6,303 research outputs found
Phase-Remapping Attack in Practical Quantum Key Distribution Systems
Quantum key distribution (QKD) can be used to generate secret keys between
two distant parties. Even though QKD has been proven unconditionally secure
against eavesdroppers with unlimited computation power, practical
implementations of QKD may contain loopholes that may lead to the generated
secret keys being compromised. In this paper, we propose a phase-remapping
attack targeting two practical bidirectional QKD systems (the "plug & play"
system and the Sagnac system). We showed that if the users of the systems are
unaware of our attack, the final key shared between them can be compromised in
some situations. Specifically, we showed that, in the case of the
Bennett-Brassard 1984 (BB84) protocol with ideal single-photon sources, when
the quantum bit error rate (QBER) is between 14.6% and 20%, our attack renders
the final key insecure, whereas the same range of QBER values has been proved
secure if the two users are unaware of our attack; also, we demonstrated three
situations with realistic devices where positive key rates are obtained without
the consideration of Trojan horse attacks but in fact no key can be distilled.
We remark that our attack is feasible with only current technology. Therefore,
it is very important to be aware of our attack in order to ensure absolute
security. In finding our attack, we minimize the QBER over individual
measurements described by a general POVM, which has some similarity with the
standard quantum state discrimination problem.Comment: 13 pages, 8 figure
Dynamics of Neural Networks with Continuous Attractors
We investigate the dynamics of continuous attractor neural networks (CANNs).
Due to the translational invariance of their neuronal interactions, CANNs can
hold a continuous family of stationary states. We systematically explore how
their neutral stability facilitates the tracking performance of a CANN, which
is believed to have wide applications in brain functions. We develop a
perturbative approach that utilizes the dominant movement of the network
stationary states in the state space. We quantify the distortions of the bump
shape during tracking, and study their effects on the tracking performance.
Results are obtained on the maximum speed for a moving stimulus to be
trackable, and the reaction time to catch up an abrupt change in stimulus.Comment: 6 pages, 7 figures with 4 caption
Undulation Instability of Epithelial Tissues
Treating the epithelium as an incompressible fluid adjacent to a viscoelastic
stroma, we find a novel hydrodynamic instability that leads to the formation of
protrusions of the epithelium into the stroma. This instability is a candidate
for epithelial fingering observed in vivo. It occurs for sufficiently large
viscosity, cell-division rate and thickness of the dividing region in the
epithelium. Our work provides physical insight into a potential mechanism by
which interfaces between epithelia and stromas undulate, and potentially by
which tissue dysplasia leads to cancerous invasion.Comment: 4 pages, 3 figure
Geometrical dependence of low frequency noise in superconducting flux qubits
A general method for directly measuring the low-frequency flux noise (below
10 Hz) in compound Josephson junction superconducting flux qubits has been used
to study a series of 85 devices of varying design. The variation in flux noise
across sets of qubits with identical designs was observed to be small. However,
the levels of flux noise systematically varied between qubit designs with
strong dependence upon qubit wiring length and wiring width. Furthermore,
qubits fabricated above a superconducting ground plane yielded lower noise than
qubits without such a layer. These results support the hypothesis that
localized magnetic impurities in the vicinity of the qubit wiring are a key
source of low frequency flux noise in superconducting devices.Comment: 5 pages, 5 figure
Instability and `Sausage-String' Appearance in Blood Vessels during High Blood Pressure
A new Rayleigh-type instability is proposed to explain the `sausage-string'
pattern of alternating constrictions and dilatations formed in blood vessels
under influence of a vasoconstricting agent. Our theory involves the nonlinear
elasticity characteristics of the vessel wall, and provides predictions for the
conditions under which the cylindrical form of a blood vessel becomes unstable.Comment: 4 pages, 4 figures submitted to Physical Review Letter
Interactions among three species of Sharks and Grouper spawning aggregations in the US Virgin Islands
High bone turnover is associated with lower bone density in Hurler-Scheie and Hunter syndromes treated with enzyme replacement therapy
Beam Orientation Optimization for Intensity Modulated Radiation Therapy using Adaptive l1 Minimization
Beam orientation optimization (BOO) is a key component in the process of IMRT
treatment planning. It determines to what degree one can achieve a good
treatment plan quality in the subsequent plan optimization process. In this
paper, we have developed a BOO algorithm via adaptive l_1 minimization.
