16,284 research outputs found
Open-closed strings: Two-dimensional extended TQFTs and Frobenius algebras
We study a special sort of 2-dimensional extended Topological Quantum Field
Theories (TQFTs) which we call open-closed TQFTs. These are defined on
open-closed cobordisms by which we mean smooth compact oriented 2-manifolds
with corners that have a particular global structure in order to model the
smooth topology of open and closed string worldsheets. We show that the
category of open-closed TQFTs is equivalent to the category of knowledgeable
Frobenius algebras. A knowledgeable Frobenius algebra (A,C,i,i^*) consists of a
symmetric Frobenius algebra A, a commutative Frobenius algebra C, and an
algebra homomorphism i:C->A with dual i^*:A->C, subject to some conditions.
This result is achieved by providing a generators and relations description of
the category of open-closed cobordisms. In order to prove the sufficiency of
our relations, we provide a normal form for such cobordisms which is
characterized by topological invariants. Starting from an arbitrary such
cobordism, we construct a sequence of moves (generalized handle slides and
handle cancellations) which transforms the given cobordism into the normal
form. Using the generators and relations description of the category of
open-closed cobordisms, we show that it is equivalent to the symmetric monoidal
category freely generated by a knowledgeable Frobenius algebra. Our formalism
is then generalized to the context of open-closed cobordisms with labeled free
boundary components, i.e. to open-closed string worldsheets with D-brane labels
at their free boundaries.Comment: 47 pages; LaTeX2e with xypic and pstricks macros; corrected typo
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An evaluation framework for stereo-based driver assistance
This is the post-print version of the Article - Copyright @ 2012 Springer VerlagThe accuracy of stereo algorithms or optical flow methods is commonly assessed by comparing the results against the Middlebury
database. However, equivalent data for automotive or robotics applications
rarely exist as they are difficult to obtain. As our main contribution, we introduce an evaluation framework tailored for stereo-based driver assistance able to deliver excellent performance measures while
circumventing manual label effort. Within this framework one can combine several ways of ground-truthing, different comparison metrics, and use large image databases.
Using our framework we show examples on several types of ground truthing techniques: implicit ground truthing (e.g. sequence recorded without a crash occurred), robotic vehicles with high precision sensors, and to a small extent, manual labeling. To show the effectiveness of our evaluation framework we compare three different stereo algorithms on
pixel and object level. In more detail we evaluate an intermediate representation
called the Stixel World. Besides evaluating the accuracy of the Stixels, we investigate the completeness (equivalent to the detection rate) of the StixelWorld vs. the number of phantom Stixels. Among many findings, using this framework enables us to reduce the number of phantom Stixels by a factor of three compared to the base parametrization. This base parametrization has already been optimized by test driving vehicles for distances exceeding 10000 km
Actively Tuned and Spatially Trapped Polaritons
We report active tuning of the polariton resonance of quantum well excitons
in a semiconductor microcavity using applied stress. Starting with the quantum
well exciton energy higher than the cavity photon mode, we use stress to reduce
the exciton energy and bring it into resonance with the photon mode. At the
point of zero detuning, line narrowing and strong increase of the
photoluminescence are seen. By the same means, we create an in-plane harmonic
potential for the polaritons, which allows trapping, potentially making
Bose-Einstein condensation of polaritons analogous to trapped atoms possible.
We demonstrate drift of the polaritons into this trap.Comment: 10 pages, 5 figure
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