Open systems dynamics in closed quantum systems:
Exact relaxation in quenched quantum many-body systems
Why do systems dynamically relax to a statistical equilibrium state?
This intriguing but old question is enjoying a renaissance recently,
with new experimental techniques becoming available: The non-
equilibrium dynamics of atoms in optical lattices can be experimentally
observed. Specifically, following a quench - that is, a rapid change of
the system's parameters - the many-body system undergoes
complicated dynamics. So what happens? There is no environment, so
how could it possibly relax?
Recent work published in the Physical Review Letters [1] answers this
question rigorously for a class of models that are idealized instances
of the Bose-Hubbard model that takes center stage in this discussion
of atoms in optical lattices. They demonstrate that while the information
on the initial condition is of course stored in the system at all times, it
becomes diluted with time. Locally, for any subsystem, one obtains a
maximally entropy state compatible with the constants of motion.
Remarkably, this is true without a time average: The system just
smoothly and nicely relaxes. So when locally looking at the system,
we think that the system has reached its equilibrium. But only apparently
so, as one day, arbitrarily far in the future, a recurrence will show that
all the time, the initial condition was not forgotten.
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LH11172
Use these quantum bits if you can count to one.
Scientists are striving to build large quantum computers for simulating quantum systems and for solving difficult numerical problems such as factoring large numbers. Qubits, the bits of quantum information, differ from classical bits since according to the rules of quantum mechanics they can be both 0 and 1 at the same time. When many qubits are combined, a large number of states can be processed in parallel, giving quantum computing exponentially increased power compared to a classical computer. The task of manipulating and interconnecting a large number of spatially separated qubits is daunting, and the present state of the art is limited to less than 10 qubits. We propose a new approach which uses an ensemble of atoms, each with N internal states, to encode an N qubit register. Register values of 0 and 1 are associated with 0 and 1 unit of excitation which is shared in a collective entangled state of all atoms in a small ensemble. The register is prepared and manipulated via excitations of high-lying Rydberg states which entangle the atoms. This method encodes N qubits in N internal states in sharp contrast with previous approaches that require 2N physical states, an exponentially large number, to encode N qubits. With this approach a 14 qubit register can be simply encoded in a single cloud of 100 cesium atoms, and more exotic atomic species have the potential to extend this to registers of more than a 100 qubits.
***
LJ10880
What's inside a black hole
---the prediction of string theory
confirmed by supercomputer
Black holes not only swallow up infalling matter,
but also emit thermal radiation as discovered by Hawking.
This suggests that the black holes have a certain interior
structure, but what precisely it is has long been a mystery.
By using supercomputers, we have successfully confirmed
a theory clarifying the interior structure of the black hole.
The key was provided by string theory, which was invented
to unify Einstein's general relativity and quantum mechanics
in a consistent manner. We were able to compute accurately
the energy of a system composed of strings, which is
conjectured to be described by a black hole macroscopically.
As the temperature of the system is lowered, our data indeed
approach the energy of the black hole.
This result shows that the thermal properties of the
black hole such as the Hawking radiation can be explained
microscopically in term of strings. There are many other interesting
issues such as the black hole evaporation and the early universe,
in which string theory is expected to play a crucial role.
Therefore the establishment of a new method in string theory
utilizing supercomputers has great significance. It is expected
to boost our understanding towards profound issues such as
the origin of the universe, matter and its interactions.
***
LE11199
Why are enzymes so big ?
In our paper,
localization of high amounts of energy for long periods of time
is shown to occur in rigid parts of protein structures.
Since it was recently shown that
catalytic sites of enzymes are often found in the rigid
parts of their structure, this result strongly supports the hypothesis
that enzymes may use such high amounts of energy in order to
achieve their function. Indeed, enzymes need energy to break
and form chemical bonds. It is usually assumed that they
use chemical energy only (coming from labile chemical bonds
like those of the ATP molecule, used for energy storage
by all living cells) and that they "wait" for favourable thermal
energy fluctuations otherwise. What our work suggest is that
enzyme structures are set up so as to be able to catch and
store much more energy than "standard" thermal fluctuations
normally allow. This would also provide an explanation for
the following, puzzling question: why are enzymes so big ?
The answer being: in order to have large stiff parts, far
away enough from the water environement and its dissipative effects.
***
LH11482
Quantum Description of "Real" Black Holes Found
We have found the quantum states of a maximally rotating black hole
using string theory techniques. The number of such states agrees
exactly with an earlier prediction of Stephen Hawking. Unlike previous
work on black holes in string theory, the black holes we study are
very similar to those observed by astrophysicists. We are able to
identify and count the quantum states of these physical black holes by
using symmetries of the theory to mathematically relate them to
previously studied cases. This is exciting as it is a step towards
understanding the quantum gravitational physics of black holes.
***
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LY10632
Can aerosols be trapped in open flows?
