463 research outputs found
A Delayed Choice Quantum Eraser
This paper reports a "delayed choice quantum eraser" experiment proposed by
Scully and Dr\"{u}hl in 1982. The experimental results demonstrated the
possibility of simultaneously observing both particle-like and wave-like
behavior of a quantum via quantum entanglement. The which-path or both-path
information of a quantum can be erased or marked by its entangled twin even
after the registration of the quantum.Comment: twocolumn, 4pages, submitted to PR
Signatures of non-locality in the first-order coherence of the scattered light
The spatial coherence of an atomic wavepacket can be detected in the
scattered photons, even when the center-of-mass motion is in the quantum
coherent superposition of two distant, non-overlapping wave packets. Spatial
coherence manifests itself in the power spectrum of the emitted photons, whose
spectral components can exhibit interference fringes as a function of the
emission angle. The contrast and the phase of this interference pattern provide
information about the quantum state of the center of mass of the scattering
atom.Comment: 5 pages, one figure, submitted to Laser Physics, special issue in
memory of Herbert Walthe
Harmonic generation by atoms in circularly polarized two-color laser fields with coplanar polarizations and commensurate frequencies
The generation of harmonics by atoms or ions in a two-color, coplanar field
configuration with commensurate frequencies is investigated through both, an
analytical calculation based on the Lewenstein model and the numerical ab
initio solution of the time-dependent Schroedinger equation of a
two-dimensional model ion. Through the analytical model, selection rules for
the harmonic orders in this field configuration, a generalized cut-off for the
harmonic spectra, and an integral expression for the harmonic dipole strength
is provided. The numerical results are employed to test the predictions of the
analytical model. The scaling of the cut-off as a function of both, one of the
laser intensities and frequency ratio , as well as entire spectra for
different and laser intensities are presented and analyzed. The
theoretical cut-off is found to be an upper limit for the numerical results.
Other discrepancies between analytical model and numerical results are
clarified by taking into account the probabilities of the absorption processes
involved.Comment: 8 figure
Complementarity and Young's interference fringes from two atoms
The interference pattern of the resonance fluorescence from a J=1/2 to J=1/2
transition of two identical atoms confined in a three-dimensional harmonic
potential is calculated. Thermal motion of the atoms is included. Agreement is
obtained with experiments [Eichmann et al., Phys. Rev. Lett. 70, 2359 (1993)].
Contrary to some theoretical predictions, but in agreement with the present
calculations, a fringe visibility greater than 50% can be observed with
polarization-selective detection. The dependence of the fringe visibility on
polarization has a simple interpretation, based on whether or not it is
possible in principle to determine which atom emitted the photon.Comment: 12 pages, including 7 EPS figures, RevTex. Submitted to Phys. Rev.
Two-atom dark states in electromagnetic cavities
The center-of-mass motion of two two-level atoms coupled to a single damped
mode of an electromagnetic resonator is investigated. For the case of one atom
being initially excited and the cavity mode in the vacuum state it is shown
that the atomic time evolution is dominated by the appearance of dark states.
These states, in which the initial excitation is stored in the internal atomic
degrees of freedom and the atoms become quantum mechanically entangled, are
almost immune against photon loss from the cavity. Various properties of the
dark states within and beyond the Raman-Nath approximation of atom optics are
worked out.Comment: 8 pages, 4 figure
Binding of an antimicrobial peptide to bacterial cells: interaction with different species, strains and cellular components
Antimicrobial peptides (AMPs) selectively kill bacteria by disrupting their cell membranes, and are promising compounds to fight drug-resistant microbes. Biophysical studies on model membranes have characterized AMP/membrane interactions and the mechanism of bilayer perturbation, showing that accumulation of cationic peptide molecules in the external leaflet leads to the formation of pores ("carpet" mechanism). However, similar quantitative studies on real cells are extremely limited. Here, we investigated the interaction of the dansylated PMAP23 peptide (DNS-PMAP23) with a Gram-positive bacterium, showing that 10(7) bound peptide molecules per cell are needed to kill it. This result is consistent with our previous finding for Gram-negative strains, where a similar high threshold for killing was determined, demonstrating the general relevance of the carpet model for real bacteria. However, in the case of the Gram-positive strain, this number of molecules even exceeds the total surface available on the bacterial membrane. The high affinity of DNS-PMAP23 for the anionic teichoic acids of the Gram-positive cell wall, but not for the lipopolysaccharides of Gram-negative bacteria, provides a rationale for this finding. To better define the role of anionic lipids in peptide/cell association, we studied DNS-PMAP23 interaction with E. coli mutant strains lacking phosphatidylglycerol and/or cardiolipin. Surprisingly, these strains showed a peptide affinity similar to that of the wild type. This finding was rationalized by observing that these bacteria have an increased content of other anionic lipids, thus maintaining the total membrane charge essentially constant. Finally, studies of DNS-PMAP23 association to dead bacteria showed an affinity an order of magnitude higher compared to that of live cells, suggesting strong peptide binding to intracellular components that become accessible after membrane perturbation. This effect could play a role in population resistance to AMP action, since dead bacteria could protect the surviving cells by sequestering significant amounts of peptide molecules. Overall, our data indicate that quantitative studies of peptide association to bacteria can lead to a better understanding of the mechanism of action of AMPs
