EB10605
PREDICTING WHERE AN EXTREME EVENT WILL TAKE PLACE?
There is growing interest in the study of extreme events, in view of
their paramount importance in such key areas as global environment,
sociology and finance. Ordinarily, the question asked is whether an
extreme will or will not occur globally. In this paper the issue of
prediction of extremes - and hence also of protection against them -
is put in a new perspective by inquiring on what is the particular
location in space that will witness at a given time period the
occurrence of such an event. For this purpose the dynamics of
extremes is analyzed in representative case studies and the
mechanisms by which extremes spread over space starting from an
initial location are identified. Of special interest is the
occurrence of long-range connections as opposed to step-by-step
propagation through successive neighbors.
***
AF10516
A single pulse suppressing undesired noise
A group of physicists of the University of Technology of Dortmund, Germany,
have succeeded in designing an optimum shape of a single electromagnetic pulse
for high-precision suppression of noise effects in nuclear magnetic resonance
and more generally in the manipulation of single quantum bits.
Nuclear magnetic resonance spectroscopy is a non-destructive, preeminent
technique for investigating matter and imaging of tissues. Lately, it has
also become a starting point for constructing a quantum computer.
Nuclei with a magnetic moment are coherently manipulated at a precise
resonance frequency. The aim is to reduce the noise due to the unavoidable
interaction with the environment which disturbs the detected signal.
The single application of the new proposed pulse is enough to average to zero
the disturbing noise, with an improvement of an order of magnitude compared to
previously known pulses. Thus less pulses are needed in total for an improved
precision which is among the fundamental prerequisites for the realization of a
quantum memory.
***
LC12202BR
First steps towards a Wigner lattice in a quantum wire
In the 1930's, Wigner predicted
that as electrons are charged particles and repel each other, so there must be
conditions where they will stop their continual motion and form a crystalline
lattice, or array, in order to minimise their mutual repulsion. This work generated
enormous interest in the physics community, with numerous papers being produced
on the theory of the "Wigner Lattice" and many experimental
investigations, although evidence of the phenomenon has been limited.
We have been studying the properties of electrons confined by an electric field
to a one-dimensional configuration, a "quantum wire". As the electric
field compressing the electrons is weakened, we have found that the electrons relax
and form two separate rows in order to minimise repulsion. In this paper, we
demonstrate that the two rows of electrons thus formed can be coupled together,
by means of hybridization, or a mixing of the wave-functions between the two
rows. The formation of two interacting rows is a first step towards the creation
of a two-dimensional Wigner lattice in a quantum wire.
Tuesday, July 21, 2009
LT11861
O fish, where art thou?
How fish follow the invisible trace generated by other fish
Fish can detect objects and other fish under water by their lateral-line system measuring the change of pressure or velocity due to water flow generated by these objects. Have you ever observed the swirls that are released by a canoe paddle and are left behind the canoe as relatively stable objects? Many fish and also seals track the underwater invisible swirls, or wakes, arising from the fins of other
fish either to stay behind them during schooling or to hunt them. In this paper, it is shown what the sensory input resulting from wakes looks like and how fish can determine the direction of a wake̓s vortex ring and thus can follow the track of other fish. To measure water flow around their bodies, fish use their lateral-line system consisting of narrow tubes directly underneath the scales and connected to the surrounding water through small pores. Between each two neighboring pores there is a cupula, a gelatinous body covering hair-cell receptors that respond to water flow in the canal and in this way generate neuronal signals. It is shown mathematically, and supported by experimental neuronal evidence, how fish can reconstruct the orientation of a vortex ring through the information they receive
and, thus, how they can track comrades or prey.
***
BZ10860
Observing the first step of electron-hole recombination
Unlike metals, semiconductors conduct electricity not just via electrons
but two kinds of charge carriers, electrons and holes. When electrons
and holes encounter each other, they can form pairs which can decay
under release of their energy either into heat or light. This is called
recombination, and recombination is sometimes detrimental to electronic
devices - for instance in solar cells where it reduces efficiency - but
sometimes it is beneficial for technical applications, for instance in
light emitting diodes where recombination is the very process that
converts electrical energy into light. Observing recombination and the
various quantum mechanical processes which contribute to recombination
is very difficult but crucial for the understanding of this
technologically important phenomenon.
We present here an experiment which allows the observation of
electron-hole pair formation which precedes recombination. The
experiment is based on a novel electrically detected magnetic resonance
pulse sequence which forces magnetic moments of electrons and holes to
dephase apart during the pair formation process before they are forced
to rephase later. The rephasing produces a current signal (an echo)
which is different for pairs which formed before and after the
dephasing. This allows to very accurately observe the formation of
electron hole pairs.
***
LE11889
Shaking of an optical lattice reveals fine features of the quantum many-body state
For more than two decades physicist have struggled to understand why
some ceramic alloys can conduct with zero resistance up to an unusual
high temperature. More or less during the same period, physicists working with cold
atoms have managed to control and manipulate atoms in such a way that
it is now conceivable to use them to simulate crystalline structures,
such as the above mentioned superconducting alloys, with high
flexibility. In our work we show by exact simulations in one dimension that modulation of an optical lattice - an artificial crystal - can reveal essential information about the many-body quantum state of the repulsively interacting fermionic atoms it is holding; even small energy gaps are accurately resolved. This study in one dimension has relevance to the three dimensional counterpart where
antiferromagnetic states are predicted to occur, suggesting the observation of the
antiferromagnetic gap by the lattice modulation. Understanding such systems is
important due the crucial role that the magnetic properties are believed to play in high temperature superconductivity.
