225 research outputs found
Influence of the thermal history and composition on the melting/solidification process in Sn-Ag-Cu solders
Presented work shows the results of DSC measurement for six Sn based solders. The alloys
Sn96Ag4, Sn99Cu1, Sn97Cu3, Sn96.5Ag3Cu0.5, Sn95.5Ag3.8Cu0.7 and Sn63Pb37 were studied in
the temperature range from room temperature up to 400◦C. The transformation temperatures
for melting as well as for solidification were influenced by the composition and thermal history
of the alloys. The thermal history was altered by changing the maximum thermal cycle temperature
and the heating/cooling rate. It is shown that the rate of solidification is far larger
than that of the melting. The solidification rate is not influenced by neither the composition
nor the thermal history of the material. Analysis of these results is presented
Experimental implementation of the optimal linear-optical controlled phase gate
We report on the first experimental realization of optimal linear-optical
controlled phase gates for arbitrary phases. The realized scheme is entirely
flexible in that the phase shift can be tuned to any given value. All such
controlled phase gates are optimal in the sense that they operate at the
maximum possible success probabilities that are achievable within the framework
of any postselected linear-optical implementation. The quantum gate is
implemented using bulk optical elements and polarization encoding of qubit
states. We have experimentally explored the remarkable observation that the
optimum success probability is not monotone in the phase.Comment: 4 pages, 5 figures, 1 tabl
Symptom-severity-related brain connectivity alterations in functional movement disorders
Background Functional movement disorders, a common cause of neurological disabilities, can occur with heterogeneous motor manifestations including functional weakness. However, the underlying mechanisms related to brain function and connectivity are unknown. Objective To identify brain connectivity alterations related to functional weakness we assessed network centrality changes in a group of patients with heterogeneous motor manifestations using task-free functional MRI in combination with different network centrality approaches. Methods Task-free functional MRI was performed in 48 patients with heterogeneous motor manifestations including 28 patients showing functional weakness and 65 age- and sex-matched healthy controls. Functional connectivity differences were assessed using different network centrality approaches, i.e. global correlation, eigenvector centrality, and intrinsic connectivity. Motor symptom severity was assessed using The Simplified Functional Movement Disorders Rating Scale and correlated with network centrality. Results Comparing patients with and without functional weakness showed significant network centrality differences in the left temporoparietal junction and precuneus. Patients with functional weakness showed increased centrality in the same anatomical regions when comparing functional weakness with healthy controls. Moreover, in the same regions, patients with functional weakness showed a positive correlation between motor symptom severity and network centrality. This correlation was shown to be specific to functional weakness with an interaction analysis, confirming a significant difference between patients with and without functional weakness. Conclusions We identified the temporoparietal junction and precuneus as key regions involved in brain connectivity alterations related to functional weakness. We propose that both regions may be promising targets for phenotype-specific non-invasive brain stimulation
Security Proof for Quantum Key Distribution Using Qudit Systems
We provide security bounds against coherent attacks for two families of
quantum key distribution protocols that use -dimensional quantum systems. In
the asymptotic regime, both the secret key rate for fixed noise and the
robustness to noise increase with . The finite-key corrections are found to
be almost insensitive to .Comment: 5 pages, 1 figure, version 3 corrects equations (9) and (11), and
slightly modifies the figure to reflect the change to equation (11
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
Unambiguous state discrimination in quantum cryptography with weak coherent states
The use of linearly independent signal states in realistic implementations of
quantum key distribution (QKD) enables an eavesdropper to perform unambiguous
state discrimination. We explore quantitatively the limits for secure QKD
imposed by this fact taking into account that the receiver can monitor to some
extend the photon number statistics of the signals even with todays standard
detection schemes. We compare our attack to the beamsplitting attack and show
that security against beamsplitting attack does not necessarily imply security
against the attack considered here.Comment: 10 pages, 6 figures, updated version with added discussion of
