Fluvial Sedimentology and Paleoecology of Holocene Alluvial Deposits, Red River, Manitoba

Stratigraphie and paleoecological analyses at five sections, together with age determinations based on 19 previously published and 21 new radiocarbon dates, provide a detailed late Holocene history of the Red River, Manitoba. Ecological information, such as age frequency analysis, relative abundance, diversity and association of species was drawn from 19 mollusc species. These data indicate that the Red and Assiniboine rivers cut the valleys they occupy today within a thousand years of the regression of Lake Agassiz. In the south, up to 14 m of alluvium has accumulated during the last 7000 years. A decrease in the sedimentation rate at 1400 BP is coincident with the shift in the position of the Assiniboine from the valley of the La Salle River to its present position. Overbank sedimentation did not start in the northern part of the area until ca. 5200 BP. Initial rapid sedimentation rates in this area are attributed to increased precipitation and a brief eastward excursion of the Assiniboine River into the Red. In spite of increased precipitation, flood frequencies remained low in the north until 1400 BP. Increased overbank sedimentation after 1400 BP is attributed to the northward shift in the position ot the Assiniboine.Les analyses stratigraphiques et paléoécologiques de cinq coupes, ainsi qu'une chronologie fondée sur 19 datations déjà connues et 21 nouvelles datations au radiocarbone permettent de reconstituer l'évolution de la rivière Rouge à l'Holocène. Des renseignements d'ordre écologique, comme l'analyse de la fréquence d'âge, l'abondance relative, la diversité et l'association des espèces ont été colligés sur 19 espèces de mollusques. Ces données montrent que les rivières Assiniboine et Rouge ont entaillé les vallées qu'elles occupent maintenant en moins de mille ans après le retrait du Lac Agassiz. Dans le sud, 14 m d'alluvions se sont accumulés depuis 7000 ans. La diminution du taux de sédimentation à 1400 BP coïncide avec le déplacement de !'Assiniboine de la vallée de la rivière LaSaIIe à son emplacement actuel. La sédimentation alluviale d'inondation n'a pas commencé dans la partie nord de la région avant environ 5200 BP. Les taux de sédimentation rapides initiaux sont attribuables aux précipitations accrues et à une brève incursion vers l'est de !'Assiniboine vers la rivière Rouge. En dépit des précipitations accrues, la fréquence des inondations est demeurée faible dans le nord jusqu'à 1400 BP. L'accroissement de la sédimentation alluviale d'inondation après 1400 BP est attribuée au déplacement de !'Assiniboine vers le nord.Mit stratigraphischen und palàoôkologischen Analysen von 5 Schnitten sowie einer auf 19 schon bekannte Datierungen und 21 neue Radikarbondatierungen gestùtzten Chronologie IaRt sich die Entwicklung des Red River im Holozàn rekonstruieren. Okologische Informationen wie die Analyse der Altersfrequenz, relative Haufigkeit, Vielfalt und Verbindung der Arten, wurden auf 19 Molluskenarten gesammelt. Dièse Daten zeigen, daB die Flùsse Assiniboine und Red die Tàler, durch sie sie jetzt fliefîen, in weniger als tausend Jahren nach dem Rùckzug des Agassizsees eingeschnitten haben. Im Sùden wurden seit 7000 Jahren 14 m Alluvium akkumuliert. Die Abnahme der Sedimentationsrate um 1400 v.u.Z. tritt gleichzeitig mit dem Wechsel des Assiniboine vom TaI des La SalleFlusses zu seiner jetzigen Position ein. Die alluviale Ùberschwemmungssedimentation hat im nôrdlichen Teil des Gebiets nicht vor etwa 5200 v.u.Z. begonnen. Die anfànglich sehr schnellen Sedimentationsraten fùhrt man auf verstàrkte Niederschlàge und einen kurzen ostwàrtigen Ausfall des Assiniboine River in Richtung Red River zurùck. Trotz der verstàrkten Niederschlàge ist die Haufigkeit der Ùberschwemmungen im Norden bis 1400 v.u.Z. niedrig geblieben. Die Zunahme der alluvialen Uberschwemmungssedimentation nach 1400 v.u.Z. wird auf die Verlagerung des Assiniboine nach Norden zurùckgefùhrt

Search for Ferromagnetism in doped semiconductors in the absence of transition metal ions

