311 research outputs found
Universal measurement apparatus controlled by quantum software
We propose a quantum device that can approximate any projective measurement
on a qubit. The desired measurement basis is selected by the quantum state of a
"program register". The device is optimized with respect to maximal average
fidelity (assuming uniform distribution of measurement bases). An interesting
result is that if one uses two qubits in the same state as a program the
average fidelity is higher than if he/she takes the second program qubit in the
orthogonal state (with respect to the first one). The average information
obtainable by the proposed measurements is also calculated and it is shown that
it can get different values even if the average fidelity stays constant.
Possible experimental realization of the simplest proposed device is presented.Comment: 4 pages, 2 figures, reference adde
Image Analysis of Eccentric Photorefraction
This article deals with image and data analysis of the recorded video-sequences of strabistic infants. It describes a unique noninvasive measuring system based on two measuring methods (position of I. Purkynje image with relation to the centre of the lens and eccentric photorefraction) for infants. The whole process is divided into three steps. The aim of the first step is to obtain video sequences on our special system (Eye Movement Analyser). Image analysis of the recorded sequences is performed in order to obtain curves of basic eye reactions (accommodation and convergence). The last step is to calibrate of these curves to corresponding units (diopter and degrees of movement)
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
Visibility bound caused by a distinguishable noise particle
We investigate how distinguishability of a "noise" particle degrades
interference of the "signal" particle. The signal, represented by an equatorial
state of a photonic qubit, is mixed with noise, represented by another photonic
qubit, via linear coupling on the beam splitter. We report on the degradation
of the "signal" photon interference depending on the degree of
indistinguishability between "signal" and "noise" photon. When the photons are
principally completely distinguishable but technically indistinguishable the
visibility drops to the value 1/sqrt(2). As the photons become more
indistinguishable the maximal visibility increases and reaches the unit value
for completely indistinguishable photons. We have examined this effect
experimentally using setup with fiber optics two-photon Mach-Zehnder
interferometer.Comment: 5 pages, 3 figures, Accepted to Phys. Rev.
Quantum cryptography with finite resources: unconditional security bound for discrete-variable protocols with one-way post-processing
We derive a bound for the security of QKD with finite resources under one-way
post-processing, based on a definition of security that is composable and has
an operational meaning. While our proof relies on the assumption of collective
attacks, unconditional security follows immediately for standard protocols like
Bennett-Brassard 1984 and six-states. For single-qubit implementations of such
protocols, we find that the secret key rate becomes positive when at least
N\sim 10^5 signals are exchanged and processed. For any other discrete-variable
protocol, unconditional security can be obtained using the exponential de
Finetti theorem, but the additional overhead leads to very pessimistic
estimates
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
Passive sources for the Bennett-Brassard 1984 quantum key distribution protocol with practical signals
Most experimental realizations of quantum key distribution are based on the
Bennett-Brassard 1984 (so-called BB84) protocol. In a typical optical
implementation of this scheme, the sender uses an active source to produce the
required BB84 signal states. While active state preparation of BB84 signals is
a simple and elegant solution in principle, in practice passive state
preparation might be desirable in some scenarios, for instance, in those
experimental setups operating at high transmission rates. Passive schemes might
also be more robust against side-channel attacks than active sources. Typical
passive devices involve parametric down-conversion. In this paper, we show that
both coherent light and practical single photon sources are also suitable for
passive generation of BB84 signal states. Our method does not require any
external-driven element, but only linear optical components and photodetectors.
In the case of coherent light, the resulting key rate is similar to the one
delivered by an active source. When the sender uses practical single photon
sources, however, the distance covered by a passive transmitter might be longer
than the one of an active configuration.Comment: 14 pages, 11 figure
Increasing efficiency of a linear-optical quantum gate using an electronic feed forward
We have successfully used a fast electronic feed forward to increase the
success probability of a linear optical implementation of a programmable phase
gate from 25% to its theoretical limit of 50%. The feed forward applies a
conditional unitary operation which changes the incorrect output states of the
data qubit to the correct ones. The gate itself rotates an arbitrary quantum
state of the data qubit around the z-axis of the Bloch sphere with the angle of
rotation being fully determined by the state of the program qubit. The gate
implementation is based on fiber optics components. Qubits are encoded into
spatial modes of single photons. The signal from the feed-forward detector is
led directly to a phase modulator using only a passive voltage divider. We have
verified the increase of the success probability and characterized the gate
operation by means of quantum process tomography. We have demonstrated that the
use of the feed forward does not affect either the process fidelity or the
output-state fidelities
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