Specifically, we introduce a sparsity energy function term into our model which
contains weighting factors for each beam angle adaptively adjusted during the
optimization process. Such an energy term favors small number of beam angles.
By optimizing a total energy function containing a dosimetric term and the
sparsity term, we are able to identify the unimportant beam angles and
gradually remove them without largely sacrificing the dosimetric objective. In
one typical prostate case, the convergence property of our algorithm, as well
as the how the beam angles are selected during the optimization process, is
demonstrated. Fluence map optimization (FMO) is then performed based on the
optimized beam angles. The resulted plan quality is presented and found to be
better than that obtained from unoptimized (equiangular) beam orientations. We
have further systematically validated our algorithm in the contexts of 5-9
coplanar beams for 5 prostate cases and 1 head and neck case. For each case,
the final FMO objective function value is used to compare the optimized beam
orientations and the equiangular ones. It is found that, our BOO algorithm can
lead to beam configurations which attain lower FMO objective function values
than corresponding equiangular cases, indicating the effectiveness of our BOO
algorithm.Comment: 19 pages, 2 tables, and 5 figure
NMR Simulation of an Eight-State Quantum System
The propagation of excitation along a one-dimensional chain of atoms is
simulated by means of NMR. The physical system used as an analog quantum
computer is a nucleus of 133-Cs (spin 7/2) in a liquid crystalline matrix. The
Hamiltonian of migration is simulated by using a special 7-frequency pulse, and
the dynamics is monitored by following the transfer of population from one of
the 8 spin energy levels to the other.Comment: 10 pages, 3 figure
Conceptual and practical challenges for implementing the communities of practice model on a national scale - a Canadian cancer control initiative
<p>Abstract</p> <p>Background</p> <p>Cancer program delivery, like the rest of health care in Canada, faces two ongoing challenges: to coordinate a pan-Canadian approach across complex provincial jurisdictions, and to facilitate the rapid translation of knowledge into clinical practice. Communities of practice, or CoPs, which have been described by Etienne Wenger as a collaborative learning platform, represent a promising solution to these challenges because they rely on bottom-up rather than top-down social structures for integrating knowledge and practice across regions and agencies. The communities of practice model has been realized in the corporate (e.g., Royal Dutch Shell, Xerox, IBM, etc) and development (e.g., World Bank) sectors, but its application to health care is relatively new. The Canadian Partnership Against Cancer (CPAC) is exploring the potential of Wenger's concept in the Canadian health care context. This paper provides an in-depth analysis of Wenger's concept with a focus on its applicability to the health care sector.</p> <p>Discussion</p> <p>Empirical studies and social science theory are used to examine the utility of Wenger's concept. Its value lies in emphasizing learning from peers and through practice in settings where innovation is valued. Yet the communities of practice concept lacks conceptual clarity because Wenger defines it so broadly and sidelines issues of decision making within CoPs. We consider the implications of his broad definition to establishing an informed nomenclature around this specific type of collaborative group. The CoP Project under CPAC and communities of practice in Canadian health care are discussed.</p> <p>Summary</p> <p>The use of communities of practice in Canadian health care has been shown in some instances to facilitate quality improvements, encourage buy in among participants, and generate high levels of satisfaction with clinical leadership and knowledge translation among participating physicians. Despite these individual success stories, more information is required on how group decisions are made and applied to the practice world in order to leverage the potential of Wenger's concept more fully, and advance the science of knowledge translation within an accountability framework.</p
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