18,689 research outputs found
Traction force microscopy on soft elastic substrates: a guide to recent computational advances
The measurement of cellular traction forces on soft elastic substrates has
become a standard tool for many labs working on mechanobiology. Here we review
the basic principles and different variants of this approach. In general, the
extraction of the substrate displacement field from image data and the
reconstruction procedure for the forces are closely linked to each other and
limited by the presence of experimental noise. We discuss different strategies
to reconstruct cellular forces as they follow from the foundations of
elasticity theory, including two- versus three-dimensional, inverse versus
direct and linear versus non-linear approaches. We also discuss how biophysical
models can improve force reconstruction and comment on practical issues like
substrate preparation, image processing and the availability of software for
traction force microscopy.Comment: Revtex, 29 pages, 3 PDF figures, 2 tables. BBA - Molecular Cell
Research, online since 27 May 2015, special issue on mechanobiolog
Cyclic mutually unbiased bases, Fibonacci polynomials and Wiedemann's conjecture
We relate the construction of a complete set of cyclic mutually unbiased
bases, i. e., mutually unbiased bases generated by a single unitary operator,
in power-of-two dimensions to the problem of finding a symmetric matrix over
F_2 with an irreducible characteristic polynomial that has a given Fibonacci
index. For dimensions of the form 2^(2^k) we present a solution that shows an
analogy to an open conjecture of Wiedemann in finite field theory. Finally, we
discuss the equivalence of mutually unbiased bases.Comment: 11 pages, added chapter on equivalenc
Event-by-event shape and flow fluctuations of relativistic heavy-ion collision fireballs
Heavy-ion collisions create deformed quark-gluon plasma (QGP) fireballs which
explode anisotropically. The viscosity of the fireball matter determines its
ability to convert the initial spatial deformation into momentum anisotropies
that can be measured in the final hadron spectra. A quantitatively precise
empirical extraction of the QGP viscosity thus requires a good understanding of
the initial fireball deformation. This deformation fluctuates from event to
event, and so does the finally observed momentum anisotropy. We present a
harmonic decomposition of the initial fluctuations in shape and orientation of
the fireball and perform event-by-event ideal fluid dynamical simulations to
extract the resulting fluctuations in the magnitude and direction of the
corresponding harmonic components of the final anisotropic flow at midrapidity.
The final harmonic flow coefficients are found to depend non-linearly on the
initial harmonic eccentricity coefficients. We show that, on average, initial
density fluctuations suppress the buildup of elliptic flow relative to what one
obtains from a smooth initial profile of the same eccentricity, and discuss
implications for the phenomenological extraction of the QGP shear viscosity
from experimental elliptic flow data.Comment: 22 pages, 17 figures. Relative to [v2], minor changes in text. Fig. 9
redrawn. This version accepted by Phys. Rev.
Indistinguishable photons from the resonance fluorescence of a single quantum dot in a microcavity
We demonstrate purely resonant continuous-wave optical laser excitation to
coherently prepare an excitonic state of a single semiconductor quantum dot
(QDs) inside a high quality pillar microcavity. As a direct proof of QD
resonance fluorescence, the evolution from a single emission line to the
characteristic Mollow triplet10 is observed under increasing pump power. By
controlled utilization of weak coupling between the emitter and the fundamental
cavity mode through Purcell-enhancement of the radiative decay, a strong
suppression of pure dephasing is achieved, which reflects in close to Fourier
transform-limited and highly indistinguishable photons with a visibility
contrast of 90%. Our experiments reveal the model-like character of the coupled
QD-microcavity system as a promising source for the generation of ideal photons
at the quantum limit. From a technological perspective, the vertical cavity
symmetry -- with optional dynamic tunability -- provides strongly directed
light emission which appears very desirable for future integrated emitter
devices.Comment: 24 pages, 6 figure
Mach-Zehnder interferometry with interacting trapped Bose-Einstein condensates
We theoretically analyze a Mach-Zehnder interferometer with trapped
condensates, and find that it is surprisingly stable against the nonlinearity
induced by inter-particle interactions. The phase sensitivity, which we study
for number squeezed input states, can overcome the shot noise limit and be
increased up to the Heisenberg limit provided that a Bayesian or
Maximum-Likelihood phase estimation strategy is used. We finally demonstrate
robustness of the Mach-Zehnder interferometer in presence of interactions
against condensate oscillations and a realistic atom counting error.Comment: 4 pages, 5 figures, minor revision
Large variations in the hole spin splitting of quantum-wire subband edges
We study Zeeman splitting of zone-center subband edges in a cylindrical hole
wire subject to a magnetic field parallel to its axis. The g-factor turns out
to fluctuate strongly as a function of wire-subband index, assuming values that
differ substantially from those found in higher-dimensional systems. We analyze
the spin properties of hole-wire states using invariants of the spin-3/2
density matrix and find a strong correlation between g-factor value and the
profile of hole-spin polarization density. Our results suggest possibilities
for confinement engineering of hole spin splittings.Comment: 4 pages, 3 figures, RevTex4, to appear in PR
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