705 research outputs found
A dynamic scheme for generating number squeezing in Bose-Einstein condensates through nonlinear interactions
We develop a scheme to generate number squeezing in a Bose-Einstein
condensate by utilizing interference between two hyperfine levels and nonlinear
atomic interactions. We describe the scheme using a multimode quantum field
model and find agreement with a simple analytic model in certain regimes. We
demonstrate that the scheme gives strong squeezing for realistic choices of
parameters and atomic species. The number squeezing can result in noise well
below the quantum limit, even if the initial noise on the system is classical
and much greater than that of a poisson distribution.Comment: 4 pages, 3 figure
Multimode quantum limits to the linewidth of an atom laser
The linewidth of an atom laser can be limited by excitation of higher energy
modes in the source Bose-Einstein condensate, energy shifts in that condensate
due to the atomic interactions, or phase diffusion of the lasing mode due to
those interactions. The first two are effects that can be described with a
semiclassical model, and have been studied in detail for both pumped and
unpumped atom lasers. The third is a purely quantum statistical effect, and has
been studied only in zero dimensional models. We examine an unpumped atom laser
in one dimension using a quantum field theory using stochastic methods based on
the truncated Wigner approach. This allows spatial and statistical effects to
be examined simultaneously, and the linewidth limit for unpumped atom lasers is
quantified in various limits.Comment: 8 Figure
Squeezing and entanglement delay using slow light
We examine the interaction of a weak probe with atoms in a lambda-level
configuration under the conditions of electromagnetically induced transparency
(EIT). In contrast to previous works on EIT, we calculate the output state of
the resultant slowly propagating light field while taking into account the
effects of ground state dephasing and atomic noise for a more realistic model.
In particular, we propose two experiments using slow light with a nonclassical
probe field and show that two properties of the probe, entanglement and
squeezing, characterizing the quantum state of the probe field, can be
well-preserved throughout the passage.Comment: 2 figures; v2: fixed some minor typographical errors in a couple of
equations and corrected author spelling in one reference. v3: Added three
authors; changed the entaglement definition to conform to a more accepted
standard (Duan's entanglement measure); altered the abstract slightly. v4:
fixed formatting of figure
Erratum : Squeezing and entanglement delay using slow light
An inconsistency was found in the equations used to calculate the variance of
the quadrature fluctuations of a field propagating through a medium
demonstrating electromagnetically induced transparency (EIT). The decoherence
term used in our original paper introduces inconsistency under weak probe
approximation. In this erratum we give the Bloch equations with the correct
dephasing terms. The conclusions of the original paper remain the same. Both
entanglement and squeezing can be delayed and preserved using EIT without
adding noise when the decoherence rate is small.Comment: 1 page, no figur
Maximal Sharing in the Lambda Calculus with letrec
Increasing sharing in programs is desirable to compactify the code, and to
avoid duplication of reduction work at run-time, thereby speeding up execution.
We show how a maximal degree of sharing can be obtained for programs expressed
as terms in the lambda calculus with letrec. We introduce a notion of `maximal
compactness' for lambda-letrec-terms among all terms with the same infinite
unfolding. Instead of defined purely syntactically, this notion is based on a
graph semantics. lambda-letrec-terms are interpreted as first-order term graphs
so that unfolding equivalence between terms is preserved and reflected through
bisimilarity of the term graph interpretations. Compactness of the term graphs
can then be compared via functional bisimulation.
We describe practical and efficient methods for the following two problems:
transforming a lambda-letrec-term into a maximally compact form; and deciding
whether two lambda-letrec-terms are unfolding-equivalent. The transformation of
a lambda-letrec-term into maximally compact form proceeds in three
steps:
(i) translate L into its term graph ; (ii) compute the maximally
shared form of as its bisimulation collapse ; (iii) read back a
lambda-letrec-term from the term graph with the property . This guarantees that and have the same unfolding, and that
exhibits maximal sharing.
The procedure for deciding whether two given lambda-letrec-terms and
are unfolding-equivalent computes their term graph interpretations and , and checks whether these term graphs are bisimilar.
For illustration, we also provide a readily usable implementation.Comment: 18 pages, plus 19 pages appendi
Federated learning for performance prediction in multi-operator environments
Telecom vendors and operators deliver services with strict requirements on performance, over complex and sometimes partly shared network infrastructures. A key enabler for network and service management in such environments is knowledge sharing, and the use of data-driven models for performance prediction, forecasting, and troubleshooting. In this paper, we outline a multi-operator service metrics prediction framework using federated learning that allows privacy-preserved knowledge-sharing across operators for improved model performance, and also reduced requirements on data transfer within an operator network. Federated learning is compared against local and central learning strategies for multi-operator performance prediction, and it is shown to balance the requirements on data privacy, model performance, and the network overhead. Further, the paper provides insights on how data heterogeneity affects model performance, where the conclusion is that standard federated learning has certain robustness to data heterogeneity. Finally, we discuss the challenges related to training a federated learning model with a limited budget on the communication rounds. The evaluation is performed using a set of realistic publicly available data traces, that are adapted specifically for the purpose of studying multi-operator service performance prediction
Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2
Neutron scattering studies on powder and single crystals have provided new
evidences for unconventional magnetism in Cu2Te2O5Cl2. The compound is built
from tetrahedral clusters of S=1/2 Cu2+ spins located on a tetragonal lattice.
Magnetic ordering, emerging at TN=18.2 K, leads to a very complex multi-domain,
most likely degenerate, ground state, which is characterized by an
incommensurate (ICM) wave vector k ~ [0.15, 0.42,1/2]. The Cu2+ ions carry a
magnetic moment of 0.67(1) mB/ Cu2+ at 1.5 K and form a four helices spin
arrangement with two canted pairs within the tetrahedra. A domain
redistribution is observed when a magnetic field is applied in the tetragonal
plane (Hc≈0.5 T), but not for H||c up to 4 T. The excitation spectrum is
characterized by two well-defined modes, one completely dispersionless at 6.0
meV, the other strongly dispersing to a gap of 2 meV. The reason for such
complex ground state and spin excitations may be geometrical frustration of the
Cu2+ spins within the tetrahedra, intra- and inter-tetrahedral couplings having
similar strengths and strong Dzyaloshinski-Moriya anisotropy. Candidates for
the dominant intra- and inter-tetrahedral interactions are proposed
Attosecond electron spectroscopy using a novel interferometric pump-probe technique
We present an interferometric pump-probe technique for the characterization
of attosecond electron wave packets (WPs) that uses a free WP as a reference to
measure a bound WP. We demonstrate our method by exciting helium atoms using an
attosecond pulse with a bandwidth centered near the ionization threshold, thus
creating both a bound and a free WP simultaneously. After a variable delay, the
bound WP is ionized by a few-cycle infrared laser precisely synchronized to the
original attosecond pulse. By measuring the delay-dependent photoelectron
spectrum we obtain an interferogram that contains both quantum beats as well as
multi-path interference. Analysis of the interferogram allows us to determine
the bound WP components with a spectral resolution much better than the inverse
of the attosecond pulse duration.Comment: 5 pages, 4 figure
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