1,372 research outputs found
Fano Lineshapes Revisited: Symmetric Photoionization Peaks from Pure Continuum Excitation
In a photoionization spectrum in which there is no excitation of the discrete
states, but only the underlying continuum, we have observed resonances which
appear as symmetric peaks, not the commonly expected window resonances.
Furthermore, since the excitation to the unperturbed continuum vanishes, the
cross section expected from Fano's configuration interaction theory is
identically zero. This shortcoming is removed by the explicit introduction of
the phase shifted continuum, which demonstrates that the shape of a resonance,
by itself, provides no information about the relative excitation amplitudes to
the discrete state and the continuum.Comment: 4 pages, 3 figure
Spatial separation in a thermal mixture of ultracold Yb and Rb atoms
We report on the observation of unusually strong interactions in a thermal
mixture of ultracold atoms which cause a significant modification of the
spatial distribution. A mixture of Rb and Yb with a temperature
of a few K is prepared in a hybrid trap consisting of a bichromatic
optical potential superimposed on a magnetic trap. For suitable trap parameters
and temperatures, a spatial separation of the two species is observed. We infer
that the separation is driven by a large interaction strength between
Yb and Rb accompanied by a large three-body recombination rate.
Based on this assumption we have developed a diffusion model which reproduces
our observations
Deterministic reordering of 40Ca+ ions in a linear segmented Paul trap
In the endeavour to scale up the number of qubits in an ion-based quantum
computer several groups have started to develop miniaturized ion traps for
extended spatial control and manipulation of the ions. Shuttling and separation
of ion strings have been the foremost issues in linear-trap arrangements and
some prototypes of junctions have been demonstrated for the extension of ion
motion to two dimensions (2D). While junctions require complex trap structures,
small extensions to the 1D motion can be accomplished in simple linear trap
arrangements. Here, control of the extended field in a planar, linear chip trap
is used to shuttle ions in 2D. With this approach, the order of ions in a
string is deterministically reversed. Optimized potentials are theoretically
derived and simulations show that the reordering can be carried out
adiabatically. The control over individual ion positions in a linear trap
presents a new tool for ion-trap quantum computing. The method is also expected
to work with mixed crystals of different ion species and as such could have
applications for sympathetic cooling of an ion string.Comment: 18 pages, 9 figures. Added section on possibility of adiabatic turn.
Added appendix on point charge model. Other minor alterations/clarifications.
Version now published (http://www.iop.org/EJ/abstract/1367-2630/11/10/103008
A Q-Ising model application for linear-time image segmentation
A computational method is presented which efficiently segments digital
grayscale images by directly applying the Q-state Ising (or Potts) model. Since
the Potts model was first proposed in 1952, physicists have studied lattice
models to gain deep insights into magnetism and other disordered systems. For
some time, researchers have realized that digital images may be modeled in much
the same way as these physical systems (i.e., as a square lattice of numerical
values). A major drawback in using Potts model methods for image segmentation
is that, with conventional methods, it processes in exponential time. Advances
have been made via certain approximations to reduce the segmentation process to
power-law time. However, in many applications (such as for sonar imagery),
real-time processing requires much greater efficiency. This article contains a
description of an energy minimization technique that applies four Potts
(Q-Ising) models directly to the image and processes in linear time. The result
is analogous to partitioning the system into regions of four classes of
magnetism. This direct Potts segmentation technique is demonstrated on
photographic, medical, and acoustic images.Comment: 7 pages, 8 figures, revtex, uses subfigure.sty. Central European
Journal of Physics, in press (2010
Process tomography of ion trap quantum gates
A crucial building block for quantum information processing with trapped ions
is a controlled-NOT quantum gate. In this paper, two different sequences of
laser pulses implementing such a gate operation are analyzed using quantum
process tomography. Fidelities of up to 92.6(6)% are achieved for single gate
operations and up to 83.4(8)% for two concatenated gate operations. By process
tomography we assess the performance of the gates for different experimental
realizations and demonstrate the advantage of amplitude--shaped laser pulses
over simple square pulses. We also investigate whether the performance of
concatenated gates can be inferred from the analysis of the single gates
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