Optical trapping of ultracold dysprosium atoms: transition probabilities, dynamic dipole polarizabilities and van der Waals C6C_6 coefficients

The efficiency of optical trapping of ultracold atoms depend on the atomic dynamic dipole polarizability governing the atom-field interaction. In this article, we have calculated the real and imaginary parts of the dynamic dipole polarizability of dysprosium in the ground and first excited level. Due to the high electronic angular momentum of those two states, the polarizabilities possess scalar, vector and tensor contributions that we have computed, on a wide range of trapping wavelengths, using the sum-over-state formula. Using the same formalism, we have also calculated the $C_6$ coefficients characterizing the van der Waals interaction between two dysprosium atoms in the two lowest levels. We have computed the energies of excited states and the transition probabilities appearing in the sums, using a combination of \textit{ab initio} and least-square-fitting techniques provided by the Cowan codes and extended in our group. Regarding the real part of the polarizability, for field frequencies far from atomic resonances, the vector and tensor contributions are two-order-of-magnitude smaller than the scalar contribution, whereas for the imaginary part, the vector and tensor contributions represent a noticeable fraction of the scalar contribution. This offers the possibility to control the decoherence and trap losses due to spontaneous emission

Probing quantum criticality and symmetry breaking at the microscopic level

We report on an experimental study of the Lipkin-Meshkov-Glick model of quantum spins interacting at infinite range in a transverse magnetic field, which exhibits a ferromagnetic phase transition in the thermodynamic limit. We use Dysprosium atoms of electronic spin $J=8$, subjected to a quadratic Zeeman light shift, to simulate $2J=16$ interacting spins $1/2$. We probe the system microscopically using single magnetic sublevel resolution, giving access to the spin projection parity, which is the collective observable characterizing the underlying $\mathbb{Z}_2$ symmetry. We measure the thermodynamic properties and dynamical response of the system, and study the quantum critical behavior around the transition point. In the ferromagnetic phase, we achieve coherent tunneling between symmetry-broken states, and test the link between symmetry breaking and the appearance of a finite order parameter.Comment: 13 pages, 13 figure

Optical cooling and trapping of highly magnetic atoms: The benefits of a spontaneous spin polarization

From the study of long-range-interacting systems to the simulation of gauge fields, open-shell Lanthanide atoms with their large magnetic moment and narrow optical transitions open novel directions in the field of ultracold quantum gases. As for other atomic species, the magneto-optical trap (MOT) is the working horse of experiments but its operation is challenging, due to the large electronic spin of the atoms. Here we present an experimental study of narrow-line Dysprosium MOTs. We show that the combination of radiation pressure and gravitational forces leads to a spontaneous polarization of the electronic spin. The spin composition is measured using a Stern-Gerlach separation of spin levels, revealing that the gas becomes almost fully spin-polarized for large laser frequency detunings. In this regime, we reach the optimal operation of the MOT, with samples of typically $3\times 10^8$ atoms at a temperature of 15\,$\mu$K. The spin polarization reduces the complexity of the radiative cooling description, which allows for a simple model accounting for our measurements. We also measure the rate of density-dependent atom losses, finding good agreement with a model based on light-induced Van der Waals forces. A minimal two-body loss rate $\beta\sim 2\times10^{-11}\,$cm$^{3}$/s is reached in the spin-polarized regime. Our results constitute a benchmark for the experimental study of ultracold gases of magnetic Lanthanide atoms.Comment: 21 pages, 9 figure

Dynamic Optical Lattices of Subwavelength Spacing for Ultracold Atoms.

We propose a scheme for realizing lattice potentials of subwavelength spacing for ultracold atoms. It is based on spin-dependent optical lattices with a time-periodic modulation. We show that the atomic motion is well described by the combined action of an effective, time-independent lattice of small spacing, together with a micromotion associated with the time modulation. A numerical simulation shows that an atomic gas can be adiabatically loaded into the effective lattice ground state, for time scales comparable to the ones required for adiabatic loading of standard optical lattices. We generalize our scheme to a two-dimensional geometry, leading to Bloch bands with nonzero Chern numbers. The realization of lattices of subwavelength spacing allows for the enhancement of energy scales, which could facilitate the achievement of strongly correlated (topological) states

Laughlin's topological charge pump in an atomic Hall cylinder

The quantum Hall effect occuring in two-dimensional electron gases was first explained by Laughlin, who envisioned a thought experiment that laid the groundwork for our understanding of topological quantum matter. His proposal is based on a quantum Hall cylinder periodically driven by an axial magnetic field, resulting in the quantized motion of electrons. We realize this milestone experiment with an ultracold gas of dysprosium atoms, the cyclic dimension being encoded in the electronic spin and the axial field controlled by the phases of laser-induced spin-orbit couplings. Our experiment provides a straightforward manifestation of the non-trivial topology of quantum Hall insulators, and could be generalized to strongly-correlated topological systems.Comment: 10 pages, 10 figure