beamsplitting attac
Upper bounds for the secure key rate of decoy state quantum key distribution
The use of decoy states in quantum key distribution (QKD) has provided a
method for substantially increasing the secret key rate and distance that can
be covered by QKD protocols with practical signals. The security analysis of
these schemes, however, leaves open the possibility that the development of
better proof techniques, or better classical post-processing methods, might
further improve their performance in realistic scenarios. In this paper, we
derive upper bounds on the secure key rate for decoy state QKD. These bounds
are based basically only on the classical correlations established by the
legitimate users during the quantum communication phase of the protocol. The
only assumption about the possible post-processing methods is that double click
events are randomly assigned to single click events. Further we consider only
secure key rates based on the uncalibrated device scenario which assigns
imperfections such as detection inefficiency to the eavesdropper. Our analysis
relies on two preconditions for secure two-way and one-way QKD: 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. Both criteria have been previously applied to evaluate single-photon
implementations of QKD. Here we use them to investigate a realistic source of
weak coherent pulses. The resulting upper bounds can be formulated as a convex
optimization problem known as a semidefinite program which can be efficiently
solved. For the standard four-state QKD protocol, they are quite close to known
lower bounds, thus showing that there are clear limits to the further
improvement of classical post-processing techniques in decoy state QKD.Comment: 10 pages, 3 figure
Abnormal activity in the precuneus during time perception in Parkinson’s disease: An fMRI study
Background Parkinson's disease (PD) patients are deficient in time estimation. This deficit improves after dopamine (DA) treatment and it has been associated with decreased internal timekeeper speed, disruption of executive function and memory retrieval dysfunction. Methodology/Findings The aim of the present study was to explore the neurophysiologic correlates of this deficit. We performed functional magnetic resonance imaging on twelve PD patients while they were performing a time reproduction task (TRT). The TRT consisted of an encoding phase (during which visual stimuli of durations from 5s to 16.6s, varied at 8 levels were presented) and a reproduction phase (during which interval durations were reproduced by a button pressing). Patients were scanned twice, once while on their DA medication (ON condition) and once after medication withdrawal (OFF condition). Differences in Blood-Oxygenation-Level-Dependent (BOLD) signal in ON and OFF conditions were evaluated. The time course of activation in the brain areas with different BOLD signal was plotted. There were no significant differences in the behavioral results, but a trend toward overestimation of intervals ≤11.9s and underestimation of intervals ≥14.1s in the OFF condition (p<0.088). During the reproduction phase, higher activation in the precuneus was found in the ON condition (p<0.05 corrected). Time course was plotted separately for long (≥14.1s) and short (≤11.9s) intervals. Results showed that there was a significant difference only in long intervals, when activity gradually decreased in the OFF, but remained stable in the ON condition. This difference in precuneus activation was not found during random button presses in a control task. Conclusions/Significance Our results show that differences in precuneus activation during retrieval of a remembered duration may underlie some aspects of time perception deficit in PD patients. We suggest that DA medication may allow compensatory activation in the precuneus, which results in a more accurate retrieval of remembered interval duration
Passive-scheme analysis for solving untrusted source problem in quantum key distribution
As a practical method, the passive scheme is useful to monitor the photon
statistics of an untrusted source in a "Plug & Play" quantum key distribution
(QKD) system. In a passive scheme, three kinds of monitor mode can be adopted:
average photon number (APN) monitor, photon number analyzer (PNA) and photon
number distribution (PND) monitor. In this paper, the security analysis is
rigorously given for APN monitor, while for PNA, the analysis including
statistical fluctuation and random noise, is addressed with a confidence level.
The results show that the PNA can achieve better performance than the APN
monitor and can asymptotically approach the theoretical limit of the PND
monitor. Also, the passive scheme with the PNA works efficiently when the
signal-to-noise ratio () is not too low and so is highly applicable to
solve the untrusted source problem in the QKD system.Comment: 8 pages, 6 figures, published versio
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