4,542 research outputs found
Quantum cryptographic ranging
We present a system to measure the distance between two parties that allows
only trusted people to access the result. The security of the protocol is
guaranteed by the complementarity principle in quantum mechanics. The protocol
can be realized with available technology, at least as a proof of principle
experiment.Comment: 2 pages, 1 figure. Contribution to the proceedings of the IV edition
of the Garda Lake Workshop "Mysteries, Puzzles and Paradoxes in Quantum
Mechanics
The Step-Harmonic Potential
We analyze the behavior of a quantum system described by a one-dimensional
asymmetric potential consisting of a step plus a harmonic barrier. We solve the
eigenvalue equation by the integral representation method, which allows us to
classify the independent solutions as equivalence classes of homotopic paths in
the complex plane. We then consider the propagation of a wave packet reflected
by the harmonic barrier and obtain an expression for the interaction time as a
function of the peak energy. For high energies we recover the classical
half-period limit.Comment: 19 pages, 7 figure
Forecasting isocurvature models with CMB lensing information: axion and curvaton scenarios
Some inflationary models predict the existence of isocurvature primordial
fluctuations, in addition to the well known adiabatic perturbation. Such mixed
models are not yet ruled out by available data sets. In this paper we explore
the possibility of obtaining better constraints on the isocurva- ture
contribution from future astronomical data. We consider the axion and curvaton
inflationary scenarios, and use Planck satellite experimental specifications
together with SDSS galaxy survey to forecast for the best parameter error
estimation by means of the Fisher information matrix formal- ism. In
particular, we consider how CMB lensing information can improve this forecast.
We found substantial improvements for all the considered cosmological
parameters. In the case of isocurvature amplitude this improvement is strongly
model dependent, varying between less than 1% and above 20% around its fiducial
value. Furthermore, CMB lensing enables the degeneracy break between the
isocurvature amplitude and correlation phase in one of the models. In this
sense, CMB lensing information will be crucial in the analysis of future data.Comment: Accepted for publication in PR
Dispersive analysis of the experimental data on the electromagnetic form factor of charged pions at spacelike momenta
The experimental data on the electromagnetic form factor of charged pions
available at spacelike momenta are analyzed using the Dispersive Matrix (DM)
approach, which describes the momentum dependence of hadronic form factors
without introducing any explicit parameterization and includes properly the
constraints coming from unitarity and analyticity. The unitary bound is
evaluated nonperturbatively making use of the results of lattice QCD
simulations of suitable two-point correlation functions contributing to the HVP
term of the muon. Thanks to the DM method we determine the pion charge radius
from existing spacelike data in a completely model-independent way and
consistently with the unitary bound, obtaining fm. This finding differs by standard deviations from the
latest PDG value fm, which is dominated by
the very precise results of dispersive analyses of timelike data coming from
measurements of the cross section of the process. We
have analyzed the spacelike data using also traditional -expansions, like
the Boyd-Grinstein-Lebed (BGL) or Bourrely-Caprini-Lellouch (BCL) fitting
functions and adopting a simple procedure that incorporates ab initio the
non-perturbative unitary bound in the fitting process. We get fm and fm in nice
agreement with the DM result. We have addressed also the issue of the onset of
perturbative QCD by performing a sensitivity study of the pion form factor at
large spacelike momenta, based only on experimental spacelike data and
unitarity. Hence, although the leading pQCD behaviour is found to set in only
at very large momenta, our DM bands may provide information about the
pre-asymptotic effects related to the scale dependence of the pion distribution
amplitude.Comment: 39 pages, 21 figures, 4 table
Inter-edge strong-to-weak scattering evolution at a constriction in the fractional quantum Hall regime
Gate-voltage control of inter-edge tunneling at a split-gate constriction in
the fractional quantum Hall regime is reported. Quantitative agreement with the
behavior predicted for out-of-equilibrium quasiparticle transport between
chiral Luttinger liquids is shown at low temperatures at specific values of the
backscattering strength. When the latter is lowered by changing the gate
voltage the zero-bias peak of the tunneling conductance evolves into a minimum
and a non-linear quasihole-like characteristic emerges. Our analysis emphasizes
the role of the local filling factor in the split-gate constriction region.Comment: 4 pages, 4 figure
4e-condensation in a fully frustrated Josephson junction diamond chain
Fully frustrated one-dimensional diamond Josephson chains have been shown [B.
Dou\c{c}ot and J. Vidal, Phys. Rev. Lett. {\bf 88}, 227005 (2002)] to posses a
remarkable property: The superfluid phase occurs through the condensation of
pairs of Cooper pairs. By means of Monte Carlo simulations we analyze
quantitatively the Insulator to -Superfluid transition. We determine the
location of the critical point and discuss the behaviour of the phase-phase
correlators. For comparison we also present the case of a diamond chain at zero
and 1/3 frustration where the standard -condensation is observed.Comment: 5 pages, 7 figure
Bosonic Memory Channels
We discuss a Bosonic channel model with memory effects. It relies on a
multi-mode squeezed (entangled) environment's state. The case of lossy Bosonic
channels is analyzed in detail. We show that in the absence of input energy
constraints the memory channels are equivalent to their memoryless
counterparts. In the case of input energy constraint we provide lower and upper
bounds for the memory channel capacity.Comment: 6 pages, 2 figure
Optimal estimation of quantum observables
We consider the problem of estimating the ensemble average of an observable
on an ensemble of equally prepared identical quantum systems. We show that,
among all kinds of measurements performed jointly on the copies, the optimal
unbiased estimation is achieved by the usual procedure that consists in
performing independent measurements of the observable on each system and
averaging the measurement outcomes.Comment: Submitted to J. Math Phy
Effect of charging on CdSe/CdS dot-in-rods single-photon emission
The photon statistics of CdSe/CdS dot-in-rods nanocrystals is studied with a
method involving post-selection of the photon detection events based on the
photoluminescence count rate. We show that flickering between two states needs
to be taken into account to interpret the single-photon emission properties.
With post-selection we are able to identify two emitting states: the exciton
and the charged exciton (trion), characterized by different lifetimes and
different second order correlation functions. Measurements of the second order
autocorrelation function at zero delay with post- selection shows a degradation
of the single photon emission for CdSe/CdS dot-in-rods in a charged state that
we explain by deriving the neutral and charged biexciton quantum yields.Comment: 10 pages, 5 figure
Normal form decomposition for Gaussian-to-Gaussian superoperators
In this paper we explore the set of linear maps sending the set of quantum
Gaussian states into itself. These maps are in general not positive, a feature
which can be exploited as a test to check whether a given quantum state belongs
to the convex hull of Gaussian states (if one of the considered maps sends it
into a non positive operator, the above state is certified not to belong to the
set). Generalizing a result known to be valid under the assumption of complete
positivity, we provide a characterization of these Gaussian-to-Gaussian (not
necessarily positive) superoperators in terms of their action on the
characteristic function of the inputs. For the special case of one-mode
mappings we also show that any Gaussian-to-Gaussian superoperator can be
expressed as a concatenation of a phase-space dilatation, followed by the
action of a completely positive Gaussian channel, possibly composed with a
transposition. While a similar decomposition is shown to fail in the multi-mode
scenario, we prove that it still holds at least under the further hypothesis of
homogeneous action on the covariance matrix
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