13,874 research outputs found
Fundamental limits on low-temperature quantum thermometry with finite resolution
While the ability to measure low temperatures accurately in quantum systems
is important in a wide range of experiments, the possibilities and the
fundamental limits of quantum thermometry are not yet fully understood
theoretically. Here we develop a general approach to low-temperature quantum
thermometry, taking into account restrictions arising not only from the sample
but also from the measurement process. We derive a fundamental bound on the
minimal uncertainty for any temperature measurement that has a finite
resolution. A similar bound can be obtained from the third law of
thermodynamics. Moreover, we identify a mechanism enabling sub-exponential
scaling, even in the regime of finite resolution. We illustrate this effect in
the case of thermometry on a fermionic tight-binding chain with access to only
two lattice sites, where we find a quadratic divergence of the uncertainty. We
also give illustrative examples of ideal quantum gases and a square-lattice
Ising model, highlighting the role of phase transitions.Comment: Published version. Main text: 12 pages, 5 figures; see also related
work by K. Hovhannisyan and L. A. Correa at arXiv:1712.0308
Permutation branes and linear matrix factorisations
All the known rational boundary states for Gepner models can be regarded as
permutation branes. On general grounds, one expects that topological branes in
Gepner models can be encoded as matrix factorisations of the corresponding
Landau-Ginzburg potentials. In this paper we identify the matrix factorisations
associated to arbitrary B-type permutation branes.Comment: 43 pages. v2: References adde
On the singular values and eigenvalues of the Fox–Li and related operators
The Fox–Li operator is a convolution operator over a finite
interval with a special highly oscillatory kernel. It plays an important
role in laser engineering. However, the mathematical analysis of its spectrum
is still rather incomplete. In this expository paper we survey part
of the state of the art, and our emphasis is on showing how standard
Wiener–Hopf theory can be used to obtain insight into the behaviour of
the singular values of the Fox–Li operator. In addition, several approximations
to the spectrum of the Fox–Li operator are discussed and results
on the singular values and eigenvalues of certain related operators are
derived
Tensor Product and Permutation Branes on the Torus
We consider B-type D-branes in the Gepner model consisting of two minimal
models at k=2. This Gepner model is mirror to a torus theory. We establish the
dictionary identifying the B-type D-branes of the Gepner model with A-type
Neumann and Dirichlet branes on the torus.Comment: 26 page
Ideal Linear Chain Polymers with Fixed Angular Momentum
The statistical mechanics of a linear non-interacting polymer chain with a
large number of monomers is considered with fixed angular momentum. The radius
of gyration for a linear polymer is derived exactly by functional integration.
This result is then compared to simulations done with a large number of
non-interacting rigid links at fixed angular momentum. The simulation agrees
with the theory up to finite size corrections. The simulations are also used to
investigate the anisotropic nature of a spinning polymer. We find universal
scaling of the polymer size along the direction of the angular momentum, as a
function of rescaled angular momentum.Comment: 7 pages, 3 figure
Quantum Thermal Machine as a Thermometer
We propose the use of a quantum thermal machine for low-temperature
thermometry. A hot thermal reservoir coupled to the machine allows for
simultaneously cooling the sample while determining its temperature without
knowing the model-dependent coupling constants. In its most simple form, the
proposed scheme works for all thermal machines which perform at Otto efficiency
and can reach Carnot efficiency. We consider a circuit QED implementation which
allows for precise thermometry down to 15 mK with realistic parameters.
