10,845 research outputs found
Optimal decision making for sperm chemotaxis in the presence of noise
For navigation, microscopic agents such as biological cells rely on noisy
sensory input. In cells performing chemotaxis, such noise arises from the
stochastic binding of signaling molecules at low concentrations. Using
chemotaxis of sperm cells as application example, we address the classic
problem of chemotaxis towards a single target. We reveal a fundamental
relationship between the speed of chemotactic steering and the strength of
directional fluctuations that result from the amplification of noise in the
chemical input signal. This relation implies a trade-off between slow, but
reliable, and fast, but less reliable, steering.
By formulating the problem of optimal navigation in the presence of noise as
a Markov decision process, we show that dynamic switching between reliable and
fast steering substantially increases the probability to find a target, such as
the egg. Intriguingly, this decision making would provide no benefit in the
absence of noise. Instead, decision making is most beneficial, if chemical
signals are above detection threshold, yet signal-to-noise ratios of gradient
measurements are low. This situation generically arises at intermediate
distances from a target, where signaling molecules emitted by the target are
diluted, thus defining a `noise zone' that cells have to cross.
Our work addresses the intermediate case between well-studied perfect
chemotaxis at high signal-to-noise ratios close to a target, and random search
strategies in the absence of navigation cues, e.g. far away from a target. Our
specific results provide a rational for the surprising observation of decision
making in recent experiments on sea urchin sperm chemotaxis. The general theory
demonstrates how decision making enables chemotactic agents to cope with high
levels of noise in gradient measurements by dynamically adjusting the
persistence length of a biased persistent random walk.Comment: 9 pages, 5 figure
Experimental Monte Carlo Quantum Process Certification
Experimental implementations of quantum information processing have now
reached a level of sophistication where quantum process tomography is
impractical. The number of experimental settings as well as the computational
cost of the data post-processing now translates to days of effort to
characterize even experiments with as few as 8 qubits. Recently a more
practical approach to determine the fidelity of an experimental quantum process
has been proposed, where the experimental data is compared directly to an ideal
process using Monte Carlo sampling. Here we present an experimental
implementation of this scheme in a circuit quantum electrodynamics setup to
determine the fidelity of two qubit gates, such as the cphase and the cnot
gate, and three qubit gates, such as the Toffoli gate and two sequential cphase
gates
Dumbbell diffusion in a spatially periodic potential
We present a numerical investigation of the Brownian motion and diffusion of
a dumbbell in a two-dimensional periodic potential. Its dynamics is described
by a Langevin model including the hydrodynamic interaction. With increasing
values of the amplitude of the potential we find along the modulated spatial
directions a reduction of the diffusion constant and of the impact of the
hydrodynamic interaction. For modulation amplitudes of the potential in the
range of the thermal energy the dumbbell diffusion exhibits a pronounced local
maximum at a wavelength of about 3/2 of the dumbbell extension. This is
especially emphasized for stiff springs connecting the two beads.Comment: 4 pages, 7 figures, published in Phys. Rev. E (2008
Lithium abundance and 6Li/7Li ratio in the active giant HD123351 I. A comparative analysis of 3D and 1D NLTE line-profile fits
Current three-dimensional (3D) hydrodynamical model atmospheres together with
NLTE spectrum synthesis, permit to derive reliable atomic and isotopic chemical
abundances from high-resolution stellar spectra. Not much is known about the
presence of the fragile 6Li isotope in evolved solar-metallicity RGB stars, not
to mention its production in magnetically active targets like HD123351. From
fits of the observed CFHT spectrum with synthetic line profiles based on 1D and
3D model atmospheres, we seek to estimate the abundance of the 6Li isotope and
to place constraints on its origin. We derive A(Li) and the 6Li/7Li isotopic
ratio by fitting different synthetic spectra to the Li-line region of a
high-resolution CFHT spectrum (R=120 000, S/R=400). The synthetic spectra are
computed with four different line lists, using in parallel 3D hydrodynamical
CO5BOLD and 1D LHD model atmospheres and treating the line formation of the
lithium components in non-LTE (NLTE). We find A(Li)=1.69+/-0.11 dex and
6Li/7Li=8.0+/-4.4 % in 3D-NLTE, using the line list of Mel\'endez et al.
