1,386 research outputs found
Interference of Fock states in a single measurement
We study analytically the structure of an arbitrary order correlation
function for a pair of Fock states and prove without any approximations that in
a single measurement of particle positions interference effects must occur as
experimentally observed with Bose-Einstein condensates. We also show that the
noise level present in the statistics is slightly lower than for a respective
measurement of phase states.Comment: 4 page
Parameterization of Dark-Energy Properties: a Principal-Component Approach
Considerable work has been devoted to the question of how to best
parameterize the properties of dark energy, in particular its equation of state
w. We argue that, in the absence of a compelling model for dark energy, the
parameterizations of functions about which we have no prior knowledge, such as
w(z), should be determined by the data rather than by our ingrained beliefs or
familiar series expansions. We find the complete basis of orthonormal
eigenfunctions in which the principal components (weights of w(z)) that are
determined most accurately are separated from those determined most poorly.
Furthermore, we show that keeping a few of the best-measured modes can be an
effective way of obtaining information about w(z).Comment: Unfeasibility of a truly model-independent reconstruction of w at z>1
illustrated. f(z) left out, and w(z) discussed in more detail. Matches the
PRL versio
Exploiting entanglement in communication channels with correlated noise
We develop a model for a noisy communication channel in which the noise
affecting consecutive transmissions is correlated. This model is motivated by
fluctuating birefringence of fiber optic links. We analyze the role of
entanglement of the input states in optimizing the classical capacity of such a
channel. Assuming a general form of an ensemble for two consecutive
transmissions, we derive tight bounds on the classical channel capacity
depending on whether the input states used for communication are separable or
entangled across different temporal slots. This result demonstrates that by an
appropriate choice, the channel capacity may be notably enhanced by exploiting
entanglement.Comment: 9 pages, 5 figure
Effect of Photometric Redshift Uncertainties on Weak Lensing Tomography
We perform a systematic analysis of the effects of photometric redshift
uncertainties on weak lensing tomography. We describe the photo-z distribution
with a bias and Gaussian scatter that are allowed to vary arbitrarily between
intervals of dz = 0.1 in redshift.While the mere presence of bias and scatter
does not substantially degrade dark energy information, uncertainties in both
parameters do. For a fiducial next-generation survey each would need to be
known to better than about 0.003-0.01 in redshift for each interval in order to
lead to less than a factor of 1.5 increase in the dark energy parameter errors.
The more stringent requirement corresponds to a larger dark energy parameter
space, when redshift variation in the equation of state of dark energy is
allowed.Of order 10^4-10^5 galaxies with spectroscopic redshifts fairly sampled
from the source galaxy distribution will be needed to achieve this level of
calibration. If the sample is composed of multiple galaxy types, a fair sample
would be required for each. These requirements increase in stringency for more
ambitious surveys; we quantify such scalings with a convenient fitting formula.
No single aspect of a photometrically binned selection of galaxies such as
their mean or median suffices, indicating that dark energy parameter
determinations are sensitive to the shape and nature of outliers in the photo-z
redshift distribution.Comment: 10 pages, 12 figures, accepted by Ap
Experimental demonstration of entanglement-enhanced classical communication over a quantum channel with correlated noise
We present an experiment demonstrating entanglement-enhanced classical
communication capacity of a quantum channel with correlated noise. The channel
is modelled by a fiber optic link exhibiting random birefringence that
fluctuates on a time scale much longer than the temporal separation between
consecutive uses of the channel. In this setting, introducing entanglement
between two photons travelling down the fiber allows one to encode reliably up
to one bit of information into their joint polarization degree of freedom. When
no quantum correlations between two separate uses of the channel are allowed,
this capacity is reduced by a factor of more than three. We demonstrated this
effect using a fiber-coupled source of entagled photon pairs based on
spontaneous parametric down-conversion, and a linear-optics Bell state
measurement.Comment: 4 pages, 2 figures, REVTe
Qubit-Initialisation and Readout with Finite Coherent Amplitudes in Cavity QED
We consider a unitary transfer of an arbitrary state of a two-level atomic
qubit in a cavity to the finite amplitude coherent state cavity field. Such
