LV11595
Data Storage by heat
Tradition computers carry and process information by electrons. Recently Wang and Li from National University of Singapore (NUS) have demonstrated that heat pulse or phonons, can be used to perform, in principal, all logic operations.
One year later, the same team (Wang Lei is now with Renmin University of China, Beijing) has gone one step further by presenting the feasibility of data storage by heat - the counterpart of another indispensable element for computation and information process, memory. Like an electronic memory that records each bit of data by maintaining voltage in a capacitance, the thermal memory stores data by keeping temperature somewhere. Due to the unavoidable perturbation from the thermometer when the temperatures are measured (i.e., data are read), anything thermally insulated, although seems to be a good candidate at the first glance, does not work. Wang and Li thus turn to build up the thermal circuit exhibiting bi-stable states by applying nonlinear lattices with Negative Differential Thermal Resistance. Via computer simulation, they have demonstrated that those two states can both last very long time and, more importantly are self-recoverable under the not-very-small perturbation introduced by the thermometer. This means that this thermal circuit can act as a thermal memory that store data by heat. Their work will soon be published in Phys. Rev. Lett. Since the related results rely only on very general principles, and given the fact that the solid state thermal rectifier has been realized experimentally in 2006, just a few years after the theoretical models, it is thus reasonably believed that, the thermal memory should be realized, e.g., in nanoscale systems experimentally, in a foreseeable future..
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LS11443AR
How can entanglement be preserved?
In real world entanglement between quantum systems tends to quickly
disappear because of effects of the surroundings. This work shows that
this disappearance can be suppressed by embedding the entangled systems in
appropriate environments such as photonic crystals.
Entanglement represents the spooky correlations that distinguish quantum
systems from classical ones and is an essential resource to develop
quantum computers and for quantum cryptography. Entanglement is very
sensitive to its surroundings and its fragility is a serious obstacle to
its exploitation in real systems.
There are materials, such as photonic crystals, structured so to present
photonic band gaps, that is ranges of frequencies where transmission of
radiation is forbidden.
This work shows that by placing entangled quantum systems, such as quantum
dots, in these materials, when the frequency of their transition falls
inside the band gap, then entanglement may be preserved for times long
enough so that its practical use may be allowed.
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LQ11936
Symmetry retards a phase transition
Phase transitions play a key role in modern data storage devices like CDs and DVDs. The speed of data storage is ultimately limited by the speed of phase transitions, for instance between crystalline and amorphous phases. Even though it is known that some phase transitions are slower than others, the reasons for this are not very clear and present an interesting fundamental problem. This paper shows that symmetry relations between adjoined phases may play a crucial role. In particular it is suggested that phase transitions involving an increase of the symmetry of the material may occur on a very fast time scale, whereas in the opposite case the phase transition is intrinsically slow. The particular example discussed in this paper is the optically induced ultrafast destruction of the magnetic order in yttrium vanadate, which is contrasted to the slow reorientation of the orbital (electronic) order in this material. The proposed symmetry rule is expected to have severe consequences for the development of ultrafast rewritable phase change memories.
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BW10545
Creating and manipulating anyons : a challenge
In two-dimensional physical systems, particles usually behave as bosons or
fermions, but they may also exhibit exotic intermediate quantum statistics.
However, such exotic particles, dubbed anyons, have never been directly observed
nor manipulated experimentally. Recent theoretical proposals of topological
quantum computation, based on the very existence of anyons, have attracted
interest from experimentalists. The possibility to design anyon-standing models
with cold atoms loaded in optical lattices is certainly one of the most
suitable ways to demonstrate that such weird objects are not merely a
theoreticians' construction. In our paper we discuss, in a pedagogical way, some
important theoretical and experimental issues concerning this problematics. We
first give a detailed explanation of how low-energy anyons, but also high-energy
fermions, emerge as collective spin-excitations in a realistic spin model known
as the Kitaev honeycomb model. This should be useful for anyone interested in
learning the physics of anyons in a very simple framework. We then show that
manipulating anyons experimentally should be rather delicate. It is indeed
difficult to create and manipulate anyons thanks to single-spin operations,
without also creating unwanted high-energy fermions, because both kinds of
particles are collective spin-excitations.
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EUR1019E
The origin of Gaussianity of velocity distribution in homogeneous
isotropic turbulence
The velocity of fluid in turbulence is very random. An idealistic
case of turbulence,
which is homogeneous and isotropic, observed behind a grid in a water channel
was discussed to have a Gaussian velocity distribution by Batchelor
(1960), based
on the central limit theorem; while a recent claim by Falkovich and
Lebedev (1997)
that the forced (not naturally decaying) turbulence should have a subGaussian
velocity distribution attracted a considerable notice. In this paper,
a reasonable closure
of Monin-Lundgren (1967) hierarchy of velocity distribution functions
in decaying
homogeneous isotropic turbulence is presented which can truncate the hierarchy
at the first equation for the one-point velocity distribution, and
the two-parameter
family of exact similarity solutions of the equation are found with a
perfect Gaussianity.
The one parameter indicates the power law index of energy decay,
while the other
is closely related with energy dissipation rate. Thus, it would be no
doubt that
the Gaussianity (or a deformed one) must be raised up from a proper
mechanism of a
statistical hydrodynamics consistent with Navier-Stokes equation, not
by a sum of
assumedly independent random numbers such as wavelets.