(2012), updated with new atomic data for V I, which results in the best fit of
the lithium line profile of HD123351. Two other line lists lead to similar
results but with inferior fit qualities. Our 2-sigma detection of the 6Li
isotope is the result of a careful statistical analysis and the visual
inspection of each achieved fit. Since the presence of a significant amount of
6Li in the atmosphere of a cool evolved star is not expected in the framework
of standard stellar evolution theory, non-standard, external lithium production
mechanisms, possibly related to stellar activity or a recent accretion of rocky
material, need to be invoked to explain the detection of 6Li in HD123351.Comment: 16 pages, 11 figures. Accepted for publication in A&
Quantum Phase Transition of Ground-state Entanglement in a Heisenberg Spin Chain Simulated in an NMR Quantum Computer
Using an NMR quantum computer, we experimentally simulate the quantum phase
transition of a Heisenberg spin chain. The Hamiltonian is generated by a
multiple pulse sequence, the nuclear spin system is prepared in its
(pseudo-pure) ground state and the effective Hamiltonian varied in such a way
that the Heisenberg chain is taken from a product state to an entangled state
and finally to a different product state.Comment: 5 pages, 5 eps figures. Accepted in Phys. Rev.
New insights into pedestrian flow through bottlenecks
Capacity estimation is an important tool for the design and dimensioning of
pedestrian facilities. The literature contains different procedures and
specifications which show considerable differences with respect to the
estimated flow values. Moreover do new experimental data indicate a stepwise
growing of the capacity with the width and thus challenge the validity of the
specific flow concept. To resolve these differences we have studied
experimentally the unidirectional pedestrian flow through bottlenecks under
laboratory conditions. The time development of quantities like individual
velocities, density and individual time gaps in bottlenecks of different width
is presented. The data show a linear growth of the flow with the width. The
comparison of the results with experimental data of other authors indicates
that the basic assumption of the capacity estimation for bottlenecks has to be
revised. In contradiction with most planning guidelines our main result is,
that a jam occurs even if the incoming flow does not overstep the capacity
defined by the maximum of the flow according to the fundamental diagram.Comment: Traffic flow, pedestrian traffic, crowd dynamics, capacity of
bottlenecks (16 pages, 8 figures); (+ 3 new figures and minor revisions
An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres I. Formation of the G-band in metal-poor dwarf stars
Recent developments in the three-dimensional (3D) spectral synthesis code
Linfor3D have meant that, for the first time, large spectral wavelength
regions, such as molecular bands, can be synthesised with it in a short amount
of time. A detailed spectral analysis of the synthetic G-band for several dwarf
turn-off-type 3D atmospheres (5850 <= T_eff [K] <= 6550, 4.0 <= log g <= 4.5,
-3.0 <= [Fe/H] <= -1.0) was conducted, under the assumption of local
thermodynamic equilibrium. We also examine carbon and oxygen molecule formation
at various metallicity regimes and discuss the impact it has on the G-band.
Using a qualitative approach, we describe the different behaviours between the
3D atmospheres and the traditional one-dimensional (1D) atmospheres and how the
different physics involved inevitably leads to abundance corrections, which
differ over varying metallicities. Spectra computed in 1D were fit to every 3D
spectrum to determine the 3D abundance correction. Early analysis revealed that
the CH molecules that make up the G-band exhibited an oxygen abundance
dependency; a higher oxygen abundance leads to weaker CH features. Nitrogen
abundances showed zero impact to CH formation. The 3D corrections are also
stronger at lower metallicity. Analysis of the 3D corrections to the G-band
allows us to assign estimations of the 3D abundance correction to most dwarf
stars presented in the literature. The 3D corrections suggest that A(C) in CEMP
stars with high A(C) would remain unchanged, but would decrease in CEMP stars
with lower A(C). It was found that the C/O ratio is an important parameter to
the G-band in 3D. Additional testing confirmed that the C/O ratio is an equally
important parameter for OH transitions under 3D. This presents a clear
interrelation between the carbon and oxygen abundances in 3D atmospheres
through their molecular species, which is not seen in 1D.Comment: 19 pages, 13 figures, 4 tables. Accepted for publication in A&
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