447 research outputs found
Quantum Repeaters based on Single Trapped Ions
We analyze the performance of a quantum repeater protocol based on single
trapped ions. At each node, single trapped ions embedded into high finesse
cavities emit single photons whose polarization is entangled with the ion
state. A specific detection of two photons at a central station located
half-way between two nodes heralds the entanglement of two remote ions.
Entanglement can be extended to long distances by applying successive
entanglement swapping operations based on two-ion gate operations that have
already been demonstrated experimentally with high precision. Our calculation
shows that the distribution rate of entanglement achievable with such an
ion-based quantum repeater protocol is higher by orders of magnitude than the
rates that are achievable with the best known schemes based on atomic ensemble
memories and linear optics. The main reason is that for trapped ions the
entanglement swapping operations are performed deterministically, in contrast
to success probabilities below 50 percent per swapping with linear optics. The
scheme requires efficient collection of the emitted photons, which can be
achieved with cavities, and efficient conversion of their wavelength, which can
be done via stimulated parametric down-conversion. We also suggest how to
realize temporal multiplexing, which offers additional significant speed-ups in
entanglement distribution, with trapped ions
Electric field noise above surfaces: a model for heating rate scaling law in ion traps
We present a model for the scaling laws of the electric field noise spectral
density as a function of the distance, , above a conducting surface. Our
analytical approach models the patch potentials by introducing a correlation
length, , of the electric potential on the surface. The predicted
scaling laws are in excellent agreement with two different classes of
experiments (cold trapped ions and cantilevers), that span at least four orders
of magnitude of . According to this model, heating rate in miniature ion
traps could be greatly reduced by proper material engineering
Breakdown of scale invariance in a quasi-two-dimensional Bose gas due to the presence of the third dimension
In this Rapid Communication, we describe how the presence of the third
dimension may break the scale invariance in a two-dimensional Bose gas in a
pancake-shaped trap. From the two-dimensional perspective, the possibility of a
weak spilling of the atomic density beyond the ground-state of the confinement
alters the two-dimensional chemical potential; in turn, this correction no
longer supports scale invariance. We compare experimental data with numerical
and analytic perturbative results and find a good agreement.Comment: 4 pages, 1 figure, published in PRA Rapid Com
Imaging the collective excitations of an ultracold gas using statistical correlations
Advanced data analysis techniques have proved to be crucial for extracting
information from noisy images. Here we show that principal component analysis
can be successfully applied to ultracold gases to unveil their collective
excitations. By analyzing the correlations in a series of images we are able to
identify the collective modes which are excited, determine their population,
image their eigenfunction, and measure their frequency. Our method allows to
discriminate the relevant modes from other noise components and is robust with
respect to the data sampling procedure. It can be extended to other dynamical
systems including cavity polariton quantum gases or trapped ions.Comment: See also the supplementary material and the video abstrac
Large 2D Coulomb crystals in a radio frequency surface ion trap
We designed and operated a surface ion trap, with an ion-substrate distance
of 500\mum, realized with standard printed-circuit-board techniques. The trap
has been loaded with up to a few thousand Sr+ ions in the Coulomb-crystal
regime. An analytical model of the pseudo-potential allowed us to determine the
parameters that drive the trap into anisotropic regimes in which we obtain
large (N>150) purely 2D ion Coulomb crystals. These crystals may open a simple
and reliable way to experiments on quantum simulations of large 2D systems.Comment: 4 pages, 4 figure
Critical Rotation of an Annular Superfluid Bose Gas
We analyze the excitation spectrum of a superfluid Bose-Einstein condensate
rotating in a ring trap. We identify two important branches of the spectrum
related to outer and inner edge surface modes that lead to the instability of
the superfluid. Depending on the initial circulation of the annular condensate,
either the outer or the inner modes become first unstable. This instability is
crucially related to the superfluid nature of the rotating gas. In particular
we point out the existence of a maximal circulation above which the superflow
decays spontaneously, which cannot be explained by invoking the average speed
of sound.Comment: 5 pages, 5 figures, PRA Rapid Com
Isotope shifts of natural Sr+ measured by laser fluorescence in a sympathetically cooled Coulomb crystal
We measured by laser spectroscopy the isotope shifts between
naturally-occurring even-isotopes of strontium ions for both the
5s\,\,^2S_{1/2}\to 5p\,\,^2P_{1/2} (violet) and the 4d\,\,^2D_{3/2}\to
5p\,\,^2P_{1/2} (infrared) dipole-allowed optical transitions. Fluorescence
spectra were taken by simultaneous measurements on a two-component Coulomb
crystal in a linear Paul trap containing -- laser-cooled Sr
ions. The isotope shifts are extracted from the experimental spectra by fitting
the data with the analytical solution of the optical Bloch equations describing
a three-level atom in interaction with two laser beams. This technique allowed
us to increase the precision with respect to previously reported data obtained
by optogalvanic spectroscopy or fast atomic-beam techniques. The results for
the 5s\,\,^2S_{1/2}\to 5p\,\,^2P_{1/2} transition are
MHz and MHz, in
agreement with previously reported measurements. In the case of the previously
unexplored 4d\,\,^2D_{3/2}\to 5p\,\,^2P_{1/2} transition we find
MHz and MHz. These
results provide more data for stringent tests of theoretical calculations of
the isotope shifts of alkali-metal-like atoms. Moreover, they simplify the
identification and the addressing of Sr isotopes for ion frequency
standards or quantum-information-processing applications in the case of
multi-isotope ion strings.Comment: 19 pages; 5 figures; accepted on Phys. Rev. A (http://pra.aps.org/
Probing superfluidity in a quasi two-dimensional Bose gas through its local dynamics
We report direct evidence of superfluidity in a quasi two-dimensional Bose
gas by observing its dynamical response to a collective excitation. Relying on
a novel local correlation analysis, we are able to probe inhomogeneous clouds
and reveal their local dynamics. We identify in this way the superfluid and
thermal phases inside the gas and locate the boundary at which the
Berezinskii--Kosterlitz--Thouless crossover occurs. This new analysis also
allows to evidence the coupling of the two fluids which induces at finite
temperatures damping rates larger than the usual Landau damping
Double-lambda microscopic model for entangled light generation by four-wave-mixing
Motivated by recent experiments, we study four-wave-mixing in an atomic
double-{\Lambda} system driven by a far-detuned pump. Using the
Heisenberg-Langevin formalism, and based on the microscopic properties of the
medium, we calculate the classical and quantum properties of seed and conju-
gate beams beyond the linear amplifier approximation. A continuous variable
approach gives us access to relative-intensity noise spectra that can be
directly compared to experiments. Restricting ourselves to the cold-atom
regime, we predict the generation of quantum-correlated beams with a
relative-intensity noise spectrum well below the standard quantum limit (down
to -6 dB). Moreover entanglement between seed and conjugate beams measured by
an inseparability down to 0.25 is expected. This work opens the way to the
generation of entangled beams by four-wave mixing in a cold atomic sample.Comment: 11 pages, 6 figures, submitted to PR
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