7,458 research outputs found
Crystallographic Analyses of Ion Channels: Lessons and Challenges
Membrane proteins fascinate at many levels, from their central functional roles in transport, energy transduction, and signal transduction processes to structural questions concerning how they fold and operate in the exotic environments of the membrane bilayer and the water-bilayer interface and to methodological issues associated with studying membrane proteins either in situ or extracted from the membrane. This interplay is beautifully exemplified by ion channels, a collection of integral membrane proteins that mediate the transmembrane passage of ions down their electrochemical potential gradient (for general reviews, see Refs. 1 and 2). Ion channels are key elements of signaling and sensing pathways, including nerve cell conduction, hormone response, and mechanosensation. The characteristic properties of ion channels reflect their conductance, ion selectivity, and gating. Ion channels are often specific for a particular type of ion (such as potassium or chloride) or a class of ions (such as anions) and are typically regulated by conformational switching of the protein structure between "open" and "closed" states. This conformational switching may be gated in response to changes in membrane potential, ligand binding, or application of mechanical forces. Detailed functional characterizations of channels and their gating mechanisms have been achieved, reflecting exquisite methodological advances such as patch clamp methods that can monitor the activities of individual channels (3). Until recently, corresponding information about the three-dimensional structures of channels was not available, reflecting difficulties in obtaining sufficient quantities of membrane proteins for crystallization trials. Happily, this situation has started to change with the structure determinations of the Streptomyces lividans K+ channel (KcsA (4)) and the Mycobacterium tuberculosis mechanosensitive channel (MscL (5)).
A variety of reviews (6-12) have appeared recently that discuss functional implications of these channel structures. This review discusses these developments from a complementary perspective, by considering the implications of these structures from within the larger framework of membrane protein structure and function. Because of space restrictions, this review necessarily emphasizes membrane proteins that are composed primarily of alpha-helical bundles, such as KcsA and MscL, rather than beta-barrel proteins, such as porins, typically found in bacterial outer membranes
Photon molecules in atomic gases trapped near photonic crystal waveguides
Realizing systems that support robust, controlled interactions between
individual photons is an exciting frontier of nonlinear optics. To this end,
one approach that has emerged recently is to leverage atomic interactions to
create strong and spatially non-local interactions between photons. In
particular, effective interactions have been successfully created via
interactions between atoms excited to Rydberg levels. Here, we investigate an
alternative approach, in which atomic interactions arise via their common
coupling to photonic crystal waveguides. This technique takes advantage of the
ability to separately tailor the strength and range of interactions via the
dispersion engineering of the structure itself, which can lead to qualitatively
new types of phenomena. As an example, we discuss the formation of correlated
transparency windows, in which photonic states of a certain number and shape
selectively propagate through the system. Through this technique, we show in
particular that one can create molecular-like potentials that lead to molecular
bound states of photon pairs
Observation of emission from chaotic lasing modes in deformed microspheres: displacement by the stable orbit modes
By combining detailed imaging measurements at different tilt angles with
simulations of ray emission from prolate deformed lasing micro-droplets, we
conclude that the probability density for the lasing modes in a
three-dimensional dielectric microcavity must reside in the chaotic region of
the ray phase space. In particular, maximum emission from such chaotic lasing
modes is not from tangent rays emerging from the highest curvature part of the
rim. The laser emission is observed and calculated to be non-tangent and
displaced from the highest curvature due to the presence of stable orbits. In
this Letter we present the first experimental evidence for this phenomenon of
``dynamical eclipsing''.Comment: 4 figure
Directional emission from asymmetric resonant cavities
Asymmetric resonant cavities (ARCs) with highly non-circular but convex
cross-sections are predicted theoretically to have high-Q whispering gallery
modes with very anisotropic emission. We develop a ray dynamics model for the
emission pattern and present numerical and experimental confirmation of the
theory.Comment: 7 pages LaTeX, 3 postscript figure
PC-CUBE: A Personal Computer Based Hypercube