Finding periodic orbits in state-dependent delay differential equations as roots of algebraic equations
In this paper we prove that periodic boundary-value problems (BVPs) for delay
differential equations are locally equivalent to finite-dimensional algebraic
systems of equations. We rely only on regularity assumptions that follow those
of the review by Hartung et al. (2006). Thus, the equivalence result can be
applied to differential equations with state-dependent delays (SD-DDEs),
transferring many results of bifurcation theory for periodic orbits to this
class of systems. We demonstrate this by using the equivalence to give an
elementary proof of the Hopf bifurcation theorem for differential equations
with state-dependent delays. This is an alternative and extension to the
original Hopf bifurcation theorem for SD-DDEs by Eichmann (2006).Comment: minor revision, correcting mistakes in formulation of Lemma 2.3 and
A.5 (which are also present in the Journal paper): center of neighborhood
must be in , which is the case for the main theore
Quantitative wave-particle duality and non-erasing quantum erasure
The notion of wave-particle duality may be quantified by the inequality
V^2+K^2 <=1, relating interference fringe visibility V and path knowledge K.
With a single-photon interferometer in which polarization is used to label the
paths, we have investigated the relation for various situations, including
pure, mixed, and partially-mixed input states. A quantum eraser scheme has been
realized that recovers interference fringes even when no which-way information
is available to erase.Comment: 6 pages, 4 figures. To appear in Phys. Rev.
State sampling dependence of the Hopfield network inference
The fully connected Hopfield network is inferred based on observed
magnetizations and pairwise correlations. We present the system in the glassy
phase with low temperature and high memory load. We find that the inference
error is very sensitive to the form of state sampling. When a single state is
sampled to compute magnetizations and correlations, the inference error is
almost indistinguishable irrespective of the sampled state. However, the error
can be greatly reduced if the data is collected with state transitions. Our
result holds for different disorder samples and accounts for the previously
observed large fluctuations of inference error at low temperatures.Comment: 4 pages, 1 figure, further discussions added and relevant references
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Dynamic Analysis of Vascular Morphogenesis Using Transgenic Quail Embryos
Background: One of the least understood and most central questions confronting biologists is how initially simple clusters or sheet-like cell collectives can assemble into highly complex three-dimensional functional tissues and organs. Due to the limits of oxygen diffusion, blood vessels are an essential and ubiquitous presence in all amniote tissues and organs. Vasculogenesis, the de novo self-assembly of endothelial cell (EC) precursors into endothelial tubes, is the first step in blood vessel formation [1]. Static imaging and in vitro models are wholly inadequate to capture many aspects of vascular pattern formation in vivo, because vasculogenesis involves dynamic changes of the endothelial cells and of the forming blood vessels, in an embryo that is changing size and shape.
Methodology/Principal Findings: We have generated Tie1 transgenic quail lines Tg(tie1:H2B-eYFP) that express H2B-eYFP in all of their endothelial cells which permit investigations into early embryonic vascular morphogenesis with unprecedented clarity and insight. By combining the power of molecular genetics with the elegance of dynamic imaging, we follow the precise patterning of endothelial cells in space and time. We show that during vasculogenesis within the vascular plexus, ECs move independently to form the rudiments of blood vessels, all while collectively moving with gastrulating tissues that flow toward the embryo midline. The aortae are a composite of somatic derived ECs forming its dorsal regions and the splanchnic derived ECs forming its ventral region. The ECs in the dorsal regions of the forming aortae exhibit variable mediolateral motions as they move rostrally; those in more ventral regions show significant lateral-to-medial movement as they course rostrally.
Conclusions/Significance: The present results offer a powerful approach to the major challenge of studying the relative role(s) of the mechanical, molecular, and cellular mechanisms of vascular development. In past studies, the advantages of the molecular genetic tools available in mouse were counterbalanced by the limited experimental accessibility needed for imaging and perturbation studies. Avian embryos provide the needed accessibility, but few genetic resources. The creation of transgenic quail with labeled endothelia builds upon the important roles that avian embryos have played in previous studies of vascular development
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