674 research outputs found
A genetic algorithm-assisted semi-adaptive MMSE multi-user detection for MC-CDMA mobile communication systems
In this work, a novel Minimum-Mean Squared-Error (MMSE) multi-user detector is proposed for MC-CDMA transmission systems working over mobile radio channels characterized by time-varying multipath fading. The proposed MUD algorithm is based on a Genetic Algorithm (GA)-assisted per-carrier MMSE criterion. The GA block works in two successive steps: a training-aided step aimed at computing the optimal receiver weights using a very short training sequence, and a decision-directed step aimed at dynamically updating the weights vector during a channel coherence period. Numerical results evidenced BER performances almost coincident with ones yielded by ideal MMSE-MUD based on the perfect knowledge of channel impulse response. The proposed GA-assisted MMSE-MUD clearly outperforms state-of-the-art adaptive MMSE receivers based on deterministic gradient algorithms, especially for high number of transmitting users
Pure down-conversion photons through sub-coherence length domain engineering
Photonic quantum technology relies on efficient sources of coherent single
photons, the ideal carriers of quantum information. Heralded single photons
from parametric down-conversion can approximate on-demand single photons to a
desired degree, with high spectral purities achieved through group-velocity
matching and tailored crystal nonlinearities.
Here we propose crystal nonlinearity engineering techniques with
sub-coherence-length domains. We first introduce a combination of two existing
methods: a deterministic approach with coherence-length domains and
probabilistic domain-width annealing. We then show how the same deterministic
domain-flip approach can be implemented with sub-coherence length domains. Both
of these complementary techniques create highly pure photons, outperforming
previous methods, in particular for short nonlinear crystals matched to
femtosecond lasers.Comment: 12 pages, 4 figures. Minor update to Fig.
A Novel Protocol-Authentication Algorithm Ruling Out a Man-in-the-Middle Attack in Quantum Cryptography
In this work we review the security vulnerability of Quantum Cryptography
with respect to "man-in-the-middle attacks" and the standard authentication
methods applied to counteract these attacks. We further propose a modified
authentication algorithm which features higher efficiency with respect to
consumption of mutual secret bits.Comment: 4 pages, submitted to the International Journal of Quantum
Information, Proceedings of the meeting "Foundations of Quantum Information",
Camerino, April 200
Efficient measurement of quantum dynamics via compressive sensing
The resources required to characterise the dynamics of engineered quantum
systems-such as quantum computers and quantum sensors-grow exponentially with
system size. Here we adapt techniques from compressive sensing to exponentially
reduce the experimental configurations required for quantum process tomography.
Our method is applicable to dynamical processes that are known to be
nearly-sparse in a certain basis and it can be implemented using only
single-body preparations and measurements. We perform efficient, high-fidelity
estimation of process matrices on an experiment attempting to implement a
photonic two-qubit logic-gate. The data base is obtained under various
decoherence strengths. We find that our technique is both accurate and noise
robust, thus removing a key roadblock to the development and scaling of quantum
technologies.Comment: New title and authors. A new experimental section. Significant
rewrite of the theor
Tribological Behavior of Thermal Spray Coatings, Deposited by HVOF and APS Techniques, and Composite Electrodeposits Ni/SiC at Both Room Temperature and 300 °C
The Both the thermal spray and the electroplating coatings are widely used because of their high wear resistance combined with good corrosion resistance. In particular the addition of both micro particles or nano‐particles to the electro deposited coatings could lead to an increase of the mechanical properties, caused by the change of the coating microstructure. The thermal spray coatings were deposited following industrial standards procedures, while the Ni/SiC composite coatings were produced at laboratory scale using both micro‐and nano‐sized ceramic particles. All the produced coatings were characterized regarding their microstructure,mechanical properties and the wear resistance. The tribological properties were analyzed using a tribometer under ball on disk configuration at both room temperature and 300oC. The results showed that the cermet thermal spray coatings have a high wear resistance, while the Ni nano‐composite showed good anti wear properties compared to the harder ceramic/cermet coatings deposited by thermal spray technique
Two-photon quantum walks in an elliptical direct-write waveguide array
Integrated optics provides an ideal test bed for the emulation of quantum
systems via continuous-time quantum walks. Here we study the evolution of
two-photon states in an elliptic array of waveguides. We characterise the
photonic chip via coherent-light tomography and use the results to predict
distinct differences between temporally indistinguishable and distinguishable
two-photon inputs which we then compare with experimental observations. Our
work highlights the feasibility for emulation of coherent quantum phenomena in
three-dimensional waveguide structures.Comment: 8 pages, 7 figure
Discrete single-photon quantum walks with tunable decoherence
Quantum walks have a host of applications, ranging from quantum computing to
the simulation of biological systems. We present an intrinsically stable,
deterministic implementation of discrete quantum walks with single photons in
space. The number of optical elements required scales linearly with the number
of steps. We measure walks with up to 6 steps and explore the
quantum-to-classical transition by introducing tunable decoherence. Finally, we
also investigate the effect of absorbing boundaries and show that decoherence
significantly affects the probability of absorption.Comment: Published version, 5 pages, 4 figure
Information complementarity in quantum physics
We demonstrate that the concept of information offers a more complete
description of complementarity than the traditional approach based on
observables. We present the first experimental test of information
complementarity for two-qubit pure states, achieving close agreement with
theory; We also explore the distribution of information in a comprehensive
range of mixed states. Our results highlight the strange and subtle properties
of even the simplest quantum systems: for example, entanglement can be
increased by reducing correlations between two subsystems.Comment: 6 pages, 7 figures (including supplementary material
Practical Quantum Key Distribution with Polarization-Entangled Photons
We present an entangled-state quantum cryptography system that operated for
the first time in a real world application scenario. The full key generation
protocol was performed in real time between two distributed embedded hardware
devices, which were connected by 1.45 km of optical fiber, installed for this
experiment in the Vienna sewage system. The generated quantum key was
immediately handed over and used by a secure communication application.Comment: 5 pages, 3 figure
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