Artificial gauge fields in materials and engineered systems

Artificial gauge fields are currently realized in a wide range of physical settings. This includes solid-state devices but also engineered systems, such as photonic crystals, ultracold gases and mechanical setups. It is the aim of this review to offer, for the first time, a unified view on these various forms of artificial electromagnetic fields and spin-orbit couplings for matter and light. This topical review provides a general introduction to the universal concept of engineered gauge fields, in a form that is accessible to young researchers entering the field. Moreover, this work aims to connect different communities, by revealing explicit links between the diverse forms and realizations of artificial gauge fields

Afonso de Albuquerque and the consumption of material culture in the Indian Ocean : 1506-1515

Nesta dissertação pretendem-se identificar as práticas de Afonso de Albuquerque enquanto consumidor de arte e avaliar até que ponto são paradigmáticas do seu tempo ou constituem um marco taxativo na periodização do consumo de arte. O governador (entre 1509 e 1515, mas na Ásia desde 1506) do que viria a ser o Estado da Índia teve um papel fundamental enquanto receptor e distribuidor de presentes diplomáticos, mas são também inteligíveis nos textos coevos apontamentos sobre as suas estratégias pessoais de usufruto e exibição de objectos artísticos. O texto explora como eram tomadas as decisões quanto à cultura material num momento de trocas intensas e sem precedentes com a Ásia. Argumenta-se que as práticas alteraram-se significativamente durante o período de governo de Albuquerque, motivadas pela sua (rápida) apreensão da geopolítica asiática. A dissertação divide-se em duas partes. Na primeira produz-se uma leitura historiográfica do interesse português na Ásia durante os anos finais do século XV e os primeiros do XVI. Esta síntese serve para mapear a conjuntura em que as situações descritas no segundo capítulo tiveram lugar. O segundo capítulo, mais extenso do que o precedente, produz uma leitura crítica das estratégias de consumo (aquisição, manutenção, exibição, e transferência de posse) de objectos por Afonso de Albuquerque. Divide-se em três tendências fundamentais que, de acordo com o que é proposto, formataram o interesse português por objectos asiáticos: os saques, as ofertas diplomáticas, e o consumo de corte. Na parte final do capítulo esboça-se uma proposta de interpretação de como foram recebidos em Portugal os objectos artísticos enviados, com diversos propósitos, por Afonso de Albuquerque.This thesis attempts to understand the practices of material culture consumption performed by Afonso de Albuquerque, and to assess if they served as a paradigm or a new tendency in sixteenth-century art consumption. The governor (from 1509-1515, but in Asia since 1506) of the future ‘Estado da Índia’ had a central role as a receiver and distributor of diplomatic gifts, but contemporary documents hint at a personal strategy in the use of art. This text explores how decisions were made in a moment of unprecedented and intensive material culture exchange with Asia. It will try to argue that practices changed in the course of Albuquerque’s government, following his (fast) apprehension of Asian geopolitics. This thesis is divided into two chapters. The first consists in a historiographical reading of the Portuguese interest in Asia during the late 15th and early 16th centuries. This summary serves as a basis to understand the stage where the episodes described in chapter II took place. Chapter II, far more extensive than the former, consists in a critical reading of the consumption strategies (acquisition, maintenance, display, and transfer) used by Afonso de Albuquerque. It is divided into three fundamental tendencies which, it is argued, shaped the Portuguese interest for Asiatica: looting, diplomatic gift-exchange, and courtly consumption. The final part of the chapter provides some suggestions on how the material culture sent by Albuquerque with various intents was received in Portugal

Farmland biodiversity and agricultural management on 237 farms in 13 European and two African regions

Farmland is a major land cover type in Europe and Africa and provides habitat for numerous species. The severe decline in farmland biodiversity of the last decades has been attributed to changes in farming practices, and organic and low-input farming are assumed to mitigate detrimental effects of agricultural intensification on biodiversity. Since the farm enterprise is the primary unit of agricultural decision making, management-related effects at the field scale need to be assessed at the farm level. Therefore, in this study, data were collected on habitat characteristics, vascular plant, earthworm, spider, and bee communities and on the corresponding agricultural management in 237 farms in 13 European and two African regions. In 15 environmental and agricultural homogeneous regions, 6–20 farms with the same farm type (e.g., arable crops, grassland, or specific permanent crops) were selected. If available, an equal number of organic and non-organic farms were randomly selected. Alternatively, farms were sampled along a gradient of management intensity. For all selected farms, the entire farmed area was mapped, which resulted in total in the mapping of 11 338 units attributed to 194 standardized habitat types, provided together with additional descriptors. On each farm, one site per available habitat type was randomly selected for species diversity investigations. Species were sampled on 2115 sites and identified to the species level by expert taxonomists. Species lists and abundance estimates are provided for each site and sampling date (one date for plants and earthworms, three dates for spiders and bees). In addition, farmers provided information about their management practices in face-to-face interviews following a standardized questionnaire. Farm management indicators for each farm are available (e.g., nitrogen input, pesticide applications, or energy input). Analyses revealed a positive effect of unproductive areas and a negative effect of intensive management on biodiversity. Communities of the four taxonomic groups strongly differed in their response to habitat characteristics, agricultural management, and regional circumstances. The data has potential for further insights into interactions of farmland biodiversity and agricultural management at site, farm, and regional scale