211 research outputs found
Heralded qubit amplifiers for practical device-independent quantum key distribution
Device-independent quantum key distribution does not need a precise quantum
mechanical model of employed devices to guarantee security. Despite of its
beauty, it is still a very challenging experimental task. We compare a recent
proposal by Gisin et al. [Phys. Rev. Lett. 105, 070501 (2010)] to close the
detection loophole problem with that of a simpler quantum relay based on
entanglement swapping with linear optics. Our full-mode analysis for both
schemes confirms that, in contrast to recent beliefs, the second scheme can
indeed provide a positive key rate which is even considerably higher than that
of the first alternative. The resulting key rates and required detection
efficiencies of approx. 95% for both schemes, however, strongly depend on the
underlying security proof.Comment: 5 pages, 3 figure
Entanglement and nonclassical properties of hypergraph states
Hypergraph states are multi-qubit states that form a subset of the locally
maximally entangleable states and a generalization of the well--established
notion of graph states. Mathematically, they can conveniently be described by a
hypergraph that indicates a possible generation procedure of these states;
alternatively, they can also be phrased in terms of a non-local stabilizer
formalism. In this paper, we explore the entanglement properties and
nonclassical features of hypergraph states. First, we identify the equivalence
classes under local unitary transformations for up to four qubits, as well as
important classes of five- and six-qubit states, and determine various
entanglement properties of these classes. Second, we present general conditions
under which the local unitary equivalence of hypergraph states can simply be
decided by considering a finite set of transformations with a clear
graph-theoretical interpretation. Finally, we consider the question whether
hypergraph states and their correlations can be used to reveal contradictions
with classical hidden variable theories. We demonstrate that various
noncontextuality inequalities and Bell inequalities can be derived for
hypergraph states.Comment: 29 pages, 5 figures, final versio
Analysing multiparticle quantum states
The analysis of multiparticle quantum states is a central problem in quantum
information processing. This task poses several challenges for experimenters
and theoreticians. We give an overview over current problems and possible
solutions concerning systematic errors of quantum devices, the reconstruction
of quantum states, and the analysis of correlations and complexity in
multiparticle density matrices.Comment: 20 pages, 4 figures, prepared for proceedings of the "Quantum
[Un]speakables II" conference (Vienna, 2014
Biomechanical analysis of the effect of congruence, depth and radius on the stability ratio of a simplistic ‘ball-and-socket’ joint model
Objectives The bony shoulder stability ratio (BSSR) allows for quantification
of the bony stabilisers in vivo. We aimed to biomechanically validate the
BSSR, determine whether joint incongruence affects the stability ratio (SR) of
a shoulder model, and determine the correct parameters (glenoid concavity
versus humeral head radius) for calculation of the BSSR in vivo. Methods Four
polyethylene balls (radii: 19.1 mm to 38.1 mm) were used to mould four fitting
sockets in four different depths (3.2 mm to 19.1mm). The SR was measured in
biomechanical congruent and incongruent experimental series. The experimental
SR of a congruent system was compared with the calculated SR based on the BSSR
approach. Differences in SR between congruent and incongruent experimental
conditions were quantified. Finally, the experimental SR was compared with
either calculated SR based on the socket concavity or plastic ball radius.
Results The experimental SR is comparable with the calculated SR (mean
difference 10%, sd 8%; relative values). The experimental incongruence study
observed almost no differences (2%, sd 2%). The calculated SR on the basis of
the socket concavity radius is superior in predicting the experimental SR
(mean difference 10%, sd 9%) compared with the calculated SR based on the
plastic ball radius (mean difference 42%, sd 55%). Conclusion The present
biomechanical investigation confirmed the validity of the BSSR. Incongruence
has no significant effect on the SR of a shoulder model. In the event of an
incongruent system, the calculation of the BSSR on the basis of the glenoid
concavity radius is recommended
One-way quantum key distribution: Simple upper bound on the secret key rate
We present a simple method to obtain an upper bound on the achievable secret
key rate in quantum key distribution (QKD) protocols that use only
unidirectional classical communication during the public-discussion phase. This
method is based on a necessary precondition for one-way secret key
distillation; the legitimate users need to prove that there exists no quantum
state having a symmetric extension that is compatible with the available
measurements results. The main advantage of the obtained upper bound is that it
can be formulated as a semidefinite program, which can be efficiently solved.
