224 research outputs found
The interface of gravity and quantum mechanics illuminated by Wigner phase space
We provide an introduction into the formulation of non-relativistic quantum
mechanics using the Wigner phase-space distribution function and apply this
concept to two physical situations at the interface of quantum theory and
general relativity: (i) the motion of an ensemble of cold atoms relevant to
tests of the weak equivalence principle, and (ii) the Kasevich-Chu
interferometer. In order to lay the foundations for this analysis we first
present a representation-free description of the Kasevich-Chu interferometer
based on unitary operators.Comment: 69 pages, 6 figures, minor changes to match the published version.
The original publication is available at
http://en.sif.it/books/series/proceedings_fermi or
http://ebooks.iospress.nl/volumearticle/3809
Light shifts in atomic Bragg diffraction
Bragg diffraction of an atomic wave packet in a retroreflective geometry with
two counterpropagating optical lattices exhibits a light shift induced phase.
We show that the temporal shape of the light pulse determines the behavior of
this phase shift: In contrast to Raman diffraction, Bragg diffraction with
Gaussian pulses leads to a significant suppression of the intrinsic phase shift
due to a scaling with the third power of the inverse Doppler frequency.
However, for box-shaped laser pulses, the corresponding shift is twice as large
as for Raman diffraction. Our results are based on approximate, but analytical
expressions as well as a numerical integration of the corresponding
Schr\"odinger equation.Comment: 6 pages, 5 figure
A high-gain Quantum free-electron laser: emergence & exponential gain
We derive an effective Dicke model in momentum space to describe collective
effects in the quantum regime of a free-electron laser (FEL). The resulting
exponential gain from a single passage of electrons allows the operation of a
Quantum FEL in the high-gain mode and avoids the experimental challenges of an
X-ray FEL oscillator. Moreover, we study the intensity fluctuations of the
emitted radiation which turn out to be super-Poissonian
Regimes of atomic diffraction: Raman versus Bragg diffraction in retroreflective geometries
We provide a comprehensive study of atomic Raman and Bragg diffraction when
coupling to a pair of counterpropagating light gratings (double diffraction) or
to a single one (single diffraction) and discuss the transition from one case
to the other in a retroreflective geometry as the Doppler detuning changes. In
contrast to single diffraction, double Raman loses its advantage of high
diffraction efficiency for short pulses and has to be performed in a Bragg-type
regime. Moreover, the structure of double diffraction leads to further
limitations for broad momentum distributions on the efficiency of mirror
pulses, making the use of (ultra) cold ensembles essential for high diffraction
efficiency.Comment: 16 pages, 14 figure
Influence of cell shape, inhomogeneities and diffusion barriers in cell polarization models
In silico experiments bear the potential to further the understanding of biological transport processes by allowing a systematic modification of any spatial property and providing immediate simulation results for the chosen models. We consider cell polarization and spatial reorganization of membrane proteins which are fundamental for cell division, chemotaxis and morphogenesis. Our computational study is motivated by mating and budding processes of S. cerevisiae. In these processes a key player during the initial phase of polarization is the GTPase Cdc42 which occurs in an active membrane-bound form and an inactive cytosolic form. We use partial differential equations to describe the membrane-cytosol shuttling of Cdc42 during budding as well as mating of yeast. The membrane is modeled as a thin layer that only allows lateral diffusion and the cytosol is modeled as a volume. We investigate how cell shape and diffusion barriers like septin structures or bud scars influence Cdc42 cluster formation and subsequent polarization of the yeast cell. Since the details of the binding kinetics of cytosolic proteins to the membrane are still controversial, we employ two conceptual models which assume different binding kinetics. An extensive set of in silico experiments with different modeling hypotheses illustrate the qualitative dependence of cell polarization on local membrane curvature, cell size and inhomogeneities on the membrane and in the cytosol. We examine that spatial inhomogenities essentially determine the location of Cdc42 cluster formation and spatial properties are crucial for the realistic description of the polarization process in cells. In particular, our computer simulations suggest that diffusion barriers are essential for the yeast cell to grow a protrusion
High-gain quantum free-electron laser: long-time dynamics and requirements
We solve the long-time dynamics of a high-gain free-electron laser in the
quantum regime. In this regime each electron emits at most one photon on
average, independently of the initial field. In contrast, the variance of the
photon statistics shows a qualitatively different behavior for different
initial states of the field. We find that the realization of a seeded Quantum
FEL is more feasible than self-amplified spontaneous emission
Interference of Clocks: A Quantum Twin Paradox
The phase of matter waves depends on proper time and is therefore susceptible
to special-relativistic (kinematic) and gravitational time dilation (redshift).