***
LB11867
Dynamic chirp control of attosecond pulses demonstrated
High-order harmonic Generation (HHG) and the attosecond pulse emission based on HHG have been extensively investigated in recent years. The attainable shortest harmonic pulse duration is limited by the intrinsic chirp of the harmonic emission. Consequently, the phase control and chirp compensation of harmonic pulses is of vital importance for producing the transform-limited harmonic pulses. We demonstrate a novel scheme of chirp control by exploiting the dispersion characteristic controlled by the laser field, which is referred as the dynamic control of chirp, different from previous approaches based on the static dispersion of the materials. To control the material dispersion dynamically, we add a weak second harmonic laser pulse to the fundamental driving laser pulse for HHG. The demonstrated method, as opposed to other demonstrated methods, can provide positive chirp. Using this technique, we have compensated the negative chirp for the first time. Additionally it has the advantage that the chirp of the attosecond pulses can be varied easily and continuously by simply changing the time delay between the two color pulses, which could open new opportunities for attosecond coherent control.
O fish, where art thou?
How fish follow the invisible trace generated by other fish
Fish can detect objects and other fish under water by their lateral-line system measuring the change of pressure or velocity due to water flow generated by these objects. Have you ever observed the swirls that are released by a canoe paddle and are left behind the canoe as relatively stable objects? Many fish and also seals track the underwater invisible swirls, or wakes, arising from the fins of other
fish either to stay behind them during schooling or to hunt them. In this paper, it is shown what the sensory input resulting from wakes looks like and how fish can determine the direction of a wake̓s vortex ring and thus can follow the track of other fish. To measure water flow around their bodies, fish use their lateral-line system consisting of narrow tubes directly underneath the scales and connected to the surrounding water through small pores. Between each two neighboring pores there is a cupula, a gelatinous body covering hair-cell receptors that respond to water flow in the canal and in this way generate neuronal signals. It is shown mathematically, and supported by experimental neuronal evidence, how fish can reconstruct the orientation of a vortex ring through the information they receive
and, thus, how they can track comrades or prey.
***
BZ10860
Observing the first step of electron-hole recombination
Unlike metals, semiconductors conduct electricity not just via electrons
but two kinds of charge carriers, electrons and holes. When electrons
and holes encounter each other, they can form pairs which can decay
under release of their energy either into heat or light. This is called
recombination, and recombination is sometimes detrimental to electronic
devices - for instance in solar cells where it reduces efficiency - but
sometimes it is beneficial for technical applications, for instance in
light emitting diodes where recombination is the very process that
converts electrical energy into light. Observing recombination and the
various quantum mechanical processes which contribute to recombination
is very difficult but crucial for the understanding of this
technologically important phenomenon.
We present here an experiment which allows the observation of
electron-hole pair formation which precedes recombination. The
experiment is based on a novel electrically detected magnetic resonance
pulse sequence which forces magnetic moments of electrons and holes to
dephase apart during the pair formation process before they are forced
to rephase later. The rephasing produces a current signal (an echo)
which is different for pairs which formed before and after the
dephasing. This allows to very accurately observe the formation of
electron hole pairs.
***
LE11889
Shaking of an optical lattice reveals fine features of the quantum many-body state
For more than two decades physicist have struggled to understand why
some ceramic alloys can conduct with zero resistance up to an unusual
high temperature. More or less during the same period, physicists working with cold
atoms have managed to control and manipulate atoms in such a way that
it is now conceivable to use them to simulate crystalline structures,
such as the above mentioned superconducting alloys, with high
flexibility. In our work we show by exact simulations in one dimension that modulation of an optical lattice - an artificial crystal - can reveal essential information about the many-body quantum state of the repulsively interacting fermionic atoms it is holding; even small energy gaps are accurately resolved. This study in one dimension has relevance to the three dimensional counterpart where
antiferromagnetic states are predicted to occur, suggesting the observation of the
antiferromagnetic gap by the lattice modulation. Understanding such systems is
important due the crucial role that the magnetic properties are believed to play in high temperature superconductivity.
***
LB11867
Dynamic chirp control of attosecond pulses demonstrated
High-order harmonic Generation (HHG) and the attosecond pulse emission based on HHG have been extensively investigated in recent years. The attainable shortest harmonic pulse duration is limited by the intrinsic chirp of the harmonic emission. Consequently, the phase control and chirp compensation of harmonic pulses is of vital importance for producing the transform-limited harmonic pulses. We demonstrate a novel scheme of chirp control by exploiting the dispersion characteristic controlled by the laser field, which is referred as the dynamic control of chirp, different from previous approaches based on the static dispersion of the materials. To control the material dispersion dynamically, we add a weak second harmonic laser pulse to the fundamental driving laser pulse for HHG. The demonstrated method, as opposed to other demonstrated methods, can provide positive chirp. Using this technique, we have compensated the negative chirp for the first time. Additionally it has the advantage that the chirp of the attosecond pulses can be varied easily and continuously by simply changing the time delay between the two color pulses, which could open new opportunities for attosecond coherent control.
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