We illustrate our results by analysing two well-known qubit-based QKD
protocols: the four-state protocol and the six-state protocol. Recent results
by Renner et al., Phys. Rev. A 72, 012332 (2005), also show that the given
precondition is only necessary but not sufficient for unidirectional secret key
distillation.Comment: 11 pages, 1 figur
Conclusive quantum steering with superconducting transition edge sensors
Quantum steering allows two parties to verify shared entanglement even if one
measurement device is untrusted. A conclusive demonstration of steering through
the violation of a steering inequality is of considerable fundamental interest
and opens up applications in quantum communication. To date all experimental
tests with single photon states have relied on post-selection, allowing
untrusted devices to cheat by hiding unfavourable events in losses. Here we
close this "detection loophole" by combining a highly efficient source of
entangled photon pairs with superconducting transition edge sensors. We achieve
an unprecedented ~62% conditional detection efficiency of entangled photons and
violate a steering inequality with the minimal number of measurement settings
by 48 standard deviations. Our results provide a clear path to practical
applications of steering and to a photonic loophole-free Bell test.Comment: Preprint of 7 pages, 3 figures; the definitive version is published
in Nature Communications, see below. Also, see related experimental work by
A. J. Bennet et al., arXiv:1111.0739 and B. Wittmann et al., arXiv:1111.076
Rank-based model selection for multiple ions quantum tomography
The statistical analysis of measurement data has become a key component of
many quantum engineering experiments. As standard full state tomography becomes
unfeasible for large dimensional quantum systems, one needs to exploit prior
information and the "sparsity" properties of the experimental state in order to
reduce the dimensionality of the estimation problem. In this paper we propose
model selection as a general principle for finding the simplest, or most
parsimonious explanation of the data, by fitting different models and choosing
the estimator with the best trade-off between likelihood fit and model
complexity. We apply two well established model selection methods -- the Akaike
information criterion (AIC) and the Bayesian information criterion (BIC) -- to
models consising of states of fixed rank and datasets such as are currently
produced in multiple ions experiments. We test the performance of AIC and BIC
on randomly chosen low rank states of 4 ions, and study the dependence of the
selected rank with the number of measurement repetitions for one ion states. We
then apply the methods to real data from a 4 ions experiment aimed at creating
a Smolin state of rank 4. The two methods indicate that the optimal model for
describing the data lies between ranks 6 and 9, and the Pearson test
is applied to validate this conclusion. Additionally we find that the mean
square error of the maximum likelihood estimator for pure states is close to
that of the optimal over all possible measurements.Comment: 24 pages, 6 figures, 3 table
Towards the “ultimate earthquake-proof” building: Development of an integrated low-damage system
The 2010–2011 Canterbury earthquake sequence has highlighted the
severe mismatch between societal expectations over the reality of seismic performance
of modern buildings. A paradigm shift in performance-based design criteria
and objectives towards damage-control or low-damage design philosophy and
technologies is urgently required. The increased awareness by the general public,
tenants, building owners, territorial authorities as well as (re)insurers, of the severe
socio-economic impacts of moderate-strong earthquakes in terms of damage/dollars/
downtime, has indeed stimulated and facilitated the wider acceptance and
implementation of cost-efficient damage-control (or low-damage) technologies.
The ‘bar’ has been raised significantly with the request to fast-track the development
of what the wider general public would hope, and somehow expect, to live
in, i.e. an “earthquake-proof” building system, capable of sustaining the shaking of
a severe earthquake basically unscathed.
The paper provides an overview of recent advances through extensive research,
carried out at the University of Canterbury in the past decade towards the development
of a low-damage building system as a whole, within an integrated
performance-based framework, including the skeleton of the superstructure, the
non-structural components and the interaction with the soil/foundation system.
Examples of real on site-applications of such technology in New Zealand, using
concrete, timber (engineered wood), steel or a combination of these materials, and
featuring some of the latest innovative technical solutions developed in the laboratory
are presented as examples of successful transfer of performance-based seismic
design approach and advanced technology from theory to practice
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