Hence, it is conceivable that atom interferometers measure general-relativistic
time-dilation effects. In contrast to this intuition, we show that light-pulse
interferometers without internal transitions are not sensitive to gravitational
time dilation, whereas they can constitute a quantum version of the
special-relativistic twin paradox. We propose an interferometer geometry
isolating the effect that can be used for quantum-clock interferometry.Comment: 9 Pages, 2 Figure
Rayleigh-to-shear wave conversion at the tunnel face - from 3D-FD modeling to ahead-of-drill exploration
For a safe tunnel excavation it is important to predict lithological and structural heterogeneities ahead of the construction. conventional tunnel seismic prediction systems utilize body waves (P- and S-waves) that are directly generated at the tunnel walls or near the cutter head of the tunnel boring machine (TBM). In this work we propose a new prediction strategy that has been discovered by 3-D elastic finite-difference (FD) modeling: Rayleigh waves arriving at the front face are converted into high amplitude S-waves propagating further ahead. Reflected or backscattered S-waves are converted back into Rayleigh waves which can be recorded along the side walls. We name these waves RSSR waves. In our approach the front face acts as a S-wave transceiver. One technical advantage is that both the sources and the receivers may be placed behind the cutter head of the TBM. The modeling reveals that the RSSR waves exhibit significantly higher amplitudes than the directly reflected body waves. The excavation damage zone causes dispersion of the RSSR wave leading to multi-modal reflection response. For the detection of geological interfaces ahead RSSR waves recorded along the side walls are corrected for dispersion and stacked. From the arrival times the distance to the S-S reflection point can be estimated. A recurrent application, while the tunnel approaches the interface, allows one to quantify the orientation of the reflecting interfaces as well. Our approach has been successfully verified in a field experiment at the Piora adit of the Gotthard base tunnel. The distance to the Piora fault zone estimated from stacked RSSR events
agrees well with the information obtained by geological surveying and exploratory drilling
Proper time in atom interferometers: Diffractive versus specular mirrors
We compare a conventional Mach-Zehnder light-pulse atom interferometer based
on diffractive mirrors with one that uses specular reflection. In contrast to
diffractive mirrors that generate a symmetric configuration, specular mirrors
realized, for example, by evanescent fields lead under the influence of gravity
to an asymmetric geometry. In such an arrangement the interferometer phase
contains nonrelativistic signatures of proper time.Comment: 7 pages, 1 figure, 1 tabl
Influence of cell shape, inhomogeneities and diffusion barriers in cell polarization models
In silico experiments bear the potential for further understanding of biological transport processes by
allowing a systematic modification of any spatial property and providing immediate simulation
results. Cell polarization and spatial reorganization of membrane proteins are fundamental for cell
division, chemotaxis and morphogenesis.Wechose the yeast Saccharomyces cerevisiae as an exemplary
model system which entails the shuttling of small Rho GTPases such as Cdc42 and Rho, between an
active membrane-bound form and an inactive cytosolic form.Weused partial differential equations
to describe the membrane-cytosol shuttling of proteins. In this study, a consistent extension of a class
of 1D reaction-diffusion systems into higher space dimensions is suggested. The membrane is
modeled as a thin layer to allow for lateral diffusion and the cytosol is modeled as an enclosed volume.
Two well-known polarization mechanisms were considered. One shows the classical Turinginstability
patterns, the other exhibits wave-pinning dynamics. For both models, we investigated how
cell shape and diffusion barriers like septin structures or bud scars influence the formation of signaling
molecule clusters and subsequent polarization. An extensive set of in silico experiments with different
modeling hypotheses illustrated the dependence of cell polarization models on local membrane
curvature, cell size and inhomogeneities on the membrane and in the cytosol. In particular, the results
of our computer simulations suggested that for both mechanisms, local diffusion barriers on the
membrane facilitate Rho GTPase aggregation, while diffusion barriers in the cytosol and cell
protrusions limit spontaneous molecule aggregations of active Rho GTPase locally
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