28,744 research outputs found
Controlled manipulation of light by cooperative response of atoms in an optical lattice
We show that a cooperative atom response in an optical lattice to resonant
incident light can be employed for precise control and manipulation of light on
a subwavelength scale. Specific collective excitation modes of the system that
result from strong light-mediated dipole-dipole interactions can be addressed
by tailoring the spatial phase-profile of the incident light. We demonstrate
how the collective response can be used to produce optical excitations at
well-isolated sites on the lattice.Comment: 8 pages, 1 figur
The 2p yields 1s pionic transition
Pion-atomic transitions, perturbation theory, S waves, and P wave
Fluctuations and the Effective Moduli of an Isotropic, Random Aggregate of Identical, Frictionless Spheres
We consider a random aggregate of identical frictionless elastic spheres that
has first been subjected to an isotropic compression and then sheared. We
assume that the average strain provides a good description of how stress is
built up in the initial isotropic compression. However, when calculating the
increment in the displacement between a typical pair of contaction particles
due to the shearing, we employ force equilibrium for the particles of the pair,
assuming that the average strain provides a good approximation for their
interactions with their neighbors. The incorporation of these additional
degrees of freedom in the displacement of a typical pair relaxes the system,
leading to a decrease in the effective moduli of the aggregate. The
introduction of simple models for the statistics of the ordinary and
conditional averages contributes an additional decrease in moduli. The
resulting value of the shear modulus is in far better agreement with that
measured in numerical simulations
A temporal switch model for estimating transcriptional activity in gene expression
Motivation: The analysis and mechanistic modelling of time series gene expression data provided by techniques such as microarrays, NanoString, reverse transcription–polymerase chain reaction and advanced sequencing are invaluable for developing an understanding of the variation in key biological processes. We address this by proposing the estimation of a flexible dynamic model, which decouples temporal synthesis and degradation of mRNA and, hence, allows for transcriptional activity to switch between different states.
Results: The model is flexible enough to capture a variety of observed transcriptional dynamics, including oscillatory behaviour, in a way that is compatible with the demands imposed by the quality, time-resolution and quantity of the data. We show that the timing and number of switch events in transcriptional activity can be estimated alongside individual gene mRNA stability with the help of a Bayesian reversible jump Markov chain Monte Carlo algorithm. To demonstrate the methodology, we focus on modelling the wild-type behaviour of a selection of 200 circadian genes of the model plant Arabidopsis thaliana. The results support the idea that using a mechanistic model to identify transcriptional switch points is likely to strongly contribute to efforts in elucidating and understanding key biological processes, such as transcription and degradation
A scalable architecture for quantum computation with molecular nanomagnets
A proposal for a magnetic quantum processor that consists of individual
molecular spins coupled to superconducting coplanar resonators and transmission
lines is carefully examined. We derive a simple magnetic quantum
electrodynamics Hamiltonian to describe the underlying physics. It is shown
that these hybrid devices can perform arbitrary operations on each spin qubit
and induce tunable interactions between any pair of them. The combination of
these two operations ensures that the processor can perform universal quantum
computations. The feasibility of this proposal is critically discussed using
the results of realistic calculations, based on parameters of existing devices
and molecular qubits. These results show that the proposal is feasible,
provided that molecules with sufficiently long coherence times can be developed
and accurately integrated into specific areas of the device. This architecture
has an enormous potential for scaling up quantum computation thanks to the
microscopic nature of the individual constituents, the molecules, and the
possibility of using their internal spin degrees of freedom.Comment: 27 pages, 6 figure
Coupling single molecule magnets to quantum circuits
In this work we study theoretically the coupling of single molecule magnets
(SMMs) to a variety of quantum circuits, including microwave resonators with
and without constrictions and flux qubits. The main results of this study is
that it is possible to achieve strong and ultrastrong coupling regimes between
SMM crystals and the superconducting circuit, with strong hints that such a
coupling could also be reached for individual molecules close to constrictions.
Building on the resulting coupling strengths and the typical coherence times of
these molecules (of the order of microseconds), we conclude that SMMs can be
used for coherent storage and manipulation of quantum information, either in
the context of quantum computing or in quantum simulations. Throughout the work
we also discuss in detail the family of molecules that are most suitable for
such operations, based not only on the coupling strength, but also on the
typical energy gaps and the simplicity with which they can be tuned and
oriented. Finally, we also discuss practical advantages of SMMs, such as the
possibility to fabricate the SMMs ensembles on the chip through the deposition
of small droplets.Comment: 23 pages, 12 figure
Bosonic Operator Methods for the Quark Model
Quark model matrix elements can be computed using bosonic operators and the
holomorphic representation for the harmonic oscillator. The technique is
illustrated for normal and exotic baryons for an arbitrary number of colors.
The computations are much simpler than those using conventional quark model
wavefunctions
Propagation in the atmosphere of ultrahigh-energy charmed hadrons
Charmed mesons may be produced when a primary cosmic ray or the leading
hadron in an air shower collide with an atmospheric nucleon. At energies \ge
10^8 GeV their decay length becomes larger than 10 km, which implies that they
tend to interact in the air instead of decaying. We study the collisions of
long-lived charmed hadrons in the atmosphere. We show that (\Lambda_c,D)-proton
diffractive processes and partonic collisions of any q^2 where the charm quark
is an spectator have lower inelasticity than (p,\pi)-proton collisions. In
particular, we find that a D meson deposits in each interaction just around 55%
of the energy deposited by a pion. On the other hand, collisions involving the
valence c quark (its annihilation with a sea cbar quark in the target or
c-quark exchange in the t channel) may deposit most of D meson energy, but
their frequency is low (below 0.1% of inelastic interactions). As a
consequence, very energetic charmed hadrons may keep a significant fraction of
their initial energy after several hadronic interactions, reaching much deeper
in the atmosphere than pions or protons of similar energy.Comment: 13 pages, version to appear in PR
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