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BW10476
Polaron problem: an entanglement perspective
The polaron concept – a particle (electron, hole) surrounded by the quanta of the host-lattice vibrations (phonons) – had been conceived by Landau as far back as 1933, and remains squarely among the central notions in condensed-matter physics. As the interaction between the particle and phonons becomes stronger, leading to an increased ``phonon-dressing'' of the particle, its motion invariably changes from being rather delocalized to being restricted to a single unit cell of the crystal. This change is not accompanied by a breaking of symmetry – that is, a phase transition. In this paper, we show that quantitative measures of entanglement, describing correlations between parts of a quantum system, provide us with the means to better understand the inner workings of polaron physics. This appears possible for the so-called Peierls-type particle-phonon interaction, of importance in molecular crystals, whereby phonons directly affect particle's hopping amplitude. We demonstrate that the entanglement measures change abruptly in the physical regime where such interaction leads to the spatially-distant particle-phonon correlations. As an interesting sidetrack, our results exemplify that such behavior does not necessarily coincide with the onset of (zero-temperature) quantum phase transitions – a point of contention in the current literature.
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AS1034
It from Bit!
Quantum theory is an extraordinarily successful physical theory. But
what exactly is it telling us about how nature works? Traditionally,
this has been very difficult to answer in any convincing way, as the
physical content of quantum theory is locked up in mathematical
language that is difficult to decipher.
In the last two decades, several physicists have proposed that the
concept of information might be the hitherto missing concept which
might unlock the physical content of quantum theory. This proposal,
perhaps most clearly made by John Wheeler under the slogan "It for
Bit", asserts that information is as fundamental --- or perhaps more
fundamental --- than the concepts of space, time, mass, and energy
that form the basis of classical physics.
In this paper, it is shown that the full mathematical language and
machinery of quantum theory can be built up by making essential use of
the primitive idea of information, thereby strongly supporting
Wheeler's contention.
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EW10364
Predict properties of colloid systems
Colloids, nanoparticles or globular proteins in various solvents are typically charged, and weakly screened long-range electrostatic repulsion between them competes with solvent-mediated short-range attraction as well as with thermal motion. The three competing tendencies: for keeping particles far apart, or very close, or randomly distributed, lead to much richer phase diagram than in molecular systems. The new stable phases consist of spherical, or elongated, or slab-like clusters of particles. The clusters may be arranged in ordered, crystal-like structures which occur for low volume fractions of particles (~0.1), because the clusters tend to be well separated to minimize the repulsion. Despite the complexity of the phase diagram, it has a universal skeleton in variables: volume fraction of particles and properly scaled temperature. Thanks to the universality, one can predict properties of many system by studying just one. The universality predicted in this work resembles the law of corresponding states in the van der Waals theory of gas-liquid separation. Since temperature is proportional to the kinetic energy, the proper temperature scale in molecular systems is the ratio between the kinetic energy of a molecule and the interaction potential at the optimal distance between two molecules. Ratio between the kinetic and the potential energy is also a proper temperature scale for the charged particles, but as found in this work the relevant potential energy is the energy associated with a formation of a dense layer followed by the depleted-density layer of optimal thickness. This is because the system tends to maximize the attraction and minimize the repulsion between the particles. The found universal sequence of bcc, hexagonal, lamellar, inverted hexagonal, inverted bcc phases for increasing volume fraction is the same as in micellar and block-copolymer systems. The universal skeleton of the diagram is decorated with more complex, system-dependent structures that occur near the coexistence of the above dominant phases for narrow volume-fraction intervals. In some systems a gyroid phase, where a regular branched cluster forms an infinite network, may occur between the hexagonal and lamellar phases.
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LW10993
Attracted by repulsion: Exotic superfluidity in an expanding atomic cloud
Cold quantum particles can display frictionless flow, known as
superfluidity. For fermions this happens when pairs are formed by an
attractive interaction. Here we propose and study a paradoxical situation
where superfluidity of fermions occurs as a result of a very strong
repulsion, instead of attraction. The resulting superfluid state is
exotic, because the total momentum of each pair is non-zero, in contrast
to conventional superfluids. The recipe for creating such a state relies
on recent advances in atomic physics and is deceptively simple: If a dense
cold cloud of fermionic atoms is slowly expanded in the presence of a
lattice created by laser beams, then a superfluid state of doubly occupied
lattice sites emerges naturally. The reason behind this miracle is that
the pairs, although high in energy, are metastable because energy
conservation prohibits them to decay.
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LT11699
Alfven instability in a compressible flow
A previously unknown macroscopic instability in flowing plasmas is
presented. Macroscopic instabilities modify the global structure and dynamics
in laboratory and space plasmas. Well-known examples include the
Rayleigh-Taylor and the Kelvin-Helmholtz (wind over water) instabilities. The
new instability does not have an analogue in hydrodynamics. It may only arise
in the presence of a compressible plasma flow embedded in an ambient magnetic
field. The kinetic energy of the flow is extracted and fed into transverse
disturbances that propagate along the magnetic field. The existence of such
transverse disturbances known as Alfven waves was established over sixty years
ago in laboratory conditions. There is an increasing volume of evidence to
suggest the presence of Alfven waves in space plasmas. However, so far little
has been known about their origin. The energetic and dynamic significance of
such waves cannot be underestimated. Possible implications include the heating
of the solar corona and the acceleration of the solar wind. The presented new
instability mechanism offers a unique and efficient way for the generation of
large amplitude Alfven waves. No flow shears or super-Alfvenic flow speeds are
required.
