Boundary between the thermal and statistical polarization regimes in a nuclear spin ensemble

As the number of spins in an ensemble is reduced, the statistical uctuations in its polarization eventually exceed the mean thermal polarization. This transition has now been surpassed in a number of recent nuclear magnetic resonance experiments, which achieve nanometer-scale detection volumes. Here, we measure nanometer- scale ensembles of nuclear spins in a KPF6 sample using magnetic resonance force microscopy. In particular, we investigate the transition between regimes dominated by thermal and statistical nuclear polarization. The ratio between the two types of polarization provides a measure of the number of spins in the detected ensemble

Decoherence times of universal two-qubit gates in the presence of broad-band noise

The controlled generation of entangled states of two quantum bits is a fundamental step toward the implementation of a quantum information processor. In nano-devices this operation is counteracted by the solid-state environment, characterized by a broadband and non-monotonic power spectrum, often 1/f at low frequencies. For single-qubit gates, incoherent processes due to fluctuations acting on different time scales result in peculiar short- and long-time behavior. Markovian noise gives rise to exponential decay with relaxation and decoherence times, T1 and T2, simply related to the symmetry of the qubit-environment coupling Hamiltonian. Noise with the 1/f power spectrum at low frequencies is instead responsible for defocusing processes and algebraic short-time behavior. In this paper, we identify the relevant decoherence times of an entangling operation due to the different decoherence channels originating from solid-state noise. Entanglement is quantified by concurrence, which we evaluate in an analytic form employing a multi-stage approach. The 'optimal' operating conditions of reduced sensitivity to noise sources are identified. We apply this analysis to a superconducting \sqrt{i-SWAP} gate for experimental noise spectra.Comment: 35 pages, 11 figure

Itinerancy and Hidden Order in URu2Si2URu_2Si_2

We argue that key characteristics of the enigmatic transition at $T_0= 17.5K$ in $URu_2Si_2$ indicate that the hidden order is a density wave formed within a band of composite quasiparticles, whose detailed structure is determined by local physics. We expand on our proposal (with J.A. Mydosh) of the hidden order as incommnesurate orbital antiferromagnetism and present experimental predictions to test our ideas. We then turn towards a microscopic description of orbital antiferromagnetism, exploring possible particle-hole pairings within the context of a simple one-band model. We end with a discussion of recent high-field and thermal transport experiment, and discuss their implications for the nature of the hidden order.Comment: 18 pages, 7 figures. v2 contains added referenc

Magnetic string contribution to hadron dynamics in QCD

Dynamics of a light quark in the field of static source (heavy-light meson) is studied using the nonlinear Dirac equation, derived recently. Special attention is paid to the contribution of magnetic correlators and it is found that it yields a significant increase of string tension at intermediate distances. The spectrum of heavy-light mesons is computed with account of this contribution and compared to experimental and lattice data.Comment: 10 pages Revte

Tree conifers invasion in steppe areas and Austrocedus chilensis forests in NW Patagonia

