2,659 research outputs found
Quantum repeaters based on heralded qubit amplifiers
We present a quantum repeater scheme based on the recently proposed qubit
amplifier [N. Gisin, S. Pironio and N. Sangouard, Phys. Rev. Lett. 105, 070501
(2010)]. It relies on a on-demand entangled-photon pair source which uses
on-demand single-photon sources, linear optical elements and atomic ensembles.
Interestingly, the imperfections affecting the states created from this source,
caused e.g. by detectors with non-unit efficiencies, are systematically
purified from an entanglement swapping operation based on a two-photon
detection. This allows the distribution of entanglement over very long
distances with a high fidelity, i.e. without vacuum components and multiphoton
errors. Therefore, the resulting quantum repeater architecture does not
necessitate final postselections and thus achieves high entanglement
distribution rates. This also provides unique opportunities for
device-independent quantum key distribution over long distances with linear
optics and atomic ensembles.Comment: 8 pages, 4 figure
REAM intensity modulator-enabled 10Gb/s colorless upstream transmission of real-time optical OFDM signals in a single-fiber-based bidirectional PON architecture
Reflective electro-absorption modulation-intensity modulators (REAM-IMs) are utilized, for the first time, to experimentally demonstrate colorless ONUs in single-fiber-based, bidirectional, intensity-modulation and direct-detection (IMDD), optical OFDM PONs (OOFDM-PONs) incorporating 25km SSMFs and OLT-side-seeded CW optical signals. The colorlessness of the REAM-IMs is characterized, based on which optimum REAM-IM operating conditions are identified. In the aforementioned PON architecture, 10Gb/s colorless upstream transmissions of end-to-end realtime OOFDM signals are successfully achieved for various wavelengths within the entire C-band. Over such a wavelength window, corresponding minimum received optical powers at the FEC limit vary in a range as small as <0.5dB. In addition, experimental measurements also indicate that Rayleigh backscattering imposes a 2.8dB optical power penalty on the 10Gb/s over 25km upstream OOFDM signal transmission. Furthermore, making use of on-line adaptive bit and power loading, a linear trade-off between aggregated signal line rate and optical power budget is observed, which shows that, for the present PON system, a 10% reduction in signal line rate can improve the optical power budget by 2.6dB. © 2012 Optical Society of America
Emergence of thin shell structure during collapse in isotropic coordinates
Numerical studies of gravitational collapse in isotropic coordinates have
recently shown an interesting connection between the gravitational Lagrangian
and black hole thermodynamics. A study of the actual spacetime was not the main
focus of this work and in particular, the rich and interesting structure of the
interior has not been investigated in much detail and remains largely unknown.
We elucidate its features by performing a numerical study of the spacetime in
isotropic coordinates during gravitational collapse of a massless scalar field.
The most salient feature to emerge is the formation of a thin shell of matter
just inside the apparent horizon. The energy density and Ricci scalar peak at
the shell and there is a jump discontinuity in the extrinsic curvature across
the apparent horizon, the hallmark that a thin shell is present in its
vicinity. At late stages of the collapse, the spacetime consists of two vacuum
regions separated by the thin shell. The interior is described by an
interesting collapsing isotropic universe. It tends towards a vacuum (never
reaches a perfect vacuum) and there is a slight inhomogeneity in the interior
that plays a crucial role in the collapse process as the areal radius tends to
zero. The spacetime evolves towards a curvature (physical) singularity in the
interior, both a Weyl and Ricci singularity. In the exterior, our numerical
results match closely the analytical form of the Schwarzschild metric in
isotropic coordinates, providing a strong test of our numerical code.Comment: 24 pages, 10 figures. version to appear in Phys. Rev.
Percolation in suspensions of polydisperse hard rods : quasi-universality and finite-size effects
We present a study of connectivity percolation in suspensions of hard
spherocylinders by means of Monte Carlo simulation and connectedness
percolation theory. We focus attention on polydispersity in the length, the
diameter and the connectedness criterion, and invoke bimodal, Gaussian and
Weibull distributions for these. The main finding from our simulations is that
the percolation threshold shows quasi universal behaviour, i.e., to a good
approximation it depends only on certain cumulants of the full size and
connectivity distribution. Our connectedness percolation theory hinges on a
Lee-Parsons type of closure recently put forward that improves upon the
often-used second virial approximation [ArXiv e-prints, May 2015, 1505.07660].