Based on the quantum Fisher information, this is close to the optimal
achievable performance. This implementation demonstrates that our proposal is
particularly promising in systems where thermalization between different
components of an experimental setup cannot be guaranteed.Comment: Main text: 5 pages, 4 figures; Supplement: 5 page
Comparison of the epifaunal assemblage of an invasive and native macroalga
The rapid spread of introduced seaweeds is of essential concern, as they can have a deleterious
impact on coastal native seaweed and seagrass communities. However, non-indigenous species
can generate mixed responses when introduced to native assemblages, and increase habitat
complexity, depending on the spatial and ecological context. By taking advantage of the cooccurrence
of the native Ulva seaweeds and the non-native Agarophyton vermiculophyllum in
the Clonakilty estuary (Cork, Ireland) we aim to assess the differences of the epifaunal
community of the native and invasive macroalgal species and how epifaunal biodiversity may
be affected by the invasive. In four locations over four sampling occasions, a total of 253
quadrants of algae and epifaunal biomass were sampled. The Ulva dominated sections mainly
contained macroalgae with tubular morphotypes and some sporadic patches of laminar Ulva
rigida. The average algal biomass of both species was similar, however, the biomass of the red
seaweed was highly variable throughout the seasons. The native algal canopy hosted up to four
times more epifaunal biomass compared to the invasive rhodophyte. Moreover, the epifaunal
community of both canopies differed substantially, whereas deposit-feeding organisms had a
higher abundance in Ulva spp. canopies and the carnivorous crab, Carcinus maenas, was much
more abundant in A. vermiculophyllum samples. The native green macroalgae hosted more
invertebrate taxa, however no difference in biodiversity was found. Increased predation on
deposit-feeders and grazers, as well as, the structural and chemical resistance of A.
vermiculophyllum against grazing and overgrowth by epiphytes may reduce the trophic transfer
from primary production toward higher trophic levels.As introduções de espécies exóticas ocorrem já há vários séculos, de forma
intencional para ganho comercial ou involuntariamente, maioritariamente por transporte
marítimo como passageiros escondidos em águas de lastro ou agarrados ao casco do navio. A
rápida dispersão de algas não-indígenas é uma preocupação básica, já que podem ter impactos
nocivos nas comunidades algas e ervas marinhas costeiras indígenas e são, juntamente com as
alterações climáticas, um dos fatores de stress mais significativos dos ecossistemas de hoje.
Estes impactos incluem a modificação da estrutura da comunidade da fauna, redução de
biodiversidade e alteração das dinâmicas de nutrientes estuarinas, que podem, em última
instância, acelerar a mudança de macrófitas de crescimento lento para macroalgas efémeras,
aumentando o risco de afloramentos de macroalgas. No entanto, as espécies não-indígenas
podem gerar respostas mistas quando introduzidas em agrupamentos nativos, e aumentar a
complexidade do habitat, dependendo do contexto espacial e ecológico. Os factores de stress
antropogénicos como o aumento dos níveis de nutrientes podem intensificar a magnitude de
tais eventos e aumentar a sua frequência. A referida eutrofização aparente de sistemas costeiros
pode fomentar as invasões de macrófitas, assim como a sua resistência a grazing o que aumenta
ainda mais a probabilidade de afloramentos de macroalgas. Duas espécies indígenas de forma
laminar (Ulva rigida e Ulva gigantea) e duas espécies indígenas de forma tubular (Ulva
prolifera e Ulva compressa) do género Ulva bem como a rodófita não-indígena
A.vermiculophyllum foram identificadas no estuário de Clonakilty (Condado de Cork, Irlanda)
e estão a formar canópias distintas próximas umas das outras. A identificação de efeitos
positivos ou negativos de uma espécie não-indígena em ecossistemas estuarinos e a forma
como é controlada teria implicações notáveis nas estratégias de gestão de espécies invasivas
fundacionais. Os vários impactos ecológicos na diversidade da epifauna e a utilização de
nutrientes precisam de ser avaliados, especialmente no contexto do sobre-enriquecimento de
nutrientes antropogénico local e a mudança global para reconhecer e recomendar uma
estratégia apropriada (activa ou passiva). Ao tirar proveito da co-ocorrência de algas indígenas
e não-indígenas neste estuário, tivemos como objectivos: (i) a comparação de padrões espaciais
e temporais na estrutura de comunidades de epifauna em ambas as algas; (ii) caracterizar o seu papel na formação de habitats de ambas as espécies e as suas implicações em diferentes taxa
da fauna; (iii) explorar implicações ecológicas para diferentes guildas alimentares e discutir o
impacto em cascadas tróficas; e (iv) a avaliação da biodiversidade da epifauna e a riqueza das
espécies de macroalgas.(…
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