@phdthesis{Roeschard2002,
  author    = {R{\"o}schard, Jacqueline},
  title     = {Cutter, carriers and bucket brigades ...},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-2240},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2002},
  abstract  = {This study investigates the foraging behaviour of grass-cutting ants, Atta vollenweideri, with specific consideration of the following issues: (a) cutting behaviour and the determination of fragment size, (b) the effect of load size on transport economics, (c) division of labour and task-partitioning. Grass-cutting ants, Atta vollenweideri, harvest grass fragments that serve as substrate for the cultivation of a symbiotic fungus. Foragers were observed to cut grass fragments across the blade, thus resulting in longish, rectangular-shaped fragments in contrast to the semicircular fragments of leaf-cutting ants. Cutting was very time-consuming: In tough grasses like the typical grassland species Paspallum intermedium and Cyperus entrerrianus, cutting times lasted up to more than 20 minutes per fragment and roughly half of all initiated cutting attempts were given up by the ants. Foragers harvesting the softer grass Leersia hexandra were smaller than those foraging on the hard grasses. Fragment size determination and the extent of size-matching between ant body size and fragment size was investigated regarding possible effects of tissue toughness on decision-making and as a function of the distance from the nest. Tissue toughness affected decision-making such that fragment width correlated with ant body mass for the hard grass but not for the soft one, suggesting that when cutting is difficult, larger ants tend to select wider grasses to initiate cutting. The length of the fragments cut out of the two grass species differed statistically, but showed a large overlap in their distribution. Distance from the nest affected load size as well as the extent of size-matching: Fragments collected directly after cutting were significantly larger than those carried on the trail. This indicates that fragments were cut once again on their way to the nest. Size-matching depended on the trail sector considered, and was stronger in ants sampled closer to the nest, suggesting that carriers either cut fragments in sizes corresponding to their body mass prior transport, or transferred them to nestmates of different size after a short carrying distance. During transport, a worker takes a fragment with its mandibles at one end and carries it in a more or less vertical position. Thus, load length might particularly affect maneuverability, because of the marked displacement of the gravitational center. Conversely, based on the energetic of cutting, workers might maximise their individual harvesting rate by cutting long grass fragments, since the longer a grass fragment, the larger is the amount of material harvested per unit cutting effort. I therefore investigated the economics of load transport by focusing on the effects of load size (mass and length) on gross material transport rate to the nest. When controlling for fragment mass, both running speed of foragers and gross material transport rate was observed to be higher for short fragments. In contrast, if fragment mass was doubled and length maintained, running speed differed according to the mass of the loads, with the heavier fragments being transported at the lower pace. For the sizes tested, heavy fragments yielded a higher transport rate in spite of the lower speed of transport, as they did not slow down foragers so much that it counterbalanced the positive effects of fragment mass on material transport rate. The sizes of the fragments cut by grass-cutting ants under natural conditions therefore may represent the outcome of an evolutionary trade-off between maximising harvesting rate at the cutting site and minimising the effects of fragment size on material transport rates. I investigated division of labour and task partitioning during foraging by recording the behaviour of marked ants while cutting, and by monitoring the transport of fragments from the cutting until they reached the nest. A. vollenweideri foragers showed division of labour between cutting and carrying, with larger workers cutting the fragments, and smaller ones transporting them. This division was absent for food sources very close to the nest, when no physical trail was present. Along the trail, the transport of fragment was a partitioned task, i.e., workers formed bucket brigades composed of 2 to 5 carriers. This sequential load transport occurred more often on long than on short trails. The first carriers of a bucket brigade covered only short distances before dropping their fragments, turned back and continued foraging at the same food source. The last carriers covered the longest distance. There was no particular location on the trail for load dropping , i.e., fragments were not cached. I tested the predictions of two hypotheses about the causes of bucket brigades: First, bucket brigades might occur because of load-carriage effects: A load that is too big for an ant to be carried is dropped and carried further by nestmates. Second, fragments carried by bucket brigades might reach the nest quicker