A Multi-Faceted Biogeochemical Approach to Analyzing Hypoxia in Green Bay, Lake Michigan

Green Bay, Lake Michigan is a large freshwater estuary that has experienced seasonal hypoxia for decades. Hypoxia, or dissolved oxygen concentrations less than 2 mg L-1, is a problem in coastal ecosystems around the world because it has a negative impact on ecosystem health by decreasing biodiversity and fisheries. In order to create adequate management policies for hypoxia, it is important to understand the sources and sinks of oxygen within Green Bay. This study utilizes a number of traditional and novel field methods to measure the production and respiration of oxygen within lower Green Bay, defined as south of Chambers Island, which is the area that experiences hypoxia. Primary production was measured using light-dark bottles and via in-situ diel oxygen fluctuation calculations. The epilimnetic waters are slightly net autotrophic during the summer months, meaning that they accumulate organic matter that can drive oxygen respiration in the hypolimnion. Hypolimnetic oxygen consumption was calculated as the loss of hypolimnetic oxygen inventories between two time periods. It was also determined that the two major processes consuming oxygen within the hypolimnion are sediment oxygen demand (SOD) and water column respiration (RH). SOD was measured using core incubations and eddy covariance. RH¬ was estimated as the difference between hypolimnetic oxygen consumption and SOD. In shallow waters, close to the Fox River mouth, SOD dominates the oxygen consumption, while in mid-bay waters oxygen respiration is divided between SOD and RH. Based on natural tracer results, cool bottom waters flows southward from the Lake Michigan-Green Bay gap and begins to lose oxygen once it reaches the mid-bay. This somewhat oxygen depleted water is further pushed down the bay into shallow waters where benthic respiration consumes more oxygen and drives water hypoxic. 222Rn and CH4 were also used as natural tracers to estimate advective flow in mid-bay bottom water and apparent methane production, respectively. Advective flow was estimated at ~3 km d-1, which agrees with current profiler velocities of 1.8 km d-1. This velocity can vary though, depending on upstream 222Rn activity. Apparent methane production trends match apparent oxygen utilization trends, confirming that methane is produced when oxygen is depleted. Finally, a biogeochemical model for Green Bay was created to predict what level of nutrient reductions would sufficiently reduce hypoxia, both now and under future scenarios. This biogeochemical model was part of an integrated modeling effort. The model was successfully formed and can be used to evaluate responses, although the baseline line model needs to be much better calibrated to better replicate observations

Measurement of the Angular Dependence of the Dipole-Dipole Interaction Between Two Individual Rydberg Atoms at a F\"orster Resonance

We measure the angular dependence of the resonant dipole-dipole interaction between two individual Rydberg atoms with controlled relative positions. By applying a combination of static electric and magnetic fields on the atoms, we demonstrate the possibility to isolate a single interaction channel at a F\"orster resonance, that shows a well-defined angular dependence. We first identify spectroscopically the F\"orster resonance of choice and we then perform a direct measurement of the interaction strength between the two atoms as a function of the angle between the internuclear axis and the quantization axis. Our results show good agreement with the expected angular dependence $\propto(1-3\cos^2\theta)$, and represent an important step towards quantum state engineering in two-dimensional arrays of individual Rydberg atoms.Comment: 5 pages, 4 figure

Coherent dipole-dipole coupling between two single atoms at a F\"orster resonance

Resonant energy transfers, i.e. the non-radiative redistribution of an electronic excitation between two particles coupled by the dipole-dipole interaction, lie at the heart of a variety of chemical and biological phenomena, most notably photosynthesis. In 1948, F\"orster established the theoretical basis of fluorescence resonant energy transfer (FRET), paving the ground towards the widespread use of FRET as a "spectroscopic ruler" for the determination of nanometer-scale distances in biomolecules. The underlying mechanism is a coherent dipole-dipole coupling between particles, as already recognized in the early days of quantum mechanics, but this coherence was not directly observed so far. Here, we study, both spectroscopically and in the time domain, the coherent, dipolar-induced exchange of electronic excitations between two single Rydberg atoms separated by a controlled distance as large as 15 microns, and brought into resonance by applying a small electric field. The coherent oscillation of the system between two degenerate pair states occurs at a frequency that scales as the inverse third power of the distance, the hallmark of dipole-dipole interactions. Our results not only demonstrate, at the most fundamental level of two atoms, the basic mechanism underlying FRET, but also open exciting prospects for active tuning of strong, coherent interactions in quantum many-body systems.Comment: 4 pages, 3 figure

