An echo experiment in a strongly interacting Rydberg gas

When ground state atoms are excited to a Rydberg state, van der Waals interactions among them can lead to a strong suppression of the excitation. Despite the strong interactions the evolution can still be reversed by a simple phase shift in the excitation laser field. We experimentally prove the coherence of the excitation in the strong blockade regime by applying an `optical rotary echo' technique to a sample of magnetically trapped ultracold atoms, analogous to a method known from nuclear magnetic resonance. We additionally measured the dephasing time due to the interaction between the Rydberg atoms.Comment: 4 pages, 5 figure

Rydberg excitation of Bose-Einstein condensates

Rydberg atoms provide a wide range of possibilities to tailor interactions in a quantum gas. Here we report on Rydberg excitation of Bose-Einstein condensed 87Rb atoms. The Rydberg fraction was investigated for various excitation times and temperatures above and below the condensation temperature. The excitation is locally blocked by the van der Waals interaction between Rydberg atoms to a density-dependent limit. Therefore the abrupt change of the thermal atomic density distribution to the characteristic bimodal distribution upon condensation could be observed in the Rydberg fraction. The observed features are reproduced by a simulation based on local collective Rydberg excitations.Comment: 4 pages, 3 figure

Evidence for coherent collective Rydberg excitation in the strong blockade regime

Blockade effects on the single quantum level are at the heart of quantum devices like single-electron transistors. The blockade mechanisms are based on strong interactions like the Coulomb interaction in case of single electrons. Neutral atoms excited into a Rydberg state experience abnormally strong interactions that lead to the corresponding blockade effect for Rydberg atoms. In this paper we report on our measurements of a strong van der Waals blockade, showing that only one out of several thousand atoms within a blockade volume can be excited. In addition, our experimental results demonstrate the coherent nature of the excitation of magnetically trapped ultracold atoms into a Rydberg state, confirming the predicted dependence of the collective Rabi frequency on the square root of the mesoscopic system size. This collective coherent behaviour is generic for all mesoscopic systems which are able to carry only one single excitation quantum.Comment: 5 pages, 4 figure

Decoupled carbonate chemistry controls on the incorporation of boron into Orbulina universa

In order to fully constrain paleo-carbonate systems, proxies for two out of seven parameters, plus temperature and salinity, are required. The boron isotopic composition (δ11B) of planktonic foraminifera shells is a powerful tool for reconstructing changes in past surface ocean pH. As B(OH)4− is substituted into the biogenic calcite lattice in place of CO32−, and both borate and carbonate ions are more abundant at higher pH, it was suggested early on that B ∕ Ca ratios in biogenic calcite may serve as a proxy for [CO32−]. Although several recent studies have shown that a direct connection of B ∕ Ca to carbonate system parameters may be masked by other environmental factors in the field, there is ample evidence for a mechanistic relationship between B ∕ Ca and carbonate system parameters. Here, we focus on investigating the primary relationship to develop a mechanistic understanding of boron uptake. Differentiating between the effects of pH and [CO32−] is problematic, as they co-vary closely in natural systems, so the major control on boron incorporation remains unclear. To deconvolve the effects of pH and [CO32−] and to investigate their impact on the B ∕ Ca ratio and δ11B, we conducted culture experiments with the planktonic foraminifer Orbulina universa in manipulated culture media: constant pH (8.05), but changing [CO32−] (238, 286 and 534 µmol kg−1 CO32−) and at constant [CO32−] (276 ± 19.5 µmol kg−1) and varying pH (7.7, 7.9 and 8.05). Measurements of the isotopic composition of boron and the B ∕ Ca ratio were performed simultaneously using a femtosecond laser ablation system coupled to a MC-ICP-MS (multiple-collector inductively coupled plasma mass spectrometer). Our results show that, as expected, δ11B is controlled by pH but it is also modulated by [CO32−]. On the other hand, the B ∕ Ca ratio is driven by [HCO3−], independently of pH. This suggests that B ∕ Ca ratios in foraminiferal calcite can possibly be used as a second, independent, proxy for complete paleo-carbonate system reconstructions. This is discussed in light of recent literature demonstrating that the primary relationship between B ∕ Ca and [HCO3−] can be obscured by other environmental parameters

