5,510 research outputs found
Spontaneous-emission suppression via multiphoton quantum interference
The spontaneous emission is investigated for an effective atomic two-level
system in an intense coherent field with frequency lower than the
vacuum-induced decay width. As this additional low-frequency field is assumed
to be intense, multiphoton processes may be induced, which can be seen as
alternative transition pathways in addition to the simple spontaneous decay.
The interplay of the various interfering transition pathways influences the
decay dynamics of the two-level system and may be used to slow down the
spontaneous decay considerably. We derive from first principles an expression
for the Hamiltonian including up to three-photon processes. This Hamiltonian is
then applied to a quantum mechanical simulation of the decay dynamics of the
two-level system. Finally, we discuss numerical results of this simulation
based on a rubidium atom and show that the spontaneous emission in this system
may be suppressed substantially.Comment: 18 pages, 7 figures, latest version with minor change
Acoustic simultaneous localization and mapping (A-SLAM) of a moving microphone array and its surrounding speakers
Acoustic scene mapping creates a representation of positions of audio sources such as talkers within the surrounding environment of a microphone array. By allowing the array to move, the acoustic scene can be explored in order to improve the map. Furthermore, the spatial diversity of the kinematic array allows for estimation of the source-sensor distance in scenarios where source directions of arrival are measured. As sound source localization is performed relative to the array position, mapping of acoustic sources requires knowledge of the absolute position of the microphone array in the room. If the array is moving, its absolute position is unknown in practice. Hence, Simultaneous Localization and Mapping (SLAM) is required in order to localize the microphone array position and map the surrounding sound sources. In realistic environments, microphone arrays receive a convolutive mixture of direct-path speech signals, noise and reflections due to reverberation. A key challenge of Acoustic SLAM (a-SLAM) is robustness against reverberant clutter measurements and missing source detections. This paper proposes a novel bearing-only a-SLAM approach using a Single-Cluster Probability Hypothesis Density filter. Results demonstrate convergence to accurate estimates of the array trajectory and source positions
Towards Informative Path Planning for Acoustic SLAM
Acoustic scene mapping is a challenging task as microphone arrays can often localize sound sources only in terms of their directions. Spatial diversity can be exploited constructively to infer source-sensor range when using microphone arrays installed on moving platforms, such as robots. As the absolute location of a moving robot is often unknown in practice, Acoustic Simultaneous Localization And Mapping (a-SLAM) is required in order to localize the moving robot’s positions and jointly map the sound sources. Using a novel a-SLAM approach, this paper investigates the impact of the choice of robot paths on source mapping accuracy. Simulation results demonstrate that a-SLAM performance can be improved by informatively planning robot paths
Quantum correlations of an atomic ensemble via a classical bath
Somewhat surprisingly, quantum features can be extracted from a classical
bath. For this, we discuss a sample of three-level atoms in ladder
configuration interacting only via the surrounding bath, and show that the
fluorescence light emitted by this system exhibits non-classical properties.
Typical realizations for such an environment are thermal baths for microwave
transition frequencies, or incoherent broadband fields for optical transitions.
In a small sample of atoms, the emitted light can be switched from sub- to
super-poissonian and from anti-bunching to super-bunching controlled by the
mean number of atoms in the sample. Larger samples allow to generate
super-bunched light over a wide range of bath parameters and thus fluorescence
light intensities. We also identify parameter ranges where the fields emitted
on the two transitions are correlated or anti-correlated, such that the
Cauchy-Schwarz inequality is violated. As in a moderately strong baths this
violation occurs also for larger numbers of atoms, such samples exhibit
mesoscopic quantum effects.Comment: 4 page
Loading atom lasers by collectivity-enhanced optical pumping
The effect of collectivity on the loading of an atom laser via optical
pumping is discussed. In our model, atoms in a beam are laser-excited and
subsequently spontaneously decay into a trapping state. We consider the case of
sufficiently high particle density in the beam such that the spontaneous
emission is modified by the particle interaction. We show that the collective
effects lead to a better population of the trapping state over a wide range of
system parameters, and that the second order correlation function of the atoms
can be controlled by the applied laser field.Comment: 5 pages, 7 figure
Modelling with measures: Approximation of a mass-emitting object by a point source
We consider a linear diffusion equation on
, where
is a bounded domain. The time-dependent flux on the
boundary is prescribed. The aim of the
paper is to approximate the dynamics by the solution of the diffusion equation
on the whole of with a measure-valued point source in the origin
and provide estimates for the quality of approximation. For all time , we
derive an -bound on the difference in flux on the
boundary. Moreover, we derive for all an -bound and an
-bound for the difference of the solutions to the two
models
Double-EIT ground-state laser cooling without blue-sideband heating
We discuss a laser cooling scheme for trapped atoms or ions which is based on
double electromagnetically induced transparency (EIT) and makes use of a
four-level atom in tripod configuration. The additional fourth atomic state is
coupled by a strong coupling laser field to the usual three-level setup of
single-EIT cooling. This effectively allows to create two EIT structures in the
absorption spectrum of the system to be cooled, which may be controlled by the
coupling laser field parameters to cancel both the carrier- and the
blue-sideband excitations. In leading order of the Lamb-Dicke expansion, this
suppresses all heating processes. As a consequence, the double-EIT scheme can
be used to lower the cooling limit by almost two powers of the Lamb-Dicke
parameter as compared to single-EIT cooling.Comment: 7 pages, 3 figure
Negative refraction with tunable absorption in an active dense gas of atoms
Applications of negative index materials (NIM) presently are severely limited
by absorption. Next to improvements of metamaterial designs, it has been
suggested that dense gases of atoms could form a NIM with negligible losses. In
such gases, the low absorption is facilitated by quantum interference. Here, we
show that additional gain mechanisms can be used to tune and effectively remove
absorption in a dense gas NIM. In our setup, the atoms are coherently prepared
by control laser fields, and further driven by a weak incoherent pump field to
induce gain. We employ nonlinear optical Bloch equations to analyze the optical
response. Metastable Neon is identified as a suitable experimental candidate at
infrared frequencies to implement a lossless active negative index material.Comment: 10 pages, 9 figure
Exploring multivariate data structures with local principal curves.
A new approach to find the underlying structure of a multidimensional data cloud is proposed, which is based on a localized version of principal components analysis. More specifically, we calculate a
series of local centers of mass and move through the data in directions given by the first local principal axis.
One obtains a smooth ``local principal curve'' passing through the "middle" of a multivariate data cloud. The concept adopts to branched curves by considering the second local principal axis. Since the algorithm is based on a simple eigendecomposition, computation is fast and easy
- …