This paper describes an important and intrinsically appealing new
phenomenon in chaotic fluid flows, which is counter-intuitive and likely
to stimulate new discoveries in several fields. We feel that this paper is
worthy of news coverage by APS, AIP and the media. Below is our attempt to
provide a one-paragraph summary consistent with the editorial style of
Physical Review Focus. The text is accompanied by a color figure (see
caption below) and a movie that illustrate the phenomena described in the
paper (available at www.pks.mpg.de/~rdvilela/leapfrogging.html ). We will
be happy to accommodate changes that would make this more readable and we
will be available to answer any questions you may have about the work.
At this time we also provide the names of three noted researchers that
would be able to comment on the importance and broad impact of this work:
1. Distinguished Prof. Edward Ott (University of Maryland, edott@umd.edu )
2. Professor Zoltan Toroczkai (University of Notre Dame, toro@nd.edu )
3. Doctor Manuel A. Matias (CSIC-UIB - Spain, Manuel.Matias@ifisc.uib.es )
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TRAPPED BY SCATTERING: MOVING OUT, YET NEVER LEAVING
Place a heavy particle in a vortex of a fluid flow and the centrifugal
force will move it outwards, essentially in the same way a fast car slides
in a curve. Now imagine the particle in an open flow (the flow of a
channel, say): it would possibly encounter some vortices, spiral outwards,
and eventually move away as the fluid itself does, right?
Not necessarily, shows our upcoming Physical Review Letters paper.
In a rather counter-intuitively fashion, the motion outwards successive
vortices can drive the particles back to the point where they started,
creating a condition of permanent trapping that leads to accumulation
of particles in specific regions of the flow.
The consequences of this newly discovered phenomenon can be important
in various fields, including astrophysics, atmospheric and environmental
research. They have clear implications for the accumulation of aerosol
pollutants in the air. But they also shed new light on the longstanding
problems of rain formation and planet formation
as a mechanism for coalescence of smaller "particles" given that cloud
droplets in the atmosphere and planetesimals in primitive nebula are
essentially heavy particles in open flows.
Leaving aside potential applications, the fact that the particles
transported by a fluid can remain confined even when all the
particles of the fluid move away is very important in its own right.
It was previously assumed that only light particles
could be trapped, in that case by moving to the center of the vortices.
The heavy particles considered in this study are trapped for exhibiting
precisely the opposite behavior: each individual vortex scatters the
particles away, but does so towards the other vortices. The particles
remain trapped by continuously escaping from the vortices!
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************************************
Figure Caption (figure attached):
Trapping of aerosols (heavy particles) in the leapfrogging vortex flow:
physical space projection of the attractors (red X symbols) and
corresponding basins of attraction (colored regions) at a given instant.
Also shown are the velocity field of the fluid (black arrows) and the
positions of the vortices (black dots) at the same instant.
The red curves indicate the orbits described by the attractors.
************************************
************************************
Movie Caption (movie available at
www.pks.mpg.de/~rdvilela/leapfrogging.html ):
This movie illustrates the dynamics of aerosols in an open fluid flow
consisting of two leapfrogging vortex pairs. To allow for flow visualization,
fluid particles initially placed in a rectangular region are painted (brown dots).
In the reference frame we use, fluid particles come from the right-hand side,
are scattered by the vortices close to the origin, and eventually leave the domain.
Aerosol particles (orange dots) initially inside the same rectangular region
have a different fate. In contrast with the fluid particles, they are permanently
trapped around the vortices.
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***
LD11147
Vortices breakup in newly discovered superconductors
In a Nobel Prize winning paper, Alexei Abrikosov realized that magnetic fields
create vortices that run through superconductors like tubes, each containing
the same fixed amount of magnetic field. In this paper we show that these
vortices can break up into half vortex pairs, or ¿vortex molecules¿, in a
recently discovered type of superconductivity predicted forty years ago by
Fulde, Ferrell, Larkin, and Ovchinnikov. The consequences of these vortex
molecules on the resulting physical properties of the superconductor are quite
dramatic. This vortex molecule state twists and turns in all three space
dimensions, an intricate structure that could not have been guessed without the
insight that the vortex molecules provide. The prediction of this new state
presents a challenge to create experiments to go and find it.
***
LF11513 Wang
PERIODIC ORBITS DRIVE DIRECTED MOTION OUT OF CHAOS
D'ailleurs, ce qui nous rend ces solutions périodiques
si précieuses, c'est qu'elles sont, pour ainsi dire,
la seule brèche par où nous puissions essayer de
pénétrer dans une place jusqu'ici réputée inabordable.
(Henri Poincare', Méthodes nouvelles) (*)
A great challenge for the future technology is to extract transport from random
fluctuations. Brownian motors rectify the random motion of particles, thus
generating a current. Such phenomenon, known as the ratchet effect, may be
the key for understanding molecular motors, that is, tiny biological engines
which transform the energy produced in chemical reactions into unidirectional
motion along macroscopically flat periodic structures. Moreover, the
rectification of fluctuations is potentially useful in technological
applications such as new