The theory predicts exact universality. Theory and simulation agree
quantitatively for aspect ratios in excess of 20, if we include the
connectivity range in our definition of the aspect ratio of the particles. We
further discuss the mechanism of cluster growth that, remarkably, differs
between systems that are polydisperse in length and in width, and exhibits
non-universal aspects.Comment: 7 figure
Comment on ``Nonuniversal Exponents in Interface Growth''
Recently, Newman and Swift[T. J. Newman and M. R. Swift, Phys. Rev. Lett.
{\bf 79}, 2261 (1997)] made an interesting suggestion that the strong-coupling
exponents of the Kardar-Parisi-Zhang (KPZ) equation may not be universal, but
rather depend on the precise form of the noise distribution. We show here that
the decrease of surface roughness exponents they observed can be attributed to
a percolative effect
Microcavity quantum-dot systems for non-equilibrium Bose-Einstein condensation
We review the practical conditions required to achieve a non-equilibrium BEC
driven by quantum dynamics in a system comprising a microcavity field mode and
a distribution of localised two-level systems driven to a step-like population
inversion profile. A candidate system based on eight 3.8nm layers of
In(0.23)Ga(0.77)As in GaAs shows promising characteristics with regard to the
total dipole strength which can be coupled to the field mode.Comment: 4 pages, 4 figures, to be published in J. Phys. Conf. Ser. for QD201
Entrainment transition in populations of random frequency oscillators
The entrainment transition of coupled random frequency oscillators is
revisited. The Kuramoto model (global coupling) is shown to exhibit unusual
sample-dependent finite size effects leading to a correlation size exponent
. Simulations of locally coupled oscillators in -dimensions
reveal two types of frequency entrainment: mean-field behavior at , and
aggregation of compact synchronized domains in three and four dimensions. In
the latter case, scaling arguments yield a correlation length exponent
, in good agreement with numerical results.Comment: published versio
Quantum Repeaters with Photon Pair Sources and Multi-Mode Memories
We propose a quantum repeater protocol which builds on the well-known DLCZ
protocol [L.M. Duan, M.D. Lukin, J.I. Cirac, and P. Zoller, Nature 414, 413
(2001)], but which uses photon pair sources in combination with memories that
allow to store a large number of temporal modes. We suggest to realize such
multi-mode memories based on the principle of photon echo, using solids doped
with rare-earth ions. The use of multi-mode memories promises a speedup in
entanglement generation by several orders of magnitude and a significant
reduction in stability requirements compared to the DLCZ protocol.Comment: 4 pages, 2 figures, to appear in PRL, accepted versio
Quantum state preparation in semiconductor dots by adiabatic rapid passage
Preparation of a specific quantum state is a required step for a variety of
proposed practical uses of quantum dynamics. We report an experimental
demonstration of optical quantum state preparation in a semiconductor quantum
dot with electrical readout, which contrasts with earlier work based on Rabi
flopping in that the method is robust with respect to variation in the optical
coupling. We use adiabatic rapid passage, which is capable of inverting single
dots to a specified upper level. We demonstrate that when the pulse power
exceeds a threshold for inversion, the final state is independent of power.
This provides a new tool for preparing quantum states in semiconductor dots and
has a wide range of potential uses.Comment: 4 pages, 4 figure
Onset of collective and cohesive motion
We study the onset of collective motion, with and without cohesion, of groups
of noisy self-propelled particles interacting locally. We find that this phase
transition, in two space dimensions, is always discontinuous, including for the
minimal model of Vicsek et al. [Phys. Rev. Lett. {\bf 75},1226 (1995)] for
which a non-trivial critical point was previously advocated. We also show that
cohesion is always lost near onset, as a result of the interplay of density,
velocity, and shape fluctuations.Comment: accepted for publication in Phys. Rev. Let
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