20,299 research outputs found
Hybrid solid state qubits: the powerful role of electron spins
We review progress on the use of electron spins to store and process quantum
information, with particular focus on the ability of the electron spin to
interact with multiple quantum degrees of freedom. We examine the benefits of
hybrid quantum bits (qubits) in the solid state that are based on coupling
electron spins to nuclear spin, electron charge, optical photons, and
superconducting qubits. These benefits include the coherent storage of qubits
for times exceeding seconds, fast qubit manipulation, single qubit measurement,
and scalable methods for entangling spatially separated matter-based qubits. In
this way, the key strengths of different physical qubit implementations are
brought together, laying the foundation for practical solid-state quantum
technologies.Comment: 54 pages, 7 figure
Metabolite essentiality elucidates robustness of Escherichia coli metabolism
Complex biological systems are very robust to genetic and environmental
changes at all levels of organization. Many biological functions of Escherichia
coli metabolism can be sustained against single-gene or even multiple-gene
mutations by using redundant or alternative pathways. Thus, only a limited
number of genes have been identified to be lethal to the cell. In this regard,
the reaction-centric gene deletion study has a limitation in understanding the
metabolic robustness. Here, we report the use of flux-sum, which is the
summation of all incoming or outgoing fluxes around a particular metabolite
under pseudo-steady state conditions, as a good conserved property for
elucidating such robustness of E. coli from the metabolite point of view. The
functional behavior, as well as the structural and evolutionary properties of
metabolites essential to the cell survival, was investigated by means of a
constraints-based flux analysis under perturbed conditions. The essential
metabolites are capable of maintaining a steady flux-sum even against severe
perturbation by actively redistributing the relevant fluxes. Disrupting the
flux-sum maintenance was found to suppress cell growth. This approach of
analyzing metabolite essentiality provides insight into cellular robustness and
concomitant fragility, which can be used for several applications, including
the development of new drugs for treating pathogens.Comment: Supplements available at
http://stat.kaist.ac.kr/publication/2007/PJKim_pnas_supplement.pd
Three-dimensional parton distribution functions and in the polarized proton-antiproton Drell-Yan process
We present predictions of the unweighted and weighted double spin asymmetries
related to the transversal helicity distribution and the longitudinal
transversity distribution , two of eight leading-twist transverse
momentum dependent parton distributions (TMDs) or three-dimensional parton
distribution functions (3dPDFs), in the polarized proton-antiproton Drell-Yan
process at typical kinematics on the Facility for Antiproton and Ion Research
(FAIR). We conclude that FAIR is ideal to access the new 3dPDFs towards a
detailed picture of the nucleon structure.Comment: 6 latex pages, 5 figures, version for publication in EPJ
Optical fibers with interferometric path length stability by controlled heating for transmission of optical signals and as components in frequency standards
We present a simple method to stabilize the optical path length of an optical
fiber to an accuracy of about 1/100 of the laser wavelength. We study the
dynamic response of the path length to modulation of an electrically conductive
heater layer of the fiber. The path length is measured against the laser
wavelength by use of the Pound-Drever-Hall method; negative feedback is applied
via the heater. We apply the method in the context of a cryogenic resonator
frequency standard.Comment: Expanded introduction and outlook. 9 pages, 5 figure
Coherence of qubits based on single Ca ions
Two-level ionic systems, where quantum information is encoded in long lived
states (qubits), are discussed extensively for quantum information processing.
We present a collection of measurements which characterize the stability of a
qubit based on the -- transition of single Ca ions
in a linear Paul trap. We find coherence times of 1 ms, discuss the
main technical limitations and outline possible improvements.Comment: Proceedings of "Trapped charged particles and fundamental
interactions" submitted to Journal of Physics B (IoP
Azimuthal Spin Asymmetries of Pion Electroproduction
Azimuthal spin asymmetries, both for charged and neutral pion production in
semi-inclusive deep inelastic scattering of unpolarized charged lepton beams on
longitudinally and transversely polarized nucleon targets, are analyzed and
calculated. Various assumptions and approximations in the quark distributions
and fragmentation functions often used in these calculations are studied in
detail. It is found that different approaches to the distribution and
fragmentation functions may lead to quite different predictions on the
azimuthal asymmetries measured in the HERMES experiments, thus their effects
should be taken into account before using the available data as a measurement
of quark transversity distributions. It is also found that the unfavored quark
to pion fragmentation functions must be taken into account for
production from a proton target, although they can be neglected for and
production. Pion production from a proton target is suitable to study
the quark transversity distribution, whereas a combination of pion
production from both proton and neutron targets can measure the flavor
structure of quark transversity distributions.Comment: 31 latex pages, 13 figure, to appear in PR
The Origins of Phase Transitions in Small Systems
The identification and classification of phases in small systems, e.g.
nuclei, social and financial networks, clusters, and biological systems, where
the traditional definitions of phase transitions are not applicable, is
important to obtain a deeper understanding of the phenomena observed in such
systems. Within a simple statistical model we investigate the validity and
applicability of different classification schemes for phase transtions in small
systems. We show that the whole complex temperature plane contains necessary
information in order to give a distinct classification.Comment: 3 pages, 4 figures, revtex 4 beta 5, for further information see
http://www.smallsystems.d
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