1,181 research outputs found
Effective operator formalism for open quantum systems
We present an effective operator formalism for open quantum systems.
Employing perturbation theory and adiabatic elimination of excited states for a
weakly driven system, we derive an effective master equation which reduces the
evolution to the ground-state dynamics. The effective evolution involves a
single effective Hamiltonian and one effective Lindblad operator for each
naturally occurring decay process. Simple expressions are derived for the
effective operators which can be directly applied to reach effective equations
of motion for the ground states. We compare our method with the hitherto
existing concepts for effective interactions and present physical examples for
the application of our formalism, including dissipative state preparation by
engineered decay processes.Comment: 11 pages, 6 figure
Efficacy of new-generation antidepressants assessed with the Montgomery-Asberg depression rating scale, the gold standard clinician rating scale : a meta-analysis of randomised placebo-controlled trials
It has been claimed that efficacy estimates based on the Hamilton Depression Rating-Scale (HDRS) underestimate antidepressants true treatment effects due to the instrument's poor psychometric properties. The aim of this study is to compare efficacy estimates based on the HDRS with the gold standard procedure, the Montgomery-Asberg Depression Rating-Scale (MADRS)
Motivational climate in the home: Implications for physical activity, psychosocial outcomes and family relations
The impact of motivational climate created by coaches/teachers on children is well documented. There is little knowledge about the motivational climate in the home. This study investigated the perceived motivational climate in the home and achievement goals, physical activity involvement, and psychosocial outcomes such as empathy, family relationships, school grades, general mastery and psychological problems for 15-16 year old adolescents. Methods: Data were collected by questionnaires in a cross-sectional design. Participants were 1940 boys and 1871 girls. Gender differences necessitated separate analyses, but for both genders, mastery climate was indirectly, positively related to physical activity, mastery, empathy, family relations, and school grades via task orientation. The home climate had a direct relationship with the achievement orientation of the children and the orientation of the child had gender specific relationships to physical activity, family relationships and general mastery. Conclusion: A home mastery climate is associated with more adaptive outcomes.publishedVersio
Memory Imperfections in Atomic Ensemble-based Quantum Repeaters
Quantum repeaters promise to deliver long-distance entanglement overcoming
noise and loss in realistic quantum channels. A promising class of repeaters,
based on atomic ensemble quantum memories and linear optics, follow the
proposal by Duan et al [Nature 414, 413, 2001]. Here we analyse this protocol
in terms of a very general model for the quantum memories employed. We derive
analytical expressions for scaling of entanglement with memory imperfections,
dark counts, loss and distance, and apply our results to two specific quantum
memory protocols. Our methods apply to any quantum memory with an interaction
Hamiltonian at most quadratic in the mode operators and are in principle
extendible to more recent modifications of the original DLCZ proposal.Comment: 15 pages, 12 figure
Multi-particle entanglement of hot trapped ions
We propose an efficient method to produce multi-particle entangled states of
ions in an ion trap for which a wide range of interesting effects and
applications have been suggested. Our preparation scheme exploits the
collective vibrational motion of the ions, but it works in such a way that this
motion need not be fully controlled in the experiment. The ions may, e.g., be
in thermal motion and exchange mechanical energy with a surrounding heat bath
without detrimental effects on the internal state preparation. Our scheme does
not require access to the individual ions in the trap.Comment: 4 pages, including 3 figures. To appear in Phys. Rev. Lett. This
paper previously appeared under the name "Schrodingers cat in a hot trap".
The paper has been revised according to Phys. Rev. policy on Schrodinger
cats. No cats were harmed during the production of this manuscrip
Loss-tolerant architecture for quantum computing with quantum emitters
We develop an architecture for measurement-based quantum computing using
photonic quantum emitters. The architecture exploits spin-photon entanglement
as resource states and standard Bell measurements of photons for fusing them
into a large spin-qubit cluster state. The scheme is tailored to emitters with
limited memory capabilities since it only uses an initial non-adaptive
(ballistic) fusion process to construct a fully percolated graph state of
multiple emitters. By exploring various geometrical constructions for fusing
entangled photons from deterministic emitters, we improve the photon loss
tolerance significantly compared to similar all-photonic schemes
Three dimensional theory for light matter interaction
We present a full quantum mechanical three dimensional theory describing an
electromagnetic field interacting with an ensemble of identical atoms. The
theory is constructed such that it describes recent experiments on light-matter
quantum interfaces, where the quantum fluctuations of light are mapped onto the
atoms and back onto light. We show that the interaction of the light with the
atoms may be separated into a mean effect of the ensemble and a deviation from
the mean. The mean effect of the interaction effectively give rise to an index
of refraction of the gas. We formally change to a dressed state picture, where
the light modes are solutions to the diffraction problem, and develop a
perturbative expansion in the fluctuations. The fluctuations are due to quantum
fluctuations as well as the random positions of the atoms. In this perturbative
expansion we show how the quantum fluctuations are mapped between atoms and
light while the random positioning of the atoms give rise to decay due to
spontaneous emission. Furthermore we identify limits, where the full three
dimensional theory reduce to the one dimensional theory typically used to
describe the interaction.Comment: 32 pages, 2 figure
Bogoliubov theory of entanglement in a Bose-Einstein condensate
We consider a Bose-Einstein condensate which is illuminated by a short
resonant light pulse that coherently couples two internal states of the atoms.
We show that the subsequent time evolution prepares the atoms in an interesting
entangled state called a spin squeezed state. This evolution is analysed in
detail by developing a Bogoliubov theory which describes the entanglement of
the atoms. Our calculation is a consistent expansion in , where
is the number of particles in the condensate, and our theory predict that it is
possible to produce spin squeezing by at least a factor of . Within
the Bogoliubov approximation this result is independent of temperature.Comment: 14 pages, including 5 figures, minor changes in the presentatio
Gravity vs radiation model: on the importance of scale and heterogeneity in commuting flows
We test the recently introduced radiation model against the gravity model for
the system composed of England and Wales, both for commuting patterns and for
public transportation flows. The analysis is performed both at macroscopic
scales, i.e. at the national scale, and at microscopic scales, i.e. at the city
level. It is shown that the thermodynamic limit assumption for the original
radiation model significantly underestimates the commuting flows for large
cities. We then generalize the radiation model, introducing the correct
normalisation factor for finite systems. We show that even if the gravity model
has a better overall performance the parameter-free radiation model gives
competitive results, especially for large scales.Comment: in press Phys. Rev. E, 201
Spin Squeezing in the Ising Model
We analyze the collective spin noise in interacting spin systems. General
expressions are derived for the short time behaviour of spin systems with
general spin-spin interactions, and we suggest optimum experimental conditions
for the detection of spin squeezing. For Ising models with site dependent
nearest neighbour interactions general expressions are presented for the spin
squeezing parameter for all times. The reduction of collective spin noise can
be used to verify the entangling powers of quantum computer architectures based
on interacting spins.Comment: 7 pages, including 3 figure
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