4,854 research outputs found
Species Diversity and Population Regulation: The Importance of Environmental Feedback Dimensionality
Atomic cluster state build up with macroscopic heralding
We describe a measurement-based state preparation scheme for the efficient
build up of cluster states in atom-cavity systems. As in a recent proposal for
the generation of maximally entangled atom pairs [Metz et al., Phys. Rev. Lett.
97, 040503 (2006)], we use an electron shelving technique to avoid the
necessity for the detection of single photons. Instead, the successful fusion
of smaller into larger clusters is heralded by an easy-to-detect macroscopic
fluorescence signal. High fidelities are achieved even in the vicinity of the
bad cavity limit and are essentially independent of the concrete size of the
system parameters.Comment: 14 pages, 12 figures; minor changes, mainly clarification
Macroscopic quantum jumps and entangled state preparation
Recently we predicted a random blinking, i.e. macroscopic quantum jumps, in
the fluorescence of a laser-driven atom-cavity system [Metz et al., Phys. Rev.
Lett. 97, 040503 (2006)]. Here we analyse the dynamics underlying this effect
in detail and show its robustness against parameter fluctuations. Whenever the
fluorescence of the system stops, a macroscopic dark period occurs and the
atoms are shelved in a maximally entangled ground state. The described setup
can therefore be used for the controlled generation of entanglement. Finite
photon detector efficiencies do not affect the success rate of the state
preparation, which is triggered upon the observation of a macroscopic
fluorescence signal. High fidelities can be achieved even in the vicinity of
the bad cavity limit due to the inherent role of dissipation in the jump
process.Comment: 14 pages, 12 figures, proof of the robustness of the state
preparation against parameter fluctuations added, figure replace
Universality of soft and collinear factors in hard-scattering factorization
Universality in QCD factorization of parton densities, fragmentation
functions, and soft factors is endangered by the process dependence of the
directions of Wilson lines in their definitions. We find a choice of directions
that is consistent with factorization and that gives universality between
e^+e^- annihilation, semi-inclusive deep-inelastic scattering, and the
Drell-Yan process. Universality is only modified by a time-reversal
transformation of the soft function and parton densities between Drell-Yan and
the other processes, whose only effect is the known reversal of sign for T-odd
parton densities like the Sivers function. The modifications of the definitions
needed to remove rapidity divergences with light-like Wilson lines do not
affect the results.Comment: 4 pages. Extra references. Text and references as in published
versio
Proper definition and evolution of generalized transverse momentum distributions
We consider one of the most fundamental sets of hadronic matrix elements,
namely the generalized transverse momentum distributions (GTMDs), and argue
that their existing definitions lack proper evolution properties. By exploiting
the similarity of GTMDs with the much better understood transverse momentum
distributions, we argue that the existing definitions of GTMDs have to include
an additional dependence on soft gluon radiation in order to render them
properly defined. With this, we manage to obtain the evolution kernel of all
(un)polarized quark and gluon GTMDs, which turns out to be spin independent. As
a byproduct, all large logarithms can be resummed up to
next-to-next-to-leading-logarithmic accuracy with the currently known
perturbative ingredients.Comment: v2: typo in eq.(19) corrected. Matches published version in PLB. v1:
9 pages, 2 figure
Dispersion relation formalism for virtual Compton scattering and the generalized polarizabilities of the nucleon
A dispersion relation formalism for the virtual Compton scattering (VCS)
reaction on the proton is presented, which for the first time allows a
dispersive evaluation of 4 generalized polarizabilities at a four-momentum
transfer 0.5 GeV. The dispersive integrals are calculated using
a state-of-the-art pion photo- and electroproduction analysis. The dispersion
formalism provides a new tool to analyze VCS experiments above pion threshold,
thus increasing the sensitivity to the generalized polarizabilities of the
nucleon.Comment: 4 pages, 2 figure
Dynamics of Similar Populations: The Link Between Population Dynamics and Evolution
We provide the link between population dynamics and the dynamics of Darwinian evolution via studying the joint population dynamics of "similar" populations. Similarity implies that the "relative" dynamics of the populations is slow compared to, and decoupled from, their "aggregated" dynamics. The relative dynamics is simple, and captured by a Taylor expansion in the difference between the populations. The emerging evolution is directional, except at the "singular" points of the evolutionary state space, where "evolutionary branching" may happen
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