PC-CUBE is an ensemble of IBM PCs or close compatibles connected in the hypercube topology with ordinary computer cables. Communication occurs at the rate of 115.2 K-band via the RS-232 serial links. Available for PC-CUBE is the Crystalline Operating System III (CrOS III), Mercury Operating System, CUBIX and PLOTIX which are parallel I/O and graphics libraries. A CrOS performance monitor was developed to facilitate the measurement of communication and computation time of a program and their effects on performance. Also available are CXLISP, a parallel version of the XLISP interpreter; GRAFIX, some graphics routines for the EGA and CGA; and a general execution profiler for determining execution time spent by program subroutines. PC-CUBE provides a programming environment similar to all hypercube systems running CrOS III, Mercury and CUBIX. In addition, every node (personal computer) has its own graphics display monitor and storage devices. These allow data to be displayed or stored at every processor, which has much instructional value and enables easier debugging of applications. Some application programs which are taken from the book Solving Problems on Concurrent Processors (Fox 88) were implemented with graphics enhancement on PC-CUBE. The applications range from solving the Mandelbrot set, Laplace equation, wave equation, long range force interaction, to WaTor, an ecological simulation
Quantum dynamics of propagating photons with strong interactions: a generalized input-output formalism
There has been rapid development of systems that yield strong interactions
between freely propagating photons in one dimension via controlled coupling to
quantum emitters. This raises interesting possibilities such as quantum
information processing with photons or quantum many-body states of light, but
treating such systems generally remains a difficult task theoretically. Here,
we describe a novel technique in which the dynamics and correlations of a few
photons can be exactly calculated, based upon knowledge of the initial photonic
state and the solution of the reduced effective dynamics of the quantum
emitters alone. We show that this generalized "input-output" formalism allows
for a straightforward numerical implementation regardless of system details,
such as emitter positions, external driving, and level structure. As a specific
example, we apply our technique to show how atomic systems with infinite-range
interactions and under conditions of electromagnetically induced transparency
enable the selective transmission of correlated multi-photon states
A digital ecosystem for ICT educators, ICT industry and ICT students
Worldwide ICT education (Information Communication Technology) is facing a major challenge of declining student enrolments; battling to keep its curriculum relevant and up-to-date while trying to meet the high demand of ICT skilled workers in domain, such as resources, health, government and commerce. This paper, documenting research in progress, discusses these issues and challenges in ICT education and proposes a solution in the form of a digital ecosystem in ICT education involving three main stakeholders: academics, students and the IT industry, and how they could come together to tackle the problems faced
Steady-state Ab Initio Laser Theory: Generalizations and Analytic Results
We improve the steady-state ab initio laser theory (SALT) of Tureci et al. by
expressing its fundamental self-consistent equation in a basis set of threshold
constant flux states that contains the exact threshold lasing mode. For
cavities with non-uniform index and/or non-uniform gain, the new basis set
allows the steady-state lasing properties to be computed with much greater
efficiency. This formulation of the SALT can be solved in the single-pole
approximation, which gives the intensities and thresholds, including the
effects of nonlinear hole-burning interactions to all orders, with negligible
computational effort. The approximation yields a number of analytic
predictions, including a "gain-clamping" transition at which strong modal
interactions suppress all higher modes. We show that the single-pole
approximation agrees well with exact SALT calculations, particularly for high-Q
cavities. Within this range of validity, it provides an extraordinarily
efficient technique for modeling realistic and complex lasers.Comment: 17 pages, 11 figure
Quantum many-body models with cold atoms coupled to photonic crystals
Using cold atoms to simulate strongly interacting quantum systems represents
an exciting frontier of physics. However, as atoms are nominally neutral point
particles, this limits the types of interactions that can be produced. We
propose to use the powerful new platform of cold atoms trapped near
nanophotonic systems to extend these limits, enabling a novel quantum material
in which atomic spin degrees of freedom, motion, and photons strongly couple
over long distances. In this system, an atom trapped near a photonic crystal
seeds a localized, tunable cavity mode around the atomic position. We find that
this effective cavity facilitates interactions with other atoms within the
cavity length, in a way that can be made robust against realistic
imperfections. Finally, we show that such phenomena should be accessible using
one-dimensional photonic crystal waveguides in which coupling to atoms has
already been experimentally demonstrated
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