In contrast to semiconductors doped with transition metal magnetic elements, which become ferromagnetic at temperatures below ~ 100K, semiconductors doped with non-magnetic ions (e.g. silicon doped with phosphorous) have not shown evidence of ferromagnetism down to millikelvin temperatures. This is despite the fact that for low densities the system is expected to be well modeled by the Hubbard model, which is predicted to have a ferromagnetic ground state at T=0 on 2- or 3-dimensional bipartite lattices in the limit of strong correlation near half-filling. We examine the impurity band formed by hydrogenic centers in semiconductors at low densities, and show that it is described by a generalized Hubbard model which has, in addition to strong electron-electron interaction and disorder, an intrinsic electron-hole asymmetry. With the help of mean field methods as well as exact diagonalization of clusters around half filling, we can establish the existence of a ferromagnetic ground state, at least on the nanoscale, which is more robust than that found in the standard Hubbard model. This ferromagnetism is most clearly seen in a regime inaccessible to bulk systems, but attainable in quantum dots and 2D heterostructures. We present extensive numerical results for small systems that demonstrate the occurrence of high-spin ground states in both periodic and positionally disordered 2D systems. We consider how properties of real doped semiconductors, such as positional disorder and electron-hole asymmetry, affect the ground state spin of small 2D systems. We also discuss the relationship between this work and diluted magnetic semiconductors, such as Ga_(1-x)Mn_(x)As, which though disordered, show ferromagnetism at relatively high temperatures.Comment: 47 page

Coherent state transfer between an electron- and nuclear spin in 15N@C60

Electron spin qubits in molecular systems offer high reproducibility and the ability to self assemble into larger architectures. However, interactions between neighbouring qubits are 'always-on' and although the electron spin coherence times can be several hundred microseconds, these are still much shorter than typical times for nuclear spins. Here we implement an electron-nuclear hybrid scheme which uses coherent transfer between electron and nuclear spin degrees of freedom in order to both controllably turn on/off dipolar interactions between neighbouring spins and benefit from the long nuclear spin decoherence times (T2n). We transfer qubit states between the electron and 15N nuclear spin in 15N@C60 with a two-way process fidelity of 88%, using a series of tuned microwave and radiofrequency pulses and measure a nuclear spin coherence lifetime of over 100 ms.Comment: 5 pages, 3 figures with supplementary material (8 pages

Creating nuclear spin entanglement using an optical degree of freedom

Molecular nanostructures are promising building blocks for future quantum technologies, provided methods of harnessing their multiple degrees of freedom can be identified and implemented. Due to low decoherence rates nuclear spins are considered ideal candidates for storing quantum information while optical excitations can give rise to fast and controllable interactions for information processing. A recent paper (Physical Review Letters \textbf{104} 200501) proposed a method for entangling two nuclear spins through their mutual coupling to a transient optically excited electron spin. Building on the same idea, we here present an extended and much more detailed theoretical framework, showing that this method is in fact applicable to a much wider class of molecular structures than previously discussed in the original proposal

Implications of Electronics Constraints for Solid-State Quantum Error Correction and Quantum Circuit Failure Probability

In this paper we present the impact of classical electronics constraints on a solid-state quantum dot logical qubit architecture. Constraints due to routing density, bandwidth allocation, signal timing, and thermally aware placement of classical supporting electronics significantly affect the quantum error correction circuit's error rate. We analyze one level of a quantum error correction circuit using nine data qubits in a Bacon-Shor code configured as a quantum memory. A hypothetical silicon double quantum dot quantum bit (qubit) is used as the fundamental element. A pessimistic estimate of the error probability of the quantum circuit is calculated using the total number of gates and idle time using a provably optimal schedule for the circuit operations obtained with an integer program methodology. The micro-architecture analysis provides insight about the different ways the electronics impact the circuit performance (e.g., extra idle time in the schedule), which can significantly limit the ultimate performance of any quantum circuit and therefore is a critical foundation for any future larger scale architecture analysis.Comment: 10 pages, 7 figures, 3 table

A new model for magnetoreception

Certain migratory birds can sense the earth's magnetic field. The nature of this process is not yet properly understood. Here we offer a simple explanation according to which birds literally `see' the local magnetic field: Our model relates the well-established radical pair hypothesis to the phenomenon of Haidinger's brush, a capacity to see the polarisation of light. This new picture explains recent surprising experimental data indicating long lifetimes for the radical pair. Moreover there is a clear evolutionary path toward this field sensing mechanism: it is an enhancement of a weak effect that may be present in many species.Comment: 8 pages, 5 figures, version of final published pape