La implantación de masas forestales con fines productivos se realiza con un conjunto reducido de especies de las que se conoce su manejo silvícola e industrial. Por tal motivo se las introduce en áreas distintas a las de su origen. Potencialmente, estas introducciones pueden desencadenar procesos de invasión de los sistemas naturales o seminaturales contiguos a las plantaciones. En la región andino patagónica, la actividad forestal con especies introducidas es aún incipiente, tanto por la superficie forestada como por la edad de las plantaciones más importantes. Se estima la superficie implantada en aproximadamente 70000 hectáreas, con una tendencia creciente en el ritmo de forestación en la última década. La especie más plantada es el Pinus ponderosa Doug (Laws) (pino ponderosa), seguida por Pseudotsuga menziesii (Mirb)Franco (pino oregón) y Pinus contorta Dougl. (pino murrayana). Actualmente ha comenzado a observarse reclutamiento de renovales creciendo fuera de los macizos forestales en los sistemas lindantes, naturales o no. El presente trabajo evalúa, a escala regional, la presencia y la tasa de reclutamiento de renovales de estas especies en dos sistemas naturales, estepa y bosque de Austrocedrus chilensis (D.Don) Pic. Ser. et Bizzarri (ciprés de la cordillera), con el fin diagnosticar si existe un proceso de invasión incipiente. Se muestrearon áreas cercanas a las principales masas forestales implantadas, desde el centro de la provincia de Neuquén (39º48’S) hasta el Norte de Chubut (42º10’S). Para ello, se registró la presencia de regeneración natural, su densidad (nº indiv/ha), distancia a la plantación, edad, altura, y si producían conos o semillas. Los resultados indican que es posible que se esté ante la invasión incipiente de las comunidades adyacentes a las masas forestales por dos de las especies consideradas: el pino murrayana estaría mostrando expansión invasora sobre la estepa y el pino oregón sobre los cipresales. El pino murrayana presenta la menor edad de inicio de reclutamiento (12 años). El pino ponderosa no parece comportarse como invasora. Sobre la base de esta información se discuten las posibles causas que podrían facilitar o limitar el proceso de expansión de estas especies relacionadas con la presión de propágulos, depredación de semillas y ocurrencia de disturbios. Se proponen recomendaciones de monitoreo y control orientadas a evitar que los renuevos generen nueva descendencia.Commercial forestry is carried out with a reduced set of species, with known silvicultural and industrial managements. Thus, these species are frequently planted far away from their natural habitats and introduced in new environments, creating a risk for starting invasion processes in natural or seminatural systems adjacent to the plantations. In NW Patagonia, commercial forestry with exotic species is recent ( 300m vs. 50m) and density (420 vs. 100 indiv/ha). Also, lodgepole seedling recruitment occurred in more sites (63% vs. 36%) and in plantations with younger trees (12 years vs. 18 years). Overall, recruitment of the introduced conifers was greater at steppe sites in which cattle rearing had been reported in the past, typically with a low vegetation cover or where burned. In the native woodland sites, Douglas fir displayed a better capacity to propagate than ponderosa pine. Both species began to produce seedlings at the same age (17-18 years). Nevertheless, distances of recruitment were greater for Douglas fir (140 versus 70 m), as were densities (1500 vs. 250 individuals/ha) and percent sites with recruitment (100% vs. 67%). Open areas and disturbed places (paths, trails or clearings) displayed greater recruitment capacity than undisturbed areas. Indeed, recruitment was absent in sites highly covered and in sites with abundant understorey vegetation. In both studied systems, areas located downwind presented greater recruitment. Results of expansion distance to determine whether these exotic species have initiated an invasion process were compared with distance of advance proposed by Richardson to define invasions. Our results indicate that an incipient invasion of lodgepole pine in steppe and Douglas fir in woodlands dominated by the native conifer is occurring. Lodgepole pine produced seedlings at the earliest age (12 years). Ponderosa pine does not seem to behave like an invader to the studied communities. The information obtained from this study allowed us to identify factors that could facilitate or limit the process of expansion of these species related to propagule pressure, seed predation and disturbances. We propose recommendations on how to follow it and assure that preventive tactics are adopted to reduce the risk for the occurrence of a biological invasion

Superconducting qubit manipulated by fast pulses: experimental observation of distinct decoherence regimes

A particular superconducting quantum interference device (SQUID)qubit, indicated as double SQUID qubit, can be manipulated by rapidly modifying its potential with the application of fast flux pulses. In this system we observe coherent oscillations exhibiting non-exponential decay, indicating a non trivial decoherence mechanism. Moreover, by tuning the qubit in different conditions (different oscillation frequencies) by changing the pulse height, we observe a crossover between two distinct decoherence regimes and the existence of an "optimal" point where the qubit is only weakly sensitive to intrinsic noise. We find that this behaviour is in agreement with a model considering the decoherence caused essentially by low frequency noise contributions, and discuss the experimental results and possible issues.Comment: 16 pages, 9 figure

Water use and productivity of native and planted forests in NW Patagonia : ecohydrological and ecophysiological approaches

