3,305 research outputs found
Development of Collembolans after coversion towards organic farming
In Northern Germany, a diverse and complex experimental farm of the Federal Agricultural Research Centre (FAL) was set-up in 2001 covering all main aspects of organic farming. Previously, the 600 ha farm had been managed conventionally. Adjacent conventional farms were used as reference. The aim of this project was to study collembolans, microbial biomass and soil organic carbon in six organically farmed fields managed as a crop rotation of six different crops compared with an adjacent conventionally managed field. We hypothesised that the specific management in organic farming promotes soil biota. Soil samples were taken during the growing season in 2004. Collembolan abundances and microbial biomass were lower under organic management, but, generally, collembolan diversity was higher in organically farmed fields combined with a shifting in the dominance structure of the species. This result reveals that, even after three years, the soil biota is still changing with management conversion
The inexorable resistance of inertia determines the initial regime of drop coalescence
Drop coalescence is central to diverse processes involving dispersions of
drops in industrial, engineering and scientific realms. During coalescence, two
drops first touch and then merge as the liquid neck connecting them grows from
initially microscopic scales to a size comparable to the drop diameters. The
curvature of the interface is infinite at the point where the drops first make
contact, and the flows that ensue as the two drops coalesce are intimately
coupled to this singularity in the dynamics. Conventionally, this process has
been thought to have just two dynamical regimes: a viscous and an inertial
regime with a crossover region between them. We use experiments and simulations
to reveal that a third regime, one that describes the initial dynamics of
coalescence for all drop viscosities, has been missed. An argument based on
force balance allows the construction of a new coalescence phase diagram
Delta-like and gtl2 are reciprocally expressed, differentially methylated linked imprinted genes on mouse chromosome 12
AbstractThe distal portion of mouse chromosome 12 is imprinted. To date, however, Gtl2 is the only imprinted gene identified on chromosome 12. Gtl2 encodes multiple alternatively spliced transcripts with no apparent open reading frame. Using conceptuses with maternal or paternal uniparental disomy for chromosome 12 (UPD12), we found that Gtl2 is expressed from the maternal allele and methylated at the 5′ end of the silent paternal allele. A reciprocally imprinted gene, Delta-like (Dlk), with homology to genes involved in the Notch signalling pathway was identified 80kb upstream of Gtl2. Dlk was expressed exclusively from the paternal allele in both the embryo and placenta, but the CpG-island promoter of Dlk was completely unmethylated on both parental alleles. Rather, a paternally methylated region was identified in the last exon of the active Dlk allele. The proximity, reciprocal imprinting and methylation in this domain are reminiscent of the co-ordinately regulated Igf2–H19 imprinted domain on mouse chromosome 7. Like H19 and Igf2, Gtl2 and Dlk were found to be co-expressed in the same tissues throughout development, though not after birth. These results have implications for the regulation, function and evolution of imprinted domains
Superconductivity and magnetic order in the non-centrosymmetric Half Heusler compound ErPdBi
We report superconductivity at K and magnetic order at K in the semi-metallic noncentrosymmetric Half Heusler compound ErPdBi.
The upper critical field, , has an unusual quasi-linear temperature
variation and reaches a value of 1.6 T for . Magnetic order is
found below and is suppressed at T for . Since , the interaction of superconductivity and magnetism
is expected to give rise to a complex ground state. Moreover, electronic
structure calculations show ErPdBi has a topologically nontrivial band
inversion and thus may serve as a new platform to study the interplay of
topological states, superconductivity and magnetic order.Comment: 6 pages, 5 figures; accepted for publication in Europhysics Letter
Decoherence and Entanglement Dynamics in Fluctuating Fields
We study pure phase damping of two qubits due to fluctuating fields. As
frequently employed, decoherence is thus described in terms of random unitary
(RU) dynamics, i.e., a convex mixture of unitary transformations. Based on a
separation of the dynamics into an average Hamiltonian and a noise channel, we
are able to analytically determine the evolution of both entanglement and
purity. This enables us to characterize the dynamics in a concurrence-purity
(CP) diagram: we find that RU phase damping dynamics sets constraints on
accessible regions in the CP plane. We show that initial state and dynamics
contribute to final entanglement independently.Comment: 10 pages, 5 figures, added minor changes in order to match published
versio
Dynamics and thermalization of the nuclear spin bath in the single-molecule magnet Mn12-ac: test for the theory of spin tunneling
The description of the tunneling of a macroscopic variable in the presence of
a bath of localized spins is a subject of great fundamental and practical
interest, and is relevant for many solid-state qubit designs. Instead of
focusing on the the "central spin" (as is most often done), here we present a
detailed study of the dynamics of the nuclear spin bath in the Mn12-ac
single-molecule magnet, probed by NMR experiments down to very low temperatures
(T = 20 mK). We find that the longitudinal relaxation rate of the 55Mn nuclei
in Mn12-ac becomes roughly T-independent below T = 0.8 K, and can be strongly
suppressed with a longitudinal magnetic field. This is consistent with the
nuclear relaxation being caused by quantum tunneling of the molecular spin, and
we attribute the tunneling fluctuations to the minority of fast-relaxing
molecules present in the sample. The transverse nuclear relaxation is also
T-independent for T < 0.8 K, and can be explained qualitatively and
quantitatively by the dipolar coupling between like nuclei in neighboring
molecules. We also show that the isotopic substitution of 1H by 2H leads to a
slower nuclear longitudinal relaxation, consistent with the decreased tunneling
probability of the molecular spin. Finally, we demonstrate that, even at the
lowest temperatures, the nuclear spins remain in thermal equilibrium with the
lattice phonons, and we investigate the timescale for their thermal
equilibration. After a review of the theory of macroscopic spin tunneling in
the presence of a spin bath, we argue that most of our experimental results are
consistent with that theory, but the thermalization of the nuclear spins is
not.Comment: 24 pages, 18 figures. Experimental study of the spin bath dynamics in
quantum nanomagnets, plus an extensive review and application of the theor
Conservative Quantum Computing
Conservation laws limit the accuracy of physical implementations of
elementary quantum logic gates. If the computational basis is represented by a
component of spin and physical implementations obey the angular momentum
conservation law, any physically realizable unitary operators with size less
than n qubits cannot implement the controlled-NOT gate within the error
probability 1/(4n^2), where the size is defined as the total number of the
computational qubits and the ancilla qubits. An analogous limit for bosonic
ancillae is also obtained to show that the lower bound of the error probability
is inversely proportional to the average number of photons. Any set of
universal gates inevitably obeys a related limitation with error probability
O(1/n^2)$. To circumvent the above or related limitations yielded by
conservation laws, it is recommended that the computational basis should be
chosen as the one commuting with the additively conserved quantities.Comment: 5 pages, RevTex. Corrected to include a new statement that for
bosonic ancillae the lower bound of the error probability is inversely
proportional to the average number of photons, kindly suggested by Julio
Gea-Banacloch
Strongly Incompatible Quantum Devices
The fact that there are quantum observables without a simultaneous
measurement is one of the fundamental characteristics of quantum mechanics. In
this work we expand the concept of joint measurability to all kinds of possible
measurement devices, and we call this relation compatibility. Two devices are
incompatible if they cannot be implemented as parts of a single measurement
setup. We introduce also a more stringent notion of incompatibility, strong
incompatibility. Both incompatibility and strong incompatibility are rigorously
characterized and their difference is demonstrated by examples.Comment: 27 pages (AMSart), 6 figure
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