Single-Atom Addressing in Microtraps for Quantum-State Engineering using Rydberg Atoms

We report on the selective addressing of an individual atom in a pair of single-atom microtraps separated by $3\;\mu$m. Using a tunable light-shift, we render the selected atom off-resonant with a global Rydberg excitation laser which is resonant with the other atom, making it possible to selectively block this atom from being excited to the Rydberg state. Furthermore we demonstrate the controlled manipulation of a two-atom entangled state by using the addressing beam to induce a phase shift onto one component of the wave function of the system, transferring it to a dark state for the Rydberg excitation light. Our results are an important step towards implementing quantum information processing and quantum simulation with large arrays of Rydberg atoms.Comment: 4 pages, 3 figure

Überidentifikation von Lernstörungen bei Kindern mit Deutsch als Zweitsprache. Implikationen für die Normierung von standardisierten Schulleistungstests

This German prevalence study examined disproportionate representation of language- minority students among children identified with learning disorder (LD) according to ICD-10 (WHO, 1992). Most German school achievement tests used in LD diagnostics do not provide separate norms for language-minority students, and thus do not take these children’s second language status into account when evaluating their academic performance. Although this is likely to result in an LD over identification of language-minority students, little is known about the magnitude of this effect. Therefore, we compared the estimation of LD prevalence between native German speaking students (n = 566) and language-minority students (n = 478) when pooled versus group-specific achievement norms were used for LD classification. Three important findings emerged from our study: Firstly, and as expected, significant disproportionality effects occurred under pooled norms. Specifically, the likelihood of being diagnosed with LD amounted to 14–18 % among native German speakers and nearly doubled to 25–30 % among language-minority students. Secondly, disproportionality varied as a function of LD subtype: Whereas no disproportionate representation was revealed for arithmetic LD (F81.2), overidentification of language-minority students was found for verbal LD subtypes (namely, reading disorder [F81.0], spelling disorder [F81.1], and mixed disorder of scholastic skills [F81.3]). Thirdly, disproportionality effects were absent when group-specific norms were used for LD classification that controlled for second-language issues. Challenges that have to be met when testing language-minority students for LD are discussed. (DIPF/Orig.)Die Prävalenzstudie untersucht bei Kindern, die Deutsch als Muttersprache (DaM) bzw. als Zweitsprache (DaZ) sprechen, die Häufigkeit von Lernstörungen nach ICD-10 (WHO, 1992). Die meisten deutschen Schulleistungstests, die zur Lernstörungsdiagnose herangezogen werden, stellen keine gesonderten Normen für Kinder mit DaZ bereit. Es ist anzunehmen, dass dies zu einer Überidentifikation von Lernstörungen bei Kindern mit DaZ führt, da die besondere Spracherwerbssituation dieser Kinder nicht berücksichtigt wird. Dennoch ist bislang wenig über das Ausmaß dieses Effektes bekannt. Die vorliegende Studie vergleicht daher die Lernstörungsprävalenz zwischen Drittklässlern mit DaM (n = 566) bzw. mit DaZ (n = 478) wenn gemeinsame versus getrennte Schulleistungsnormen zur Leistungsbeurteilung herangezogen werden. Die Studie erbrachte drei wesentliche Ergebnisse: (1) Wie erwartet kam es bei Verwendung gemeinsamer Schulleistungsnormen zu einer deutlichen Erhöhung der Lernstörungsprävalenz bei Kindern mit DaZ. Die Wahrscheinlichkeit einer Lernstörungsdiagnose belief sich für diese Teilstichprobe auf 25–30 % und war damit annähernd doppelt so groß wie bei Kindern mit DaM, für die sich eine Gesamtprävalenz von 14–18 % ergab. (2) Die Gruppenunterschiede variierten dabei in Abhängigkeit des Lernstörungstypus: Während keine signifikant unterschiedlichen Prävalenzraten für die isolierte Rechenstörung (F81.2) nachweisbar waren, zeigten sich für die verbalen Lernstörungstypen (d. h. Lese-Rechtschreibstörung [F81.0], isolierte Rechtschreibstörung [F81.1] und kombinierte Störung schulischer Fertigkeiten [F81.3]) signifikant erhöhte Prävalenzraten für Kinder mit DaZ. (3) Werden hingegen getrennte Schulleistungsnormen zur Lernstörungsdiagnose herangezogen um für die besondere Spracherwerbssituation von Kindern mit DaZ zu kontrollieren, nähern sich die Prävalenzraten beider Gruppen wie erwartet auf ein vergleichbares Niveau an. Es wird diskutiert, welche Herausforderungen sich bei der Lernstörungsdiagnostik von Kindern mit DaZ ergeben. (DIPF/Orig.

