420 research outputs found
The Röntgen interaction and forces on dipoles in time-modulated optical fields
The Röntgen term is an often neglected contribution to the interaction between an atom and an electromagnetic field in the electric dipole approximation. In this work we discuss how this interaction term leads to a difference between the kinetic and canonical momentum of an atom which, in turn, leads to surprising radiation forces acting on the atom. We use a number of examples to explore the main features of this interaction, namely forces acting against the expected dipole force or accelerations perpendicular to the beam propagation axis
Vacuum Friction
We know that in empty space there is no preferred state of rest. This is true
both in special relativity but also in Newtonian mechanics with its associated
Galilean relativity. It comes as something of a surprise, therefore, to
discover the existence a friction force associated with spontaneous emission.
he resolution of this paradox relies on a central idea from special relativity
even though our derivation of it is non-relativistic. We examine the
possibility that the physics underlying this effect might be explored in an ion
trap, via the observation of a superposition of different mass states.Comment: 8 pages, 2 figures. Published in Journal of Modern Optics on 14
September 2017. Version 2 with a corrected typo on page
Local Retrodiction Models for Photon-Noise-Limited Images
Imaging technologies working at very low light levels acquire data by attempting to count the number of photons impinging on each pixel. Especially in cases with, on average, less than one photocount per pixel the resulting images are heavily corrupted by Poissonian noise and a host of successful algorithms trying to reconstruct the original image from this noisy data have been developed. Here we review a recently proposed scheme that complements these algorithms by calculating the full probability distribution for the local intensity distribution behind the noisy photocount measurements. Such a probabilistic treatment opens the way to hypothesis testing and confidence levels for conclusions drawn from image analysis
Will a decaying atom feel a friction force?
We show how a simple calculation leads to the surprising result that an excited two-level atom moving through vacuum sees a tiny friction force of first order in v/c. At first sight this seems to be in obvious contradiction to other calculations showing that the interaction with the vacuum does not change the velocity of an atom. It is yet more surprising that this change in the atom's momentum turns out to be a necessary result of energy and momentum conservation in special relativity
From retrodiction to Bayesian quantum imaging
We employ quantum retrodiction to develop a robust Bayesian algorithm for reconstructing the intensity values of an image from sparse photocount data, while also accounting for detector noise in the form of dark counts. This method yields not only a reconstructed image but also provides the full probability distribution function for the intensity at each pixel. We use simulated as well as real data to illustrate both the applications of the algorithm and the analysis options that are only available when the full probability distribution functions are known. These include calculating Bayesian credible regions for each pixel intensity, allowing an objective assessment of the reliability of the reconstructed image intensity values
Optomechanical deformation and strain in elastic dielectrics
Light forces induced by scattering and absorption in elastic dielectrics lead
to local density modulations and deformations. These perturbations in turn
modify light propagation in the medium and generate an intricate nonlinear
response. We generalise an analytic approach where light propagation in
one-dimensional media of inhomogeneous density is modelled as a result of
multiple scattering between polarizable slices. Using the Maxwell stress tensor
formalism we compute the local optical forces and iteratively approach
self-consistent density distributions where the elastic back-action balances
gradient- and scattering forces. For an optically trapped dielectric we derive
the nonlinear dependence of trap position, stiffness and total deformation on
the object's size and field configuration. Generally trapping is enhanced by
deformation, which exhibits a periodic change between stretching and
compression. This strongly deviates from qualitative expectations based on the
change of photon momentum of light crossing the surface of a dielectric. We
conclude that optical forces have to be treated as volumetric forces and that a
description using the change of photon momentum at the surface of a medium is
inappropriate
Mass-energy and anomalous friction in quantum optics
The usual multipolar Hamiltonian for atom-light interaction features a
non-relativistic moving atom interacting with electromagnetic fields which
inherently follow Lorentzian symmetry. This combination can lead to situations
where atoms appear to experience a friction force, when in fact they only
change their internal mass-energy due to the emission or absorption of a
photon. Unfortunately the simple Galilean description of the atom's motion is
not sufficient to distinguish between a change in momentum due to acceleration
and a change in momentum due to a change in internal mass-energy. In this work
we show how a low-order relativistic correction can be included in the
multipolar atom-light Hamiltonian. We also give examples how this affects the
most basic mechanical interactions between atoms and photons
The vacuum friction paradox and related puzzles
The frequency of light emitted by a moving source is shifted by a factor proportional
to its velocity. We find that this Doppler shift requires the existence of a paradoxical
effect: that a moving atom radiating in otherwise empty space feels a net or average
force acing against its direction motion and proportional in magnitude to is speed.
