351 research outputs found
On single-photon quantum key distribution in the presence of loss
We investigate two-way and one-way single-photon quantum key distribution
(QKD) protocols in the presence of loss introduced by the quantum channel. Our
analysis is based on a simple precondition for secure QKD in each case. In
particular, the legitimate users need to prove that there exists no separable
state (in the case of two-way QKD), or that there exists no quantum state
having a symmetric extension (one-way QKD), that is compatible with the
available measurements results. We show that both criteria can be formulated as
a convex optimisation problem known as a semidefinite program, which can be
efficiently solved. Moreover, we prove that the solution to the dual
optimisation corresponds to the evaluation of an optimal witness operator that
belongs to the minimal verification set of them for the given two-way (or
one-way) QKD protocol. A positive expectation value of this optimal witness
operator states that no secret key can be distilled from the available
measurements results. We apply such analysis to several well-known
single-photon QKD protocols under losses.Comment: 14 pages, 6 figure
Calibration robust entanglement detection beyond Bell inequalities
In its vast majority entanglement verification is examined either in the
complete characterized or totally device independent scenario. The assumptions
imposed by these extreme cases are often either too weak or strong for real
experiments. Here we investigate this detection task for the intermediate
regime where partial knowledge of the measured observables is known,
considering cases like orthogonal, sharp or only dimension bounded
measurements. We show that for all these assumptions it is not necessary to
violate a corresponding Bell inequality in order to detect entanglement. We
derive strong detection criteria that can be directly evaluated for
experimental data and which are robust against large classes of calibration
errors. The conditions are even capable of detecting bound entanglement under
the sole assumption of dimension bounded measurements.Comment: 10 pages + appendix, 1 figur
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
Passive decoy state quantum key distribution with practical light sources
Decoy states have been proven to be a very useful method for significantly
enhancing the performance of quantum key distribution systems with practical
light sources. While active modulation of the intensity of the laser pulses is
an effective way of preparing decoy states in principle, in practice passive
preparation might be desirable in some scenarios. Typical passive schemes
involve parametric down-conversion. More recently, it has been shown that phase
randomized weak coherent pulses (WCP) can also be used for the same purpose [M.
Curty {\it et al.}, Opt. Lett. {\bf 34}, 3238 (2009).] This proposal requires
only linear optics together with a simple threshold photon detector, which
shows the practical feasibility of the method. Most importantly, the resulting
secret key rate is comparable to the one delivered by an active decoy state
setup with an infinite number of decoy settings. In this paper we extend these
results, now showing specifically the analysis for other practical scenarios
with different light sources and photo-detectors. In particular, we consider
sources emitting thermal states, phase randomized WCP, and strong coherent
light in combination with several types of photo-detectors, like, for instance,
threshold photon detectors, photon number resolving detectors, and classical
photo-detectors. Our analysis includes as well the effect that detection
inefficiencies and noise in the form of dark counts shown by current threshold
detectors might have on the final secret ket rate. Moreover, we provide
estimations on the effects that statistical fluctuations due to a finite data
size can have in practical implementations.Comment: 17 pages, 14 figure
Iterations of nonlinear entanglement witnesses
We describe a generic way to improve a given linear entanglement witness by a
quadratic, nonlinear term. This method can be iterated, leading to a whole
sequence of nonlinear witnesses, which become stronger in each step of the
iteration. We show how to optimize this iteration with respect to a given
state, and prove that in the limit of the iteration the nonlinear witness
detects all states that can be detected by the positive map corresponding to
the original linear witness.Comment: 11 pages, 5 figure
Cold atmospheric pressure plasma for treatment of chronic wounds: drug or medical device?
Objective:
The use of cold atmospheric pressure plasma (CAPP) as a new therapeutic option to aid the healing of chronic wounds appears promising. Currently, uncertainty exists regarding their classification as medical device or medical drug. Because the classification of CAPP has medical, legal, and economic consequences as well as implications for the level of preclinical and clinical testing, the correct classification is not an academic exercise, but an ethical need.
Method:
A multidisciplinary team of physicians, surgeons, pharmacists, physicists and lawyers has analysed the physical and technical characteristics as well as legal conditions of the biological action of CAPP.
Results:
It was concluded that the mode of action of the locally generated CAPP, with its main active components being different radicals, is pharmacological and not physical in nature.
Conclusion:
Depending on the intended use, CAPP should be classified as a drug, which is generated by use of a medical device directly at the point of therapeutic application
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
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
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