1,002 research outputs found
Dynamics from seconds to hours in Hodgkin-Huxley model with time-dependent ion concentrations and buffer reservoirs
The classical Hodgkin--Huxley (HH) model neglects the time-dependence of ion
concentrations in spiking dynamics. The dynamics is therefore limited to a time
scale of milliseconds, which is determined by the membrane capacitance
multiplied by the resistance of the ion channels, and by the gating time
constants. We study slow dynamics in an extended HH framework that includes
time-dependent ion concentrations, pumps, and buffers. Fluxes across the
neuronal membrane change intra- and extracellular ion concentrations, whereby
the latter can also change through contact to reservoirs in the surroundings.
Ion gain and loss of the system is identified as a bifurcation parameter whose
essential importance was not realized in earlier studies. Our systematic study
of the bifurcation structure and thus the phase space structure helps to
understand activation and inhibition of a new excitability in ion homeostasis
which emerges in such extended models. Also modulatory mechanisms that regulate
the spiking rate can be explained by bifurcations. The dynamics on three
distinct slow times scales is determined by the cell volume-to-surface-area
ratio and the membrane permeability (seconds), the buffer time constants (tens
of seconds), and the slower backward buffering (minutes to hours). The
modulatory dynamics and the newly emerging excitable dynamics corresponds to
pathological conditions observed in epileptiform burst activity, and spreading
depression in migraine aura and stroke, respectively.Comment: 18 pages, 11 figure
Three-color Sagnac source of polarization-entangled photon pairs
We demonstrate a compact and stable source of polarization-entangled pairs of
photons, one at 810 nm wavelength for high detection efficiency and the other
at 1550 nm for long-distance fiber communication networks. Due to a novel
Sagnac-based design of the interferometer no active stabilization is needed.
Using only one 30 mm ppKTP bulk crystal the source produces photons with a
spectral brightness of 1.13x10^6 pairs/s/mW/THz with an entanglement fidelity
of 98.2%. Both photons are single-mode fiber coupled and ready to be used in
quantum key distribution (QKD) or transmission of photonic quantum states over
large distances.Comment: 7 pages, 4 figure
Bistable dynamics underlying excitability of ion homeostasis in neuron models
When neurons fire action potentials, dissipation of free energy is usually
not directly considered, because the change in free energy is often negligible
compared to the immense reservoir stored in neural transmembrane ion gradients
and the long-term energy requirements are met through chemical energy, i.e.,
metabolism. However, these gradients can temporarily nearly vanish in
neurological diseases, such as migraine and stroke, and in traumatic brain
injury from concussions to severe injuries. We study biophysical neuron models
based on the Hodgkin-Huxley (HH) formalism extended to include time-dependent
ion concentrations inside and outside the cell and metabolic energy-driven
pumps. We reveal the basic mechanism of a state of free energy-starvation (FES)
with bifurcation analyses showing that ion dynamics is for a large range of
pump rates bistable without contact to an ion bath. This is interpreted as a
threshold reduction of a new fundamental mechanism of 'ionic excitability' that
causes a long-lasting but transient FES as observed in pathological states. We
can in particular conclude that a coupling of extracellular ion concentrations
to a large glial-vascular bath can take a role as an inhibitory mechanism
crucial in ion homeostasis, while the Na/K pumps alone are insufficient
to recover from FES. Our results provide the missing link between the HH
formalism and activator-inhibitor models that have been successfully used for
modeling migraine phenotypes, and therefore will allow us to validate the
hypothesis that migraine symptoms are explained by disturbed function in ion
channel subunits, Na/K pumps, and other proteins that regulate ion
homeostasis.Comment: 14 pages, 8 figures, 4 table
On the optimality of individual entangling-probe attacks against BB84 quantum key distribution
It is shown that an existing method to study ideal individual attacks on the
BB84 QKD protocol using error discard can be adapted to reconciliation with
error correction, and that an optimal attack can be explicitly found. Moreover,
this attack fills Luetkenhaus bound, independently of whether error positions
are leaked to Eve, proving that it is tight. In addition, we clarify why the
existence of such optimal attacks is not in contradiction with the established
``old-style'' theory of BB84 individual attacks, as incorrectly suggested
recently in a news feature.Comment: 12 pages, 3 figure
Superdeformation and hyperdeformation in the Cd nucleus
The superdeformation and hyperdeformation in Cd have been studied for
the first time within the framework of the fully self-consistent cranked mean
field theory, namely, cranked relativistic mean field theory. The structure of
observed superdeformed bands 1 and 2 have been analyzed in detail. The bumps
seen in their dynamic moments of inertia are explained as arising from unpaired
band crossings. This is contrary to an explanation given earlier within the
framework of projected shell model. It was also concluded that this nucleus is
not doubly magic SD nucleus
Correlated electron tunneling through two separate quantum dot systems with strong capacitive interdot coupling
A system consisting of two independently contacted quantum dots with strong
electrostatic interaction shows interdot Coulomb blockade when the dots are
weakly tunnel coupled to their leads. It is studied experimentally how the
blockade can be overcome by correlated tunneling when tunnel coupling to the
leads increases. The experimental results are compared with numerical
renormalization group calculations using predefined (measured) parameters. Our
results indicate Kondo correlations due to the electrostatic interaction in
this double quantum dot system.Comment: 5 pages, 3 figures, published in Phys. Rev. Lett. Oct. 30t
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