Yet there is no preferred rest frame, either in relativity or in Newtonian mechanics,
so how can there be a vacuum friction force
The significance of lipid composition for membrane activity: new concepts and ways of assessing function
In the last decade or so, it has been realised that membranes do not just have a lipid-bilayer structure in which proteins
are embedded or with which they associate. Structures are dynamic and contain areas of heterogeneity which are vital for
their formation. In this review, we discuss some of the ways in which these dynamic and heterogeneous structures have
implications during stress and in relation to certain human diseases. A particular stress is that of temperature which
may instigate adaptation in poikilotherms or appropriate defensive responses during fever in mammals. Recent data
emphasise the role of membranes in sensing temperature changes and in controlling a regulatory loop with chaperone proteins.
This loop seems to need the existence of specific membrane microdomains and also includes association of chaperone
(heat stress) proteins with the membrane. The role of microdomains is then discussed further in relation to various human
pathologies such as cardiovascular disease, cancer and neurodegenerative diseases. The concept of modifying membrane
lipids (lipid therapy) as a means for treating such pathologies is then introduced. Examples are given when such methods
have been shown to have benefit.
In order to study membrane microheterogeneity in detail and to elucidate possible molecular mechanisms that account
for alteration in membrane function, new methods are needed. In the second part of the review, we discuss ultra-sensitive
and ultra-resolution imaging techniques. These include atomic force microscopy, single particle tracking, single particle
tracing and various modern fluorescence methods. Finally, we deal with computing simulation of membrane systems. Such
methods include coarse-grain techniques and Monte Carlo which offer further advances into molecular dynamics. As computational
methods advance they will have more application by revealing the very subtle interactions that take place
between the lipid and protein components of membranes – and which are so essential to their function
Novel targets of the CbrAB/Crc carbon catabolite control system revealed by transcript abundance in Pseudomonas aeruginosa.
The opportunistic human pathogen Pseudomonas aeruginosa is able to utilize a wide range of carbon and nitrogen compounds, allowing it to grow in vastly different environments. The uptake and catabolism of growth substrates are organized hierarchically by a mechanism termed catabolite repression control (Crc) whereby the Crc protein establishes translational repression of target mRNAs at CA (catabolite activity) motifs present in target mRNAs near ribosome binding sites. Poor carbon sources lead to activation of the CbrAB two-component system, which induces transcription of the small RNA (sRNA) CrcZ. This sRNA relieves Crc-mediated repression of target mRNAs. In this study, we have identified novel targets of the CbrAB/Crc system in P. aeruginosa using transcriptome analysis in combination with a search for CA motifs. We characterized four target genes involved in the uptake and utilization of less preferred carbon sources: estA (secreted esterase), acsA (acetyl-CoA synthetase), bkdR (regulator of branched-chain amino acid catabolism) and aroP2 (aromatic amino acid uptake protein). Evidence for regulation by CbrAB, CrcZ and Crc was obtained in vivo using appropriate reporter fusions, in which mutation of the CA motif resulted in loss of catabolite repression. CbrB and CrcZ were important for growth of P. aeruginosa in cystic fibrosis (CF) sputum medium, suggesting that the CbrAB/Crc system may act as an important regulator during chronic infection of the CF lung
- …