Quantum networks with chiral light--matter interaction in waveguides

We propose a scalable architecture for a quantum network based on a simple on-chip photonic circuit that performs loss-tolerant two-qubit measurements. The circuit consists of two quantum emitters positioned in the arms of an on-chip Mach-Zehnder interferometer composed of waveguides with chiral light--matter interfaces. The efficient chiral light--matter interaction allows the emitters to perform high-fidelity intranode two-qubit parity measurements within a single chip, and to emit photons to generate internode entanglement, without any need for reconfiguration. We show that by connecting multiple circuits of this kind into a quantum network, it is possible to perform universal quantum computation with heralded two-qubit gate fidelities ${\cal F} \sim 0.998$ achievable in state-of-the-art quantum dot systems.Comment: 5 pages plus supplementary materia

Entanglement in Anderson Nanoclusters

We investigate the two-particle spin entanglement in magnetic nanoclusters described by the periodic Anderson model. An entanglement phase diagram is obtained, providing a novel perspective on a central property of magnetic nanoclusters, namely the temperature dependent competition between local Kondo screening and nonlocal Ruderman-Kittel-Kasuya-Yoshida spin ordering. We find that multiparticle entangled states are present for finite magnetic field as well as in the mixed valence regime and away from half filling. Our results emphasize the role of charge fluctuations.Comment: 5 pages, 3 figure

Dual Geometric Worm Algorithm for Two-Dimensional Discrete Classical Lattice Models

We present a dual geometrical worm algorithm for two-dimensional Ising models. The existence of such dual algorithms was first pointed out by Prokof'ev and Svistunov \cite{ProkofevClassical}. The algorithm is defined on the dual lattice and is formulated in terms of bond-variables and can therefore be generalized to other two-dimensional models that can be formulated in terms of bond-variables. We also discuss two related algorithms formulated on the direct lattice, applicable in any dimension. These latter algorithms turn out to be less efficient but of considerable intrinsic interest. We show how such algorithms quite generally can be "directed" by minimizing the probability for the worms to erase themselves. Explicit proofs of detailed balance are given for all the algorithms. In terms of computational efficiency the dual geometrical worm algorithm is comparable to well known cluster algorithms such as the Swendsen-Wang and Wolff algorithms, however, it is quite different in structure and allows for a very simple and efficient implementation. The dual algorithm also allows for a very elegant way of calculating the domain wall free energy.Comment: 12 pages, 6 figures, Revtex

Effects of green manure storage and incorporation methods on greenhouse gas fluxes and N mineralization after soil application

Organic arable farming faces challenges with low crop yields, partly due to inefficient use of green manure-derived nitrogen (N). Under current farming practices, green manure leys are often cut and mulched during the growing season with the associated risk of environmental N losses, leading to eutrophication and global warming. In this 3-month incubation experiment, we tested a new green manure management strategy as part of the ICROFS project HighCrop. With the new strategy, green manure leys are instead harvested and preserved until the following spring either as compost mixed with straw (grass-clover:straw, 4:1, w:w) or as silage of harvested ley biomass. In spring, these two green manure materials can then be used for targeted fertilization of spring sown crops. The objectives of the study were to: • Assess how storage methods (compost vs. silage) affect N2O fluxes and soil respiratory CO2 emissions after soil application of preserved grass-clover green manure. • Determine whether the greenhouse gas fluxes are influenced by the incorporation method, more specifically harrowing (simulated by mixing the material into the top 5 cm soil layer) and ploughing (the material placed at 15 cm depth). • Compare composted and ensiled green manures concerning their abilities to provide plant-available N during a 3-month period. During the experiment, gas fluxes were measured at nine occasion followed by eight destructive soil harvests. In total, the study included 192 soil units that were incubated at 15 °C in darkness. Each unit consisted of a packed soil core (26 cm high × 10 cm diameter) with bulk density of 1.07 g cm-3 and gravimetric soil moisture of 20 %. The addition of compost and silage corresponded to a fertilization rate of 120 kg total N ha-1. A mineral fertilizer treatment was included as a reference and received 80 kg NH4-N ha-1. Compared to the more degraded compost, the silage material had a high content of labile compound. In addition, incorporation of green manure by harrowing was expected to improve soil microbes’ access to the materials, and thereby increase the decomposition rate. In line with this, cumulative CO2 emissions from the green manure treatments was lowest for compost incorporated by ploughing and highest for silage incorporated by harrowing. Between 32 and 54 % of the added green manure carbon was respired as CO2 during the 3-month experiment. Interestingly, mineral fertilizer suppressed soil respiratory CO2 emission. Generally, N2O emissions were higher from the silage-amended soils than from soils fertilized with compost. Especially, silage incorporated by ploughing gave rise to increased N2O effluxes, corresponding to 0.3 % of applied total N during the 3-month period. This could partly result from denitrification of initial soil nitrate, stimulated by high local oxygen consumption in the labile silage layer. In contrast, compost incorporated by harrowing caused a downwards N2O flux into the soil, presumably an effect of lacking mineral N availability in this treatment. Overall, our study showed that emissions of N2O can be reduced by incorporating green manure using harrowing instead of ploughing. Net mineralization of green manure-derived N was absent until more than three weeks after incorporation of the materials. Over the 3-month experiment, grass-clover silage provided the highest net release of inorganic N with preliminary results corresponding to 38-43 kg N ha-1, irrespective of the incorporation method used. In contrast, no increase in soil mineral N was observed for the composted grass-clover and straw mixture compared to the unfertilized control soil. In fact, soil incorporation of compost by harrowing caused immobilization of soil mineral nitrogen 1-2 months after experimental set-up

Kløvergræs ensilage og kompost som grøngødning

En ny strategi kan måske forbedre udnyttelsen af det kvælstof, som fikseres af kløverplanter i økologisk planteavl. Vores studie viste en større frigivelse af kvælstof fra ensileret kløvergræs end fra kompost af kløvergræs iblandet halm. Samtidig fandt vi, at indarbejdning af grøngødning ved harvning frem for pløjning reducerede udledningen af den stærke drivhusgas, lattergas

Nitrogen mineralization and greenhouse gas emissions after soil incorporation of ensiled and composted grass-clover as green manure

This 3-month incubation study showed that ensiled grass-clover was a better nitrogen (N) source than a composted grass-clover and straw mix (grass-clover:straw, 4:1, w:w), owing to the high content of labile compounds compared to the more degraded compost. Our study also indicated that emissions of the strong greenhouse gas nitrous oxide (N2O) can be reduced by incorporating green manure using harrowing instead of ploughing. The silage-derived N release by the end of the incubation was equivalent to 38-42 kg N ha-1, which corresponded to one third of the N applied in silage, with no difference between ploughing and harrowing. In contrast, no net release of mineral N was detected from the composted grass-clover