spectrum of Cs2 molecule

We have carried out parallel studies of the quenching process in Na(3p)+H2 collisions and the possible reactive process in Na(3p)+H2 (v
 ″=1,2,3) collisions. Rich CARS spectra which were obtained at H2 pressure of 100 mbar and oven temperature of 600 K indicate the presence of vibrationally excited H2 and photochemically produced NaH molecules. Temporal resolution of NaH CARS lines was employed in order to rule out competing collisional processes. We make use of resonantly enhanced CARS methods which enabled us to achieve very high sensitivity for NaH detection.

Goran Pichler

Papers

Asymmetric self-broadening of Cs principal lines

The 458 nm diffuse band of the lithium dimer

Photochemical production of the electronically excited NaCd excimer

Cusp satellite bands in the spectrum of Cs2 molecule

CARS investigations of quenching and photochemical reactions in the Na+H2 collision system