10,013 research outputs found
AC conductance and non-symmetrized noise at finite frequency in quantum wires and carbon nanotubes
We calculate the AC conductance and the finite-frequency non-symmetrized
noise in interacting quantum wires and single-wall carbon nanotubes in the
presence of an impurity. We observe a strong asymmetry in the frequency
spectrum of the non-symmetrized excess noise, even in the presence of the
metallic leads. We find that this asymmetry is proportional to the differential
excess AC conductance of the system, defined as the difference between the AC
differential conductances at finite and zero voltage, and thus disappears for a
linear system. In the quantum regime, for temperatures much smaller than the
frequency and the applied voltage, we find that the emission noise is exactly
equal to the impurity partition noise. For the case of a weak impurity we
expand our results for the AC conductance and the noise perturbatively. In
particular, if the impurity is located in the middle of the wire or at one of
the contacts, our calculations show that the noise exhibits oscillations with
respect to frequency, whose period is directly related to the value of the
interaction parameter
Analysis of circuit imperfections in BosonSampling
BosonSampling is a problem where a quantum computer offers a provable speedup
over classical computers. Its main feature is that it can be solved with
current linear optics technology, without the need for a full quantum computer.
In this work, we investigate whether an experimentally realistic BosonSampler
can really solve BosonSampling without any fault-tolerance mechanism. More
precisely, we study how the unavoidable errors linked to an imperfect
calibration of the optical elements affect the final result of the computation.
We show that the fidelity of each optical element must be at least , where refers to the number of single photons in the scheme. Such
a requirement seems to be achievable with state-of-the-art equipment.Comment: 20 pages, 7 figures, v2: new title, to appear in QI
Optimal incorporation of sparsity information by weighted optimization
Compressed sensing of sparse sources can be improved by incorporating prior
knowledge of the source. In this paper we demonstrate a method for optimal
selection of weights in weighted norm minimization for a noiseless
reconstruction model, and show the improvements in compression that can be
achieved.Comment: 5 pages, 2 figures, to appear in Proceedings of ISIT201
Guiding light via geometric phases
Known methods for transverse confinement and guidance of light can be grouped
into a few basic mechanisms, the most common being metallic reflection, total
internal reflection and photonic-bandgap (or Bragg) reflection. All of them
essentially rely on changes of the refractive index, that is on scalar
properties of light. Recently, processes based on "geometric Berry phases",
such as manipulation of polarization states or deflection of spinning-light
rays, have attracted considerable interest in the contexts of singular optics
and structured light. Here, we disclose a new approach to light waveguiding,
using geometric Berry phases and exploiting polarization states and their
handling. This can be realized in structured three-dimensional anisotropic
media, in which the optic axis lies orthogonal to the propagation direction and
is modulated along it and across the transverse plane, so that the refractive
index remains constant but a phase distortion can be imposed on a beam. In
addition to a complete theoretical analysis with numerical simulations, we
present a proof-of-principle experimental demonstration of this effect in a
discrete element implementation of a geometric phase waveguide. The mechanism
we introduce shows that spin-orbit optical interactions can play an important
role in integrated optics and paves the way to an entire new class of photonic
systems that exploit the vectorial nature of light.Comment: Publication supported by European Union (EU) within Horizon 2020 -
ERC-Advanced Grant PHOSPhOR, grant no. 694683. This is the final
peer-reviewed manuscript as accepted for publication (including methods and
supplementary information
Computation of Light Scattering in Young Stellar Objects
A Monte Carlo light scattering code incorporating aligned non-spherical
particles is described. The major effects on the flux distribution, linear
polarisation and circular polarisation are presented, with emphasis on the
application to Young Stellar Objects (YSOs). The need for models with
non-spherical particles in order to successfully model polarisation data is
reviewed. The ability of this type of model to map magnetic field structure in
embedded YSOs is described. The possible application to the question of the
origin of biomolecular homochirality via UV circular polarisation in star
forming regions is also briefly discussed.Comment: Accepted by The Journal of Quantitative Spectroscopy and Radiative
Transfer. Replaced version corrects an error in the definition of the sense
of Cpol in the published version and other minor errors found at the proof
stag
Bounds on the Sum Capacity of Synchronous Binary CDMA Channels
In this paper, we obtain a family of lower bounds for the sum capacity of
Code Division Multiple Access (CDMA) channels assuming binary inputs and binary
signature codes in the presence of additive noise with an arbitrary
distribution. The envelope of this family gives a relatively tight lower bound
in terms of the number of users, spreading gain and the noise distribution. The
derivation methods for the noiseless and the noisy channels are different but
when the noise variance goes to zero, the noisy channel bound approaches the
noiseless case. The behavior of the lower bound shows that for small noise
power, the number of users can be much more than the spreading gain without any
significant loss of information (overloaded CDMA). A conjectured upper bound is
also derived under the usual assumption that the users send out equally likely
binary bits in the presence of additive noise with an arbitrary distribution.
As the noise level increases, and/or, the ratio of the number of users and the
spreading gain increases, the conjectured upper bound approaches the lower
bound. We have also derived asymptotic limits of our bounds that can be
compared to a formula that Tanaka obtained using techniques from statistical
physics; his bound is close to that of our conjectured upper bound for large
scale systems.Comment: to be published in IEEE Transactions on Information Theor
Photon engineering for quantum information processing
We study distinguishing information in the context of quantum interference
involving more than one parametric downconversion (PDC) source and in the
context of polarization-entangled photon pairs based on PDC. We arrive at
specific design criteria for two-photon sources so that when used as part of
complex optical systems, such as photon-based quantum information processing
schemes, distinguishing information between the photons is eliminated
guaranteeing high visibility interference. We propose practical techniques
which lead to suitably engineered two-photon states that can be realistically
implemented with available technology. Finally, we study an implementation of
the nonlinear-sign shift (NS) logic gate with PDC sources and show the effect
of distinguishing information on the performance of the gate.Comment: 23 pages, 13 figures. submitted to Quantum Information & Computatio
The Effects of Market Structure on Industry Growth: Rivalrous Non-excludable Capital
We analyze imperfect competition in dynamic environments where firms use rivalrous but nonexcludable industry-specific capital that is provided exogenously. Capital depreciation depends on utilization, so firms influence the evolution of the capital equipment through more or less intensive supply in the final-goods market. Strategic incentives stem from, (i) a dynamic externality, arising due to the non-excludability of the capital stock, leading firms to compete for its use (rivalry), and, (ii) a market externality, leading to the classic Cournot-type supply competition. Comparing alternative market structures, we isolate the effect of these externalities on strategies and industry growth.
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