464 research outputs found
Superpositions of the Orbital Angular Momentum for Applications in Quantum Experiments
Two different experimental techniques for preparation and analyzing
superpositions of the Gaussian and Laguerre-Gassian modes are presented. This
is done exploiting an interferometric method on the one hand and using computer
generated holograms on the other hand. It is shown that by shifting the
hologram with respect to an incoming Gaussian beam different superpositions of
the Gaussian and the Laguerre-Gaussian beam can be produced. An analytical
expression between the relative phase and the amplitudes of the modes and the
displacement of the hologram is given. The application of such orbital angular
momenta superpositions in quantum experiments such as quantum cryptography is
discussed.Comment: 18 pages, 4 figures. to appear in Journal of Optics
Modally Resolved Fabry-Perot Experiment with Semiconductor Waveguides
Based on the interaction between different spatial modes, semiconductor
Bragg-reflection waveguides provide a highly functional platform for non-linear
optics. Therefore, the control and engineering of the properties of each
spatial mode is essential. Despite the multimodeness of our waveguide, the
well-established Fabry-Perot technique for recording fringes in the optical
transmission spectrum can successfully be employed for a detailed linear
optical characterization when combined with Fourier analysis. A prerequisite
for the modal sensitivity is a finely resolved transmission spectrum that is
recorded over a broad frequency band. Our results highlight how the features of
different spatial modes, such as their loss characteristics and dispersion
properties, can be separated from each other allowing their comparison. The
mode-resolved measurements are important for optimizing the performance of such
multimode waveguides by tailoring the properties of their spatial modes.Comment: 8 pages, 7 figure
Quantum entanglement distribution with 810 nm photons through telecom fibers
We demonstrate the distribution of polarization entangled photons of
wavelength 810 nm through standard telecom fibers. This technique allows
quantum communication protocols to be performed over established fiber
infrastructure, and makes use of the smaller and better performing setups
available around 800 nm, as compared to those which use telecom wavelengths
around 1550 nm. We examine the excitation and subsequent quenching of
higher-order spatial modes in telecom fibers up to 6 km in length, and perform
a distribution of high quality entanglement (visibility 95.6%). Finally, we
demonstrate quantum key distribution using entangled 810 nm photons over a 4.4
km long installed telecom fiber link.Comment: 5 pages, 5 figures, 1 tabl
Multi-dimensional laser spectroscopy of exciton-polaritons with spatial light modulators
We describe an experimental system that allows one to easily access the
dispersion curve of exciton-polaritons in a microcavity. Our approach is based
on two spatial light modulators (SLM), one for changing the excitation angles
(momenta), and the other for tuning the excitation wavelength. We show that
with this setup, an arbitrary number of states can be excited accurately and
that re-configuration of the excitation scheme can be done at high speed.Comment: 4 pages, 5 figure
Jet propulsion without inertia
A body immersed in a highly viscous fluid can locomote by drawing in and
expelling fluid through pores at its surface. We consider this mechanism of jet
propulsion without inertia in the case of spheroidal bodies, and derive both
the swimming velocity and the hydrodynamic efficiency. Elementary examples are
presented, and exact axisymmetric solutions for spherical, prolate spheroidal,
and oblate spheroidal body shapes are provided. In each case, entirely and
partially porous (i.e. jetting) surfaces are considered, and the optimal
jetting flow profiles at the surface for maximizing the hydrodynamic efficiency
are determined computationally. The maximal efficiency which may be achieved by
a sphere using such jet propulsion is 12.5%, a significant improvement upon
traditional flagella-based means of locomotion at zero Reynolds number. Unlike
other swimming mechanisms which rely on the presentation of a small cross
section in the direction of motion, the efficiency of a jetting body at low
Reynolds number increases as the body becomes more oblate, and limits to
approximately 162% in the case of a flat plate swimming along its axis of
symmetry. Our results are discussed in the light of slime extrusion mechanisms
occurring in many cyanobacteria
Solving the Einstein-Podolsky-Rosen puzzle: the origin of non-locality in Aspect-type experiments
So far no mechanism is known, which could connect the two measurements in an
Aspect-type experiment. Here, we suggest such a mechanism, based on the phase
of a photon's field during propagation. We show that two polarization
measurements are correlated, even if no signal passes from one point of
measurement to the other. The non-local connection of a photon pair is the
result of its origin at a common source, where the two fields acquire a well
defined phase difference. Therefore, it is not actually a non-local effect in
any conventional sense. We expect that the model and the detailed analysis it
allows will have a major impact on quantum cryptography and quantum
computation.Comment: 5 pages 1 figure. Added an analysis of quantum steering. The result
is that under certain conditions the experimental result at B can be
predicted if the polarization angle and the result at A are known. The paper
has been accepted for publication in Frontiers of Physics. arXiv admin note:
substantial text overlap with arXiv:1108.435
Fish schooling as a basis for vertical axis wind turbine farm design
Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the
high power coefficient (mechanical power output divided by the power of the
free-stream air through the turbine cross-sectional area) of an isolated
turbine. However when in close proximity to neighbouring turbines, HAWTs suffer
from a reduced power coefficient. In contrast, previous research on vertical
axis wind turbines (VAWTs) suggests that closely-spaced VAWTs may experience
only small decreases (or even increases) in an individual turbine's power
coefficient when placed in close proximity to neighbours, thus yielding much
higher power outputs for a given area of land. A potential flow model of
inter-VAWT interactions is developed to investigate the effect of changes in
VAWT spatial arrangement on the array performance coefficient, which compares
the expected average power coefficient of turbines in an array to a
spatially-isolated turbine. A geometric arrangement based on the configuration
of shed vortices in the wake of schooling fish is shown to significantly
increase the array performance coefficient based upon an array of 16x16 wind
turbines. Results suggest increases in power output of over one order of
magnitude for a given area of land as compared to HAWTs.Comment: Submitted for publication in BioInspiration and Biomimetics. Note:
The technology described in this paper is protected under both US and
international pending patents filed by the California Institute of Technolog
Polarization entangled photon-pair source based on a type-II PPLN waveguide emitting at a telecom wavelength
We report the realization of a fiber coupled polarization entangled
photon-pair source at 1310 nm based on a birefringent titanium in-diffused
waveguide integrated on periodically poled lithium niobate. By taking advantage
of a dedicated and high-performance setup, we characterized the quantum
properties of the pairs by measuring two-photon interference in both
Hong-Ou-Mandel and standard Bell inequality configurations. We obtained, for
the two sets of measurements, interference net visibilities reaching nearly
100%, which represent important and competitive results compared to similar
waveguide-based configurations already reported. These results prove the
relevance of our approach as an enabling technology for long-distance quantum
communication.Comment: 13 pages, 4 figures, to appear in New Journal of Physic
Uncovering dispersion properties in semiconductor waveguides to study photon-pair generation
This work was supported by the FWF through project no. I-2065-N27, the DFG Project no. SCHN1376/2-1, the ERC project EnSeNa (257531) and the State of Bavaria.We investigate the dispersion properties of ridge Bragg-reflection waveguides to deduce their phasematching characteristics. These are crucial for exploiting them as sources of parametric down-conversion (PDC). In order to estimate the phasematching bandwidth we first determine the group refractive indices of the interacting modes via Fabry–Perot experiments in two distant wavelength regions. Second, by measuring the spectra of the emitted PDC photons, we gain access to their group index dispersion. Our results offer a simple approach for determining the PDC process parameters in the spectral domain, and provide important feedback for designing such sources, especially in the broadband case.Publisher PDFPeer reviewe
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