7,216 research outputs found
Coherence-Preserving Quantum Bits
Real quantum systems couple to their environment and lose their intrinsic
quantum nature through the process known as decoherence. Here we present a
method for minimizing decoherence by making it energetically unfavorable. We
present a Hamiltonian made up solely of two-body interactions between four
two-level systems (qubits) which has a two-fold degenerate ground state. This
degenerate ground state has the property that any decoherence process acting on
an individual physical qubit must supply energy from the bath to the system.
Quantum information can be encoded into the degeneracy of the ground state and
such coherence-preserving qubits will then be robust to local decoherence at
low bath temperatures. We show how this quantum information can be universally
manipulated and indicate how this approach may be applied to a quantum dot
quantum computer.Comment: 5 pages, 1 figur
Molecular basis of gap junctional communication in the CNS of the leech Hirudo medicinalis
Gap junctions are intercellular channels that allow the passage of ions and small molecules between cells. In the nervous system, gap junctions mediate electrical coupling between neurons. Despite sharing a common topology and similar physiology, two unrelated gap junction protein families exist in the animal kingdom. Vertebrate gap junctions are formed by members of the connexin family, whereas invertebrate gap junctions are composed of innexin proteins. Here we report the cloning of two innexins from the leech Hirudo medicinalis. These innexins show a differential expression in the leech CNS: Hm-inx1 is expressed by every neuron in the CNS but not in glia, whereas Hm-inx2 is expressed in glia but not neurons. Heterologous expression in the paired Xenopus oocyte system demonstrated that both innexins are able to form functional homotypic gap junctions. Hm-inx1 forms channels that are not strongly gated. In contrast, Hm-inx2 forms channels that are highly voltage-dependent; these channels demonstrate properties resembling those of a double rectifier. In addition, Hm-inx1 and Hm-inx2 are able to cooperate to form heterotypic gap junctions in Xenopus oocytes. The behavior of these channels is primarily that predicted from the properties of the constituent hemichannels but also demonstrates evidence of an interaction between the two. This work represents the first demonstration of a functional gap junction protein from a Lophotrochozoan animal and supports the hypothesis that connexin-based communication is restricted to the deuterostome clade
Contamination
Soil contamination occurs when substances are added to soil, resulting in increases in concentrations
above background or reference levels. Pollution may follow from contamination when contaminants
are present in amounts that are detrimental to soil quality and become harmful to the environment or
human health. Contamination can occur via a range of pathways including direct application to land and
indirect application from atmospheric deposition.
Contamination was identified by SEPA (2001) as a significant threat to soil quality in many parts of
Scotland. Towers et al. (2006) identified four principal contamination threats to Scottish soils: acidification;
eutrophication; metals; and pesticides. The Scottish Soil Framework (Scottish Government, 2009) set out
the potential impact of these threats on the principal soil functions.
Severe contamination can lead to âcontaminated landâ [as defined under Part IIA of the Environmental
Protection Act (1990)]. This report does not consider the state and impacts of contaminated land on
the wider environment in detail. For further information on contaminated land, see âDealing with Land
Contamination in Scotlandâ (SEPA, 2009).
This chapter considers the causes of soil contamination and their environmental and socio-economic
impacts before going on to discuss the status of, and trends in, levels of contaminants in Scotlandâs soils
Weak Gravitational Flexion
Flexion is the significant third-order weak gravitational lensing effect
responsible for the weakly skewed and arc-like appearance of lensed galaxies.
Here we demonstrate how flexion measurements can be used to measure galaxy halo
density profiles and large-scale structure on non-linear scales, via
galaxy-galaxy lensing, dark matter mapping and cosmic flexion correlation
functions. We describe the origin of gravitational flexion, and discuss its
four components, two of which are first described here. We also introduce an
efficient complex formalism for all orders of lensing distortion. We proceed to
examine the flexion predictions for galaxy-galaxy lensing, examining isothermal
sphere and Navarro, Frenk & White (NFW) profiles and both circularly symmetric
and elliptical cases. We show that in combination with shear we can precisely
measure galaxy masses and NFW halo concentrations. We also show how flexion
measurements can be used to reconstruct mass maps in 2-D projection on the sky,
and in 3-D in combination with redshift data. Finally, we examine the
predictions for cosmic flexion, including convergence-flexion
cross-correlations, and find that the signal is an effective probe of structure
on non-linear scales.Comment: 17 pages, including 12 figures, submitted to MNRA
The Stability of Quantum Concatenated Code Hamiltonians
Protecting quantum information from the detrimental effects of decoherence
and lack of precise quantum control is a central challenge that must be
overcome if a large robust quantum computer is to be constructed. The
traditional approach to achieving this is via active quantum error correction
using fault-tolerant techniques. An alternative to this approach is to engineer
strongly interacting many-body quantum systems that enact the quantum error
correction via the natural dynamics of these systems. Here we present a method
for achieving this based on the concept of concatenated quantum error
correcting codes. We define a class of Hamiltonians whose ground states are
concatenated quantum codes and whose energy landscape naturally causes quantum
error correction. We analyze these Hamiltonians for robustness and suggest
methods for implementing these highly unnatural Hamiltonians.Comment: 18 pages, small corrections and clarification
Few-body spin couplings and their implications for universal quantum computation
Electron spins in semiconductor quantum dots are promising candidates for the
experimental realization of solid-state qubits. We analyze the dynamics of a
system of three qubits arranged in a linear geometry and a system of four
qubits arranged in a square geometry. Calculations are performed for several
quantum dot confining potentials. In the three-qubit case, three-body effects
are identified that have an important quantitative influence upon quantum
computation. In the four-qubit case, the full Hamiltonian is found to include
both three-body and four-body interactions that significantly influence the
dynamics in physically relevant parameter regimes. We consider the implications
of these results for the encoded universality paradigm applied to the
four-electron qubit code; in particular, we consider what is required to
circumvent the four-body effects in an encoded system (four spins per encoded
qubit) by the appropriate tuning of experimental parameters.Comment: 1st version: 33 pages, 25 figures. Described at APS March Meeting in
2004 (P36.010) and 2005 (B17.00009). Most figures made uglier here to reduce
file size. 2nd version: 19 pages, 9 figures. Much mathematical detail chopped
away after hearing from journal referee; a few typos correcte
Arkansas Wheat Cultivar Performance Tests 2010-2011
Wheat cultivar performance tests are conducted each year in Arkansas by the Arkansas Agricultural Experiment Station, Department of Crop, Soil and Environmental Sciences. The tests provide information to companies developing cultivars and/or marketing seed within the state and aid the Arkansas Cooperative Extension Service in formulating cultivar recommendations for small-grain producers
Using continuous measurement to protect a universal set of quantum gates within a perturbed decoherence-free subspace
We consider a universal set of quantum gates encoded within a perturbed
decoherence-free subspace of four physical qubits. Using second-order
perturbation theory and a measuring device modeled by an infinite set of
harmonic oscillators, simply coupled to the system, we show that continuous
observation of the coupling agent induces inhibition of the decoherence due to
spurious perturbations. We thus advance the idea of protecting or even creating
a decoherence-free subspace for processing quantum information.Comment: 7 pages, 1 figure. To be published in Journal of Physics A:
Mathematical and Genera
Distribution, Conservation and Current Status of Three Endemic Heterosternuta (Coleoptera: Dytiscidae: Hydroporinae) in Arkansas
- âŠ