Existe preocupación por el posible uso excesivo de agua de las forestaciones con especies exóticas de rápido crecimiento en comparación con las especies nativas a las cuales reemplazan en la región NO de la Patagonia. Debido a ello, se han realizado estudios de productividad, consumo y eficiencia en el uso del agua tanto en plantaciones de especies exóticas como en sistemas vegetales naturales en zonas con distintos niveles de déficit hídrico. Esto se llevó a cabo mediante aproximaciones ecohidrológicas y ecofisiológicas. El objetivo del presente trabajo fue analizar y sintetizar el nivel de avance logrado en el conocimiento hasta la fecha. Tanto a nivel de árbol individual como de bosques con cobertura total de dosel arbóreo, las especies exóticas [Pinus ponderosa (pino ponderosa) y Pseudotsuga menziesii (pino oregón)] poseen un mayor consumo de agua que los individuos-sistemas nativos que crecen en sitios similares [pastizal, bosque de ciprés de la cordillera (Austrocedrus chilensis) y bosque mixto de ñire (Nothofagus antarctica) y radal (Lomatia hirsuta), y otras especies leñosas, respectivamente), aunque en todos los casos también se verificó un aumento en la eºciencia en el uso del agua. Los resultados sugieren que las diferencias en productividad y uso de agua se relacionan principalmente con una mayor resistencia hidráulica tanto de flujos en estado líquido (menor conductividad de la madera) como en estado gaseoso (mayor sensibilidad estomática a la demanda atmosférica) en las especies nativas. En el caso de las plantaciones de pino ponderosa, es posible disminuir el consumo de agua mediante el manejo del área foliar (podas y densidad de plantación). A su vez, en el bosque mixto nativo es posible manejar la cantidad y la eficiencia en el uso del agua mediante la extracción selectiva de ciertas especies y tamaños de individuos. Por otro lado, las mayores diferencias en el consumo de agua se observaron entre los pastizales y los bosques densos de pino ponderosa, los que a su vez se ubican en las zonas más secas del área forestable. Por otro lado, la interceptación de lluvias en la zona más árida fue mayor en los bosques nativos que en los implantados, lo cual hizo similar la evapotranspiración total de ambos, aunque es levemente superior en los bosques exóticos en la zona de mayor precipitación anual. Esto pone de manifiesto la importancia de considerar la ubicación, la densidad y el tamaño de los parches forestados en situaciones en las que los recursos hídricos excedentes de las precipitaciones quieran ser aprovechados aguas abajo en otras actividades. En este tipo de situaciones se recomiendan plantaciones con una baja densidad arbórea. Por el contrario, dado los excedentes de agua en la zona más húmeda (de aptitud para el pino oregón), el mayorconsumo de la especie exótica no redundaría en un impacto negativo sobre los recursos hídricos. Cabe destacar que dada la mayor eficiencia en el uso del agua de los sistemas exóticos, el costo en agua de la producción de madera de estos bosques es bastante menor al de los sistemas nativos.There is a concern about the potential excessive water used of forest plantations with fast growing exotic species compared to the native vegetation systems they replace in NW Patagonia. For this reason, several studies have being conducted focused on the productivity, water consumption and water use efficiency of the different systems, in areas with different precipitation levels. The studies were carried out from ecohydrological and ecophysiological approaches. The objective of this paper was to analyze and synthesize the available knowledge about these topics. Both at the individual tree level or at forest with complete cover systems, the exotic species (Pinus ponderosa and Pseudotsuga menziesii) have a higher water consumption than all native individual tree-vegetation systems they replace (grassland- Austrocedrus chilensis forests, and mixed forests with Nothofagus antarctica, Lomatia hirsuta, and other native woody species, respectively). However, in all cases, an increase in water use efficiency was observed in exotic species compared to native species. Our results suggest that differences in productivity and water use are mainly due to a higher resistance to water flow, both in liquid (lower wood conductivity) and vapor (higher stomatal sensitivity to vapor pressure deficit) phases in native species compared to exotic ones. At least in the case of ponderosa pine, water use of plantations may be decreased managing leaf area of the system by means of green pruning and thinning. Moreover, in the native mixed forest water use and water use efºciency are able to be managed through selective extraction of individuals of certain species and sizes. On the other hand, the higher differences in water use between native and exotic systems were observed in the most xeric areas, between native grasslands and high density ponderosa pine plantations. On the other hand, rain interception was higher in native forests than in forests plantations in the xeric area, resulting in similar whole evapotranspiration of both systems, whereas it was slightly higher in the forests plantations than in native systems in the humid area. This highlight the importance of considering the spatial distribution, density and size of plantation patches in those situations in which water surplus has to be used for other purposes. In these situations, it is recommended to install low density plantations. In contrast, due to the high water surplus in the more humid areas (with aptitude for Douglas-fir plantations), the higher water consumption of this exotic species compared to the native systems, would not lead to a negative impact on water resources. Finally, we highlight that, due to the high water use efficiency of exotic species, the wood production costs in terms of water are much lower using these species than native ones

A microscopic semiclassical confining field equation for U(1)U(1) lattice gauge theory in 2+1 dimensions

We present a semiclassical nonlinear field equation for the confining field in 2+1--dimensional $U(1)$ lattice gauge theory (compact QED). The equation is derived directly from the underlying microscopic quantum Hamiltonian by means of truncation. Its nonlinearities express the dynamic creation of magnetic monopole currents leading to the confinement of the electric field between two static electric charges. We solve the equation numerically and show that it can be interpreted as a London relation in a dual superconductor.Comment: 21 pages, epsf postscript figures included, full postscript available at ftp://ftp.th.physik.uni-frankfurt.de/pub/cbest/micro.ps.Z or http://www.th.physik.uni-frankfurt.de/~cbest/pub.htm