Realizing quantum Ising models in tunable two-dimensional arrays of single Rydberg atoms

Spin models are the prime example of simplified manybody Hamiltonians used to model complex, real-world strongly correlated materials. However, despite their simplified character, their dynamics often cannot be simulated exactly on classical computers as soon as the number of particles exceeds a few tens. For this reason, the quantum simulation of spin Hamiltonians using the tools of atomic and molecular physics has become very active over the last years, using ultracold atoms or molecules in optical lattices, or trapped ions. All of these approaches have their own assets, but also limitations. Here, we report on a novel platform for the study of spin systems, using individual atoms trapped in two-dimensional arrays of optical microtraps with arbitrary geometries, where filling fractions range from 60 to 100% with exact knowledge of the initial configuration. When excited to Rydberg D-states, the atoms undergo strong interactions whose anisotropic character opens exciting prospects for simulating exotic matter. We illustrate the versatility of our system by studying the dynamics of an Ising-like spin-1/2 system in a transverse field with up to thirty spins, for a variety of geometries in one and two dimensions, and for a wide range of interaction strengths. For geometries where the anisotropy is expected to have small effects we find an excellent agreement with ab-initio simulations of the spin-1/2 system, while for strongly anisotropic situations the multilevel structure of the D-states has a measurable influence. Our findings establish arrays of single Rydberg atoms as a versatile platform for the study of quantum magnetism.Comment: This is the version of the manuscript as initially submitted to Natur

Single-atom trapping in holographic 2D arrays of microtraps with arbitrary geometries

We demonstrate single-atom trapping in two-dimensional arrays of microtraps with arbitrary geometries. We generate the arrays using a Spatial Light Modulator (SLM), with which we imprint an appropriate phase pattern on an optical dipole trap beam prior to focusing. We trap single $^{87}{\rm Rb}$ atoms in the sites of arrays containing up to $\sim100$ microtraps separated by distances as small as $3\;\mu$m, with complex structures such as triangular, honeycomb or kagome lattices. Using a closed-loop optimization of the uniformity of the trap depths ensures that all trapping sites are equivalent. This versatile system opens appealing applications in quantum information processing and quantum simulation, e.g. for simulating frustrated quantum magnetism using Rydberg atoms.Comment: 9 pages, 10 figure

Facilitation dynamics and localization phenomena in Rydberg lattice gases with position disorder

We explore the dynamics of Rydberg excitations in an optical tweezer array under anti-blockade (or facilitation) conditions. Due to the finite temperature the atomic positions are randomly spread, an effect that leads to quenched correlated disorder in the interatomic interaction strengths. This drastically affects the facilitation dynamics as we demonstrate experimentally on the elementary example of two atoms. To shed light on the role of disorder in a many-body setting we show that here the dynamics is governed by an Anderson-Fock model, i.e. an Anderson model formulated on a lattice with sites corresponding to many-body Fock states. We first consider a one-dimensional atom chain in a limit which is described by a one-dimensional Anderson-Fock model with disorder on every other site, featuring both localized and delocalized states. We then illustrate the effect of disorder experimentally in a situation in which the system maps on a two-dimensional Anderson-Fock model on a trimmed square lattice. We observe a clear suppression of excitation propagation which we ascribe to the localization of the many-body wavefunctions in Hilbert space