3,561 research outputs found
Halogenation of SiC for band-gap engineering and excitonic functionalization
The optical excitation spectra and excitonic resonances are investigated in
systematically functionalized SiC with Fluorine and/or Chlorine utilizing
density functional theory in combination with many-body perturbation theory.
The latter is required for a realistic description of the energy band-gaps as
well as for the theoretical realization of excitons. Structural, electronic and
optical properties are scrutinized and show the high stability of the predicted
two-dimensional materials. Their realization in laboratory is thus possible.
Huge band-gaps of the order of 4 eV are found in the so-called GW
approximation, with the occurrence of bright excitons, optically active in the
four investigated materials. Their binding energies vary from 0.9 eV to 1.75 eV
depending on the decoration choice and in one case, a dark exciton is foreseen
to exist in the fully chlorinated SiC. The wide variety of opto-electronic
properties suggest halogenated SiC as interesting materials with potential not
only for solar cell applications, anti-reflection coatings or high-reflective
systems but also for a possible realization of excitonic Bose-Einstein
condensation
Suitability versus fidelity for rating single-photon guns
The creation of specified quantum states is important for most, if not all,
applications in quantum computation and communication. The quality of the state
preparation is therefore an essential ingredient in any assessment of a
quantum-state gun. We show that the fidelity, under the standard definitions is
not sufficient to assess quantum sources, and we propose a new measure of
suitability that necessarily depends on the application for the source. We
consider the performance of single-photon guns in the context of quantum key
distribution (QKD) and linear optical quantum computation. Single-photon
sources for QKD need radically different properties than sources for quantum
computing. Furthermore, the suitability for single-photon guns is discussed
explicitly in terms of experimentally accessible criteria.Comment: 4 pages, 2 figures Revised per referee suggestion
Analyse morphologique de films CrN. Corrélation entre contrainte résiduelle et résistivité de films de chrome
Les films minces de nitrure de chrome ont sensiblement attiré l'attention, ces dernières années. Attrait du à leurs excellentes propriétés physiques, chimiques et mécaniques. Une partie de cette étude a porté sur la caractérisation morphologique de couches minces dures de nitrure de chrome (Cr-N), élaborées par PVD sur substrats de silicium (100) par pulvérisation magnétron. Nous avons considéré l'influence de la température de recuit sur l'adhérence et la stabilité thermique de ces films. Nous avons procédé à des caractérisations en microscopie électronique à balayage (MEB) équipé en microanalyse X (EDX).Des recuits d'une heure, entre 600°C et 1000°C sous azote N2, effectués sur des revêtements CrN,de différentes épaisseurs entre 0.5 et 1µm, ont montré la stabilité thermique de ces revêtements aux basses températures. Les résultats obtenus en MEB et EDX sont comparés aux résultats en DRX. Cette étude porte aussi sur des dépôts de couches de chrome élaborés par pulvérisation RF cathodique (PVD). L'analyse des contraintes résiduelles par le dispositif des anneaux de Newton, pour différentes épaisseurs, montre l'existence d'un pic de contraintes entre 170 et 200 nm. La méthode des quatre pointes a été utilisée pour déterminer la résistivité des couches de chrome. La résistivité décroît fortement pour les petites épaisseurs (< 100nm), pour ensuite se stabiliser à une valeur moyenne de 30µ.Ohm.cm. Une corrélation entre l'évolution des contraintes et de la résistivité nous a permis de faire un essai d'interprétation sur la croissance des films
Fast Purcell-enhanced single photon source in 1,550-nm telecom band from a resonant quantum dot-cavity coupling
High-bit-rate nanocavity-based single photon sources in the 1,550-nm telecom
band are challenges facing the development of fibre-based long-haul quantum
communication networks. Here we report a very fast single photon source in the
1,550-nm telecom band, which is achieved by a large Purcell enhancement that
results from the coupling of a single InAs quantum dot and an InP photonic
crystal nanocavity. At a resonance, the spontaneous emission rate was enhanced
by a factor of 5 resulting a record fast emission lifetime of 0.2 ns at 1,550
nm. We also demonstrate that this emission exhibits an enhanced anti-bunching
dip. This is the first realization of nanocavity-enhanced single photon
emitters in the 1,550-nm telecom band. This coupled quantum dot cavity system
in the telecom band thus provides a bright high-bit-rate non-classical single
photon source that offers appealing novel opportunities for the development of
