3,860 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
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
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
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
An optical diode made from a `flying' photonic crystal
Optical diodes controlling the flow of light are of principal significance
for optical information processing 1. They transmit light from an input to an
output, but not in reverse direction. This breaking of time reversal symmetry
is typically achieved via non-linear 2,3 or magnetic effects 4, which imposes
limits to all-optical control 5-7, on-chip integration 7-11, or single-photon
operation 12. Here, we propose an optical diode which requires neither magnetic
fields nor strong input fields. It is based on a flying photonic crystal. Due
to the Doppler effect, the crystal has a band gap with frequency depending on
the light propagation direction relative to the crystal motion.
Counter-intuitively, our setup does not involve the movement of any material
parts. Rather, the flying photonic crystal is realized by optically inducing a
spatially periodic but moving modulation of the optical properties of a
near-resonant medium. The flying crystal not only opens perspectives for
optical diodes operating at low light levels or integrated in small solid state
devices, but also enables novel photonic devices such as optically tunable
mirrors and cavities.Comment: 13 pages, 4 figures, presented in PQE 201
Localization of atomic ensembles via superfluorescence
The sub-wavelength localization of an ensemble of atoms concentrated to a
small volume in space is investigated. The localization relies on the
interaction of the ensemble with a standing wave laser field. The light
scattered in the interaction of standing wave field and atom ensemble depends
on the position of the ensemble relative to the standing wave nodes. This
relation can be described by a fluorescence intensity profile, which depends on
the standing wave field parameters, the ensemble properties, and which is
modified due to collective effects in the ensemble of nearby particles. We
demonstrate that the intensity profile can be tailored to suit different
localization setups. Finally, we apply these results to two localization
schemes. First, we show how to localize an ensemble fixed at a certain position
in the standing wave field. Second, we discuss localization of an ensemble
passing through the standing wave field.Comment: 7 pages, 6 figure
Superconductivity in Pseudo-Binary Silicide SrNixSi2-x with AlB2-Type Structure
We demonstrate the emergence of superconductivity in pseudo-binary silicide
SrNixSi2-x. The compound exhibits a structural phase transition from the cubic
SrSi2-type structure (P4132) to the hexagonal AlB2-type structure (P6/mmm) upon
substituting Ni for Si at approximately x = 0.1. The hexagonal structure is
stabilized in the range of 0.1 < x < 0.7. The superconducting phase appears in
the vicinity of the structural phase boundary. Ni acts as a nonmagnetic dopant,
as confirmed by the Pauli paramagnetic behavior.Comment: 12 pages, 5 figure
Snapshots of the EYES project
The EYES project (IST-2001-34734) is a three years European research project on self-organizing and collaborative energy-efficient sensor networks. It addresses the convergence of distributed information processing, wireless communications, and mobile computing. The goal of the project is to develop the architecture and the technology which enables the creation of a new generation of sensors that can effectively network together so as to provide a flexible platform for the support of a large variety of mobile sensor network applications. This paper provides a broad overview of the EYES project and highlights some approaches and results of the architecture
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