481 research outputs found
Trapping and coherent manipulation of a Rydberg atom on a microfabricated device: a proposal
We propose to apply atom-chip techniques to the trapping of a single atom in
a circular Rydberg state. The small size of microfabricated structures will
allow for trap geometries with microwave cut-off frequencies high enough to
inhibit the spontaneous emission of the Rydberg atom, paving the way to
complete control of both external and internal degrees of freedom over very
long times. Trapping is achieved using carefully designed electric fields,
created by a simple pattern of electrodes. We show that it is possible to
excite, and then trap, one and only one Rydberg atom from a cloud of ground
state atoms confined on a magnetic atom chip, itself integrated with the
Rydberg trap. Distinct internal states of the atom are simultaneously trapped,
providing us with a two-level system extremely attractive for atom-surface and
atom-atom interaction studies. We describe a method for reducing by three
orders of magnitude dephasing due to Stark shifts, induced by the trapping
field, of the internal transition frequency. This allows for, in combination
with spin-echo techniques, maintenance of an internal coherence over times in
the second range. This method operates via a controlled light shift rendering
the two internal states' Stark shifts almost identical. We thoroughly identify
and account for sources of imperfection in order to verify at each step the
realism of our proposal.Comment: Accepted in EPJ
Breakdown of superfluidity of an atom laser past an obstacle
The 1D flow of a continuous beam of Bose-Einstein condensed atoms in the
presence of an obstacle is studied as a function of the beam velocity and of
the type of perturbing potential (representing the interaction of the obstacle
with the atoms of the beam). We identify the relevant regimes:
stationary/time-dependent and superfluid/dissipative; the absence of drag is
used as a criterion for superfluidity. There exists a critical velocity below
which the flow is superfluid. For attractive obstacles, we show that this
critical velocity can reach the value predicted by Landau's approach. For
penetrable obstacles, it is shown that superfluidity is recovered at large beam
velocity. Finally, enormous differences in drag occur when switching from
repulsive to attractive potential.Comment: 15 pages, 6 figure
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
Trapping cold atoms near carbon nanotubes: thermal spin flips and Casimir-Polder potential
We investigate the possibility to trap ultracold atoms near the outside of a
metallic carbon nanotube (CN) which we imagine to use as a miniaturized
current-carrying wire. We calculate atomic spin flip lifetimes and compare the
strength of the Casimir-Polder potential with the magnetic trapping potential.
Our analysis indicates that the Casimir-Polder force is the dominant loss
mechanism and we compute the minimum distance to the carbon nanotube at which
an atom can be trapped.Comment: 8 pages, 3 figure
Coherence length of an elongated condensate: a study by matter-wave interferometry
We measure the spatial correlation function of Bose-Einstein condensates in
the cross-over region between phase-coherent and strongly phase-fluctuating
condensates. We observe the continuous path from a gaussian-like shape to an
exponential-like shape characteristic of one-dimensional phase-fluctuations.
The width of the spatial correlation function as a function of the temperature
shows that the condensate coherence length undergoes no sharp transition
between these two regimes.Comment: 8 pages, 6 figure, submitted to EPJ
Caries risk assessment in school children using a reduced Cariogram model without saliva tests
<p>Abstract</p> <p>Background</p> <p>To investigate the caries predictive ability of a reduced Cariogram model without salivary tests in schoolchildren.</p> <p>Methods</p> <p>The study group consisted of 392 school children, 10-11 years of age, who volunteered after informed consent. A caries risk assessment was made at baseline with aid of the computer-based Cariogram model and expressed as "the chance of avoiding caries" and the children were divided into five risk groups. The caries increment (ÎDMFS) was extracted from the dental records and bitewing radiographs after 2 years. The reduced Cariogram was processed by omitting the variables "salivary mutans streptococci", "secretion rate" and "buffer capacity" one by one and finally all three. Differences between the total and reduced models were expressed as area under the ROC-curve.</p> <p>Results</p> <p>The baseline caries prevalence in the study population was 40% (mean DMFS 0.87 ± 1.35) and the mean 2-year caries increment was 0.51 ± 1.06. Both Cariogram models displayed a statistically relationship with caries development (p < 0.05); more caries was found among those assessed with high risk compared to those with low risk. The combined sensitivity and specificity decreased after exclusion of the salivary tests and a statistically significant reduction of the area under the ROC-curve was displayed compared with the total Cariogram (p < 0.05). Among the salivary variables, omission of the mutans streptococci enumeration impaired the predictive ability the most.</p> <p>Conclusions</p> <p>The accuracy of caries prediction in school children was significantly impaired when the Cariogram model was applied without enumeration of salivary tests.</p
A Nanofiber-Based Optical Conveyor Belt for Cold Atoms
We demonstrate optical transport of cold cesium atoms over millimeter-scale
distances along an optical nanofiber. The atoms are trapped in a
one-dimensional optical lattice formed by a two-color evanescent field
surrounding the nanofiber, far red- and blue-detuned with respect to the atomic
transition. The blue-detuned field is a propagating nanofiber-guided mode while
the red-detuned field is a standing-wave mode which leads to the periodic axial
confinement of the atoms. Here, this standing wave is used for transporting the
atoms along the nanofiber by mutually detuning the two counter-propagating
fields which form the standing wave. The performance and limitations of the
nanofiber-based transport are evaluated and possible applications are
discussed
Quantum Limits of Stochastic Cooling of a Bosonic Gas
The quantum limits of stochastic cooling of trapped atoms are studied. The
energy subtraction due to the applied feedback is shown to contain an
additional noise term due to atom-number fluctuations in the feedback region.
This novel effect is shown to dominate the cooling efficiency near the
condensation point. Furthermore, we show first results that indicate that
Bose--Einstein condensation could be reached via stochastic cooling.Comment: 5 pages, 3 figures, to appear in Phys. Rev.
Robust entanglement
It is common belief among physicists that entangled states of quantum systems
loose their coherence rather quickly. The reason is that any interaction with
the environment which distinguishes between the entangled sub-systems collapses
the quantum state. Here we investigate entangled states of two trapped Ca
ions and observe robust entanglement lasting for more than 20 seconds
Highly anisotropic Bose-Einstein condensates: crossover to lower dimensionality
We develop a simple analytical model based on a variational method to explain
the properties of trapped cylindrically symmetric Bose-Einstein condensates
(BEC) of varying degrees of anisotropy well into regimes of effective one
dimension (1D) and effective two dimension (2D). Our results are accurate in
regimes where the Thomas-Fermi approximation breaks down and they are shown to
be in agreement with recent experimental data.Comment: 4 pages, 2 figures; significantly more new material added; title and
author-list changed due to changes in conten
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