a long-haul quantum telecommunication system via optical fibres.Comment: 16 pages, 4 figure
Matter wave pulses characteristics
We study the properties of quantum single-particle wave pulses created by
sharp-edged or apodized shutters with single or periodic openings. In
particular, we examine the visibility of diffraction fringes depending on
evolution time and temperature; the purity of the state depending on the
opening-time window; the accuracy of a simplified description which uses
``source'' boundary conditions instead of solving an initial value problem; and
the effects of apodization on the energy width.Comment: 11 pages, 11 figure
Probabilistic Quantum Logic Operations Using Polarizing Beam Splitters
It has previously been shown that probabilistic quantum logic operations can
be performed using linear optical elements, additional photons (ancilla), and
post-selection based on the output of single-photon detectors. Here we describe
the operation of several quantum logic operations of an elementary nature,
including a quantum parity check and a quantum encoder, and we show how they
can be combined to implement a controlled-NOT (CNOT) gate. All of these gates
can be constructed using polarizing beam splitters that completely transmit one
state of polarization and totally reflect the orthogonal state of polarization,
which allows a simple explanation of each operation. We also describe a
polarizing beam splitter implementation of a CNOT gate that is closely
analogous to the quantum teleportation technique previously suggested by
Gottesman and Chuang [Nature 402, p.390 (1999)]. Finally, our approach has the
interesting feature that it makes practical use of a quantum-eraser technique.Comment: 9 pages, RevTex; Submitted to Phys. Rev. A; additional references
inlcude
Single Photons on Pseudo-Demand from Stored Parametric Down-Conversion
We describe the results of a parametric down-conversion experiment in which
the detection of one photon of a pair causes the other photon to be switched
into a storage loop. The stored photon can then be switched out of the loop at
a later time chosen by the user, providing a single photon for potential use in
a variety of quantum information processing applications. Although the stored
single photon is only available at periodic time intervals, those times can be
chosen to match the cycle time of a quantum computer by using pulsed
down-conversion. The potential use of the storage loop as a photonic quantum
memory device is also discussed.Comment: 8 pages, 7 Figs., RevTe
Determinisitic Optical Fock State Generation
We present a scheme for the deterministic generation of N-photon Fock states
from N three-level atoms in a high-finesse optical cavity. The method applies
an external laser pulsethat generates an -photon output state while
adiabatically keeping the atom-cavity system within a subspace of optically
dark states. We present analytical estimates of the error due to amplitude
leakage from these dark states for general N, and compare it with explicit
results of numerical simulations for N \leq 5. The method is shown to provide a
robust source of N-photon states under a variety of experimental conditions and
is suitable for experimental implementation using a cloud of cold atoms
magnetically trapped in a cavity. The resulting N-photon states have potential
applications in fundamental studies of non-classical states and in quantum
information processing.Comment: 25 pages, 9 figure
Interferometric Tests of Teleportation
We investigate a direct test of teleportation efficacy based on a
Mach-Zehnder interferometer. The analysis is performed for continuous variable
teleportation of both discrete and continuous observables
An Elementary Quantum Network of Single Atoms in Optical Cavities
Quantum networks are distributed quantum many-body systems with tailored
topology and controlled information exchange. They are the backbone of
distributed quantum computing architectures and quantum communication. Here we
present a prototype of such a quantum network based on single atoms embedded in
optical cavities. We show that atom-cavity systems form universal nodes capable
of sending, receiving, storing and releasing photonic quantum information.
Quantum connectivity between nodes is achieved in the conceptually most
fundamental way: by the coherent exchange of a single photon. We demonstrate
the faithful transfer of an atomic quantum state and the creation of
entanglement between two identical nodes in independent laboratories. The
created nonlocal state is manipulated by local qubit rotation. This efficient
cavity-based approach to quantum networking is particularly promising as it
offers a clear perspective for scalability, thus paving the way towards
large-scale quantum networks and their applications.Comment: 8 pages, 5 figure
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