than if they are transported by a single carrier. Third, bucket brigades might enhance information flow among foragers: By transferring the load a worker may return earlier back to the foraging site and be able to reinforce the chemical trail, thus recruitment. In addition, the dropped fragment itself may contain information for unladen foragers about currently harvested sources and may enable them to choose between sources of different quality. I investigated load-carriage effects and possible time-saving by presenting ants with fragments of different but defined sizes. Load size did not affect frequency of load dropping nor the distance the first carrier covered before dropping, and transport time by bucket brigades was significantly longer than by single carriers. In order to study the information transfer hypothesis, I presented ants with fragments of different attractivity but constant size. Ants carrying high-quality fragments would be expected to drop them more often than workers transporting low-quality fragments, thus increasing the frequency of bucket brigades. My results show that increasing load quality increased the frequency of bucket brigades as well as it decreased the carrying distance of the first carrier. In other words, more attractive loads were dropped more frequently and after a shorter distance than less attractive ones with the first carriers returning to the foraging site to continue foraging. Summing up, neither load-carriage effects nor time-saving caused the occurrence of bucket brigades. Rather, the benefit might be found at colony level in an enhanced information flow.},
  subject      = {Atta},
  language  = {en}
}
@phdthesis{Northe2019,
  author    = {Northe, Christian},
  title     = {Interfaces and Information in Gauge/Gravity Duality},
  doi       = {10.25972/OPUS-19159},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-191594},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {This dissertation employs gauge/gravity duality to investigate features of ( 2 + 1 ) -dimensional quantum gravity in Anti-de Sitter space (AdS) and its relation to conformal field theory (CFT) in 1 + 1 dimensions. Concretely, we contribute to research on the frontier of gauge/gravity with condensed matter as well as the frontier with quantum informa- tion. The first research topic of this thesis is motivated by the Kondo model, which describes the screening of magnetic impurities in metals by conduction electrons at low temperatures. This process has a de- scription in the language of string theory via fluctuating surfaces in spacetime, called branes. At high temperatures the unscreened Kondo impurity is modelled by a stack of pointlike branes. At low tempera- tures this stack condenses into a single spherical, two-dimensional brane which embodies the screened impurity. This thesis demonstrates how this condensation process is naturally reinvoked in the holographic D1/D5 system. We find brane configu- rations mimicking the Kondo impurities at high and low energies and establish the corresponding brane condensation, where the brane grows two additional dimensions. We construct supergravity solutions, which fully take into account the effect of the brane on its surrounding space- time before and after the condensation takes place. This enables us to compute the full impurity entropies through which we confirm the validity of the g-theorem. The second research topic is rooted in the connection of geometry with quantum information. The motivation stems from the "complexity equals volume" proposal, which relates the volume of wormholes to the cicruit complexity of a thermal quantum state. We approach this proposal from a pragmatic point of view by studying the properties of certain volumes in gravity and their description in the CFT. We study subregion complexities, which are the volumes of the re- gions subtended by Ryu-Takayanagi (RT) geodesics. On the gravity side we reveal their topological properties in the vacuum and in ther- mal states, where they turn out to be temperature independent. On the field theory side we develop and proof a formula using kinematic space which computes subregion complexities without referencing the bulk. We apply our formula to global AdS 3 , the conical defect and a black hole. While entanglement, i.e. minimal boundary anchored geodesics, suffices to produce vacuum geometries, for the conical defect we also need geodesics windings non-trivially around the singularity. The black hole geometry requires additional thermal contributions.},
  subject      = {Information},
  language  = {en}
}
@article{LaihoPresslSchlageretal.2016,
  author    = {Laiho, K. and Pressl, B. and Schlager, A. and Suchomel, H. and Kamp, M. and H{\"o}fling, S. and Schneider, C. and Weihs, G.},
  title     = {Uncovering dispersion properties in semiconductor waveguides to study photon-pair generation},
  series = {Nanotechnology},
  volume    = {27},
  journal   = {Nanotechnology},
  number    = {43},
  doi       = {10.1088/0957-4484/27/43/434003},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-187025},
  pages     = {434003},
  year      = {2016},
  abstract  = {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.},
  language  = {en}
}