116 research outputs found
Hybrid Quantum Repeater Protocol With Fast Local Processing
We propose a hybrid quantum repeater protocol combining the advantages of
continuous and discrete variables. The repeater is based on the previous work
of Brask et al. [Phys. Rev. Lett. 105, 160501 (2010)] but we present two ways
of improving this protocol. In the previous protocol entangled single-photon
states are produced and grown into superpositions of coherent states, known as
two-mode cat states. The entanglement is then distributed using homodyne
detection. To improve the protocol, we replace the time-consuming non-local
growth of cat states with local growth of single-mode cat states, eliminating
the need for classical communication during growth. Entanglement is generated
in subsequent connection processes. Furthermore the growth procedure is
optimized. We review the main elements of the original protocol and present the
two modifications. Finally the two protocols are compared and the modified
protocol is shown to perform significantly better than the original protocol.Comment: 14 pages, 7 figure
Noisy metrology beyond the standard quantum limit
Parameter estimation is of fundamental importance in areas from atomic
spectroscopy and atomic clocks to gravitational wave detection. Entangled
probes provide a significant precision gain over classical strategies in the
absence of noise. However, recent results seem to indicate that any small
amount of realistic noise restricts the advantage of quantum strategies to an
improvement by at most a multiplicative constant. Here, we identify a relevant
scenario in which one can overcome this restriction and attain superclassical
precision scaling even in the presence of uncorrelated noise. We show that
precision can be significantly enhanced when the noise is concentrated along
some spatial direction, while the Hamiltonian governing the evolution which
depends on the parameter to be estimated can be engineered to point along a
different direction. In the case of perpendicular orientation, we find
superclassical scaling and identify a state which achieves the optimum.Comment: Erroneous expressions with inconsistent units have been corrected. 5
pages, 3 figures + Appendi
Exploring the Local Orthogonality Principle
Nonlocality is arguably one of the most fundamental and counterintuitive
aspects of quantum theory. Nonlocal correlations could, however, be even more
nonlocal than quantum theory allows, while still complying with basic physical
principles such as no-signaling. So why is quantum mechanics not as nonlocal as
it could be? Are there other physical or information-theoretic principles which
prohibit this? So far, the proposed answers to this question have been only
partially successful, partly because they are lacking genuinely multipartite
formulations. In Nat. Comm. 4, 2263 (2013) we introduced the principle of Local
Orthogonality (LO), an intrinsically multipartite principle which is satisfied
by quantum mechanics but is violated by non-physical correlations.
Here we further explore the LO principle, presenting new results and
explaining some of its subtleties. In particular, we show that the set of
no-signaling boxes satisfying LO is closed under wirings, present a
classification of all LO inequalities in certain scenarios, show that all
extremal tripartite boxes with two binary measurements per party violate LO,
and explain the connection between LO inequalities and unextendible product
bases.Comment: Typos corrected; data files uploade
Quantum memory for entangled two-mode squeezed states
A quantum memory for light is a key element for the realization of future
quantum information networks. Requirements for a good quantum memory are (i)
versatility (allowing a wide range of inputs) and (ii) true quantum coherence
(preserving quantum information). Here we demonstrate such a quantum memory for
states possessing Einstein-Podolsky-Rosen (EPR) entanglement. These
multi-photon states are two-mode squeezed by 6.0 dB with a variable orientation
of squeezing and displaced by a few vacuum units. This range encompasses
typical input alphabets for a continuous variable quantum information protocol.
The memory consists of two cells, one for each mode, filled with cesium atoms
at room temperature with a memory time of about 1msec. The preservation of
quantum coherence is rigorously proven by showing that the experimental memory
fidelity 0.52(2) significantly exceeds the benchmark of 0.45 for the best
possible classical memory for a range of displacements.Comment: main text 5 pages, supplementary information 3 page
Evolution of non-kin cooperation:social assortment by cooperative phenotype in guppies
© 2019 The Authors. Cooperation among non-kin constitutes a conundrum for evolutionary biology. Theory suggests that non-kin cooperation can evolve if individuals differ consistently in their cooperative phenotypes and assort socially by these, such that cooperative individuals interact predominantly with one another. However, our knowledge of the role of cooperative phenotypes in the social structuring of real-world animal populations is minimal. In this study, we investigated cooperative phenotypes and their link to social structure in wild Trinidadian guppies (Poecilia reticulata). We first investigated whether wild guppies are repeatable in their individual levels of cooperativeness (i.e. have cooperative phenotypes) and found evidence for this in seven out of eight populations, a result which was mostly driven by females. We then examined the social network structure of one of these populations where the expected fitness impact of cooperative contexts is relatively high, and found assortment by cooperativeness, but not by genetic relatedness. By contrast, and in accordance with our expectations, we did not find assortment by cooperativeness in a population where the expected fitness impact of cooperative contexts is lower. Our results provide empirical support for current theory and suggest that assortment by cooperativeness is important for the evolution and persistence of non-kin cooperation in real-world populations
Report on developing bottom-up Marginal Abatement Cost Curves (MACCS) for representative farm types.
bitstream/item/177181/1/D11.2-Report-on-developing-bottom-up-MACCs-for-representative-farm-types-0263D69D-63D3-4EA3-A333-EDA004AFDFCB1.pd
Anamnestic risk factor questionnaire as reliable diagnostic instrument for osteoporosis (reduced bone morphogenic density)
<p>Abstract</p> <p>Background</p> <p>Osteoporosis is a major health problem worldwide, and is included in the WHO list of the top 10 major diseases. However, it is often undiagnosed until the first fracture occurs, due to inadequate patient education and lack of insurance coverage for screening tests. Anamnestic risk factors like positive family anamnesis or early menopause are assumed to correlate with reduced BMD.</p> <p>Methods</p> <p>In our study of 78 patients with metaphyseal long bone fractures, we searched for a correlation between anamnestic risk factors, bone specific laboratory values, and the bone morphogenic density (BMD). Each indicator was examined as a possible diagnostic instrument for osteoporosis. The secondary aim of this study was to demonstrate the high prevalence of osteoporosis in patients with metaphyseal fractures.</p> <p>Results</p> <p>76.9% of our fracture patients had decreased bone density and 43.6% showed manifest osteoporosis in DXA (densitometry) measurements. Our questionnaire, identifying anamnestic risk factors, correlated highly significantly (p = 0.01) with reduced BMD, whereas seven bone-specific laboratory values (p = 0.046) correlated significantly.</p> <p>Conclusions</p> <p>Anamnestic risk factors correlate with pathological BMD. The medical questionnaire used in this study would therefore function as a cost-effective primary diagnostic instrument for identification of osteoporosis patients.</p
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