transfer can be used to either provide an effective readout measurement on the
atom by a subsequent measurement on the light field or as a method for
initializing a fixed atomic state - a so-called "attractor state", studied
previously for the case of an infinitely strong cavity field. We show that with
a suitable adjustment of the coherent amplitude and evolution time the qubit
transfers all its information to the field, attaining a selected state of high
purity irrespectively of the initial state.Comment: 6 pages, 4 figure
Fast approximation of centrality and distances in hyperbolic graphs
We show that the eccentricities (and thus the centrality indices) of all
vertices of a -hyperbolic graph can be computed in linear
time with an additive one-sided error of at most , i.e., after a
linear time preprocessing, for every vertex of one can compute in
time an estimate of its eccentricity such that
for a small constant . We
prove that every -hyperbolic graph has a shortest path tree,
constructible in linear time, such that for every vertex of ,
. These results are based on an
interesting monotonicity property of the eccentricity function of hyperbolic
graphs: the closer a vertex is to the center of , the smaller its
eccentricity is. We also show that the distance matrix of with an additive
one-sided error of at most can be computed in
time, where is a small constant. Recent empirical studies show that
many real-world graphs (including Internet application networks, web networks,
collaboration networks, social networks, biological networks, and others) have
small hyperbolicity. So, we analyze the performance of our algorithms for
approximating centrality and distance matrix on a number of real-world
networks. Our experimental results show that the obtained estimates are even
better than the theoretical bounds.Comment: arXiv admin note: text overlap with arXiv:1506.01799 by other author
Depolarization channels with zero-bandwidth noises
A simple model describing depolarization channels with zero-bandwidth
environment is presented and exactly solved. The environment is modelled by
Lorentzian, telegraphic and Gaussian zero-bandwidth noises. Such channels can
go beyond the standard Markov dynamics and therefore can illustrate the
influence of memory effects of the noisy communication channel on the
transmitted information. To quantify the disturbance of quantum states the
entanglement fidelity between arbitrary input and output states is
investigated.Comment: 15 pages, 3 figure
Quantum computation via measurements on the low-temperature state of a many-body system
We consider measurement-based quantum computation using the state of a
spin-lattice system in equilibrium with a thermal bath and free to evolve under
its own Hamiltonian. Any single qubit measurements disturb the system from
equilibrium and, with adaptive measurements performed at a finite rate, the
resulting dynamics reduces the fidelity of the computation. We show that it is
possible to describe the loss in fidelity by a single quantum operation on the
encoded quantum state that is independent of the measurement history. To
achieve this simple description, we choose a particular form of spin-boson
coupling to describe the interaction with the environment, and perform
measurements periodically at a natural rate determined by the energy gap of the
system. We found that an optimal cooling exists, which is a trade-off between
keeping the system cool enough that the resource state remains close to the
ground state, but also isolated enough that the cooling does not strongly
interfere with the dynamics of the computation. For a sufficiently low
temperature we obtain a fault-tolerant threshold for the couplings to the
environment.Comment: 9 pages, 3 figures; v2 published versio
Evidence for dielectric aging due to progressive 180 domain wall pinning in polydomain Pb(Zr0.45Ti0.55)O3 thin films
An evidence that the dielectric ageing in the polydomain Pb(Zr0.45Ti0.55)O3
thin films is controlled by progressive pinning of 180 domain walls is
presented. To provide such a conclusion, we use a general method, which is
based on the study of the time evolution of the nonlinear, but anhysteretic,
dielectric response of the ferroelectric to a weak electric field. A
thermodynamic model of the ferroelectric system where the dielectric response
is controlled by bending movements of pinned 180 domain walls is developed.
Within this model, the nonlinear permittivity of the ferroelectric is expressed
as a function of the microstructural parameters of the domain pattern. It is
shown that using the analysis of the time evolution of the nonlinear
permittivity, it is possible to estimate changes in the concentration of the
pinning centers that block the movements of the 180 domain walls during aging
in polydomain perovskite ferroelectrics.Comment: This version is modifed and corrected according to recently published
Erratum: Phys. Rev. B 79, 219903(E) (2009). 21 pages, 3 figure
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