Ion detection in the photoionization of a Rb Bose-Einstein condensate

Two-photon ionization of Rubidium atoms in a magneto-optical trap and a Bose-Einstein condensate (BEC) is experimentally investigated. Using 100 ns laser pulses, we detect single ions photoionized from the condenstate with a 35(10)% efficiency. The measurements are performed using a quartz cell with external electrodes, allowing large optical access for BECs and optical lattices.Comment: 14 pages, 7 figure

Observation of mesoscopic crystalline structures in a two-dimensional Rydberg gas

The ability to control and tune interactions in ultracold atomic gases has paved the way towards the realization of new phases of matter. Whereas experiments have so far achieved a high degree of control over short-ranged interactions, the realization of long-range interactions would open up a whole new realm of many-body physics and has become a central focus of research. Rydberg atoms are very well-suited to achieve this goal, as the van der Waals forces between them are many orders of magnitude larger than for ground state atoms. Consequently, the mere laser excitation of ultracold gases can cause strongly correlated many-body states to emerge directly when atoms are transferred to Rydberg states. A key example are quantum crystals, composed of coherent superpositions of different spatially ordered configurations of collective excitations. Here we report on the direct measurement of strong correlations in a laser excited two-dimensional atomic Mott insulator using high-resolution, in-situ Rydberg atom imaging. The observations reveal the emergence of spatially ordered excitation patterns in the high-density components of the prepared many-body state. They have random orientation, but well defined geometry, forming mesoscopic crystals of collective excitations delocalised throughout the gas. Our experiment demonstrates the potential of Rydberg gases to realise exotic phases of matter, thereby laying the basis for quantum simulations of long-range interacting quantum magnets.Comment: 10 pages, 7 figure

Mg/Ca-temperature and seawater-test chemistry relationships in the shallow-dwelling large benthic foraminifera <i>Operculina ammonoides</i>

The foraminifera Mg/Ca palaeothermometer contributes significantly to our understanding of palaeoceanic temperaturevariation. However, since seawater Mg/Ca has undergone large secular variation and the relationship between seawaterand test Mg/Ca has not been calibrated in detail for any species with a substantial fossil record, it is only possible to assessrelative temperature changes in pre-Pleistocene fossil samples. In order to establish the basis of accurate quantitativeMg/Ca-derived deep-time temperature reconstructions, we have calibrated the relationship between test Mg/Ca, seawaterchemistry and temperature in laboratory cultures of the shallow-dwelling large benthic species Operculina ammonoides.Operculina has a fossil range extending back to the early Paleogene and is the nearest living relative of the abundant genusNummulites. We find a temperature sensitivity of 1.7% C-1 and a linear relationship between the Mg distribution coefficientand seawater Mg/Ca (Mg=Casw) with m = -1.9 x 10-3, within error of the equivalent slope for inorganic calcite. The highertest Mg/Ca of O. ammonoides compared to inorganic calcite may be explained by an elevated pH of the calcifying fluid, implying that these foraminifera do not modify the Mg/Ca ratio of the seawater from which they calcify, differentiating them in thisrespect from most other perforate foraminifera. Applying these calibrations to previously published fossil data results inpalaeo-Mg=Casw reconstruction consistent with independent proxy evidence. Furthermore, our data enable accurate absolutepalaeotemperature reconstructions if Mg=Casw is constrained by another technique (e.g. ridge flank vein carbonate; fluidinclusions). Finally, we examine Li, Na, Sr and Ba incorporation into the test of O. ammonoides and discuss the controlexerted by temperature, seawater chemistry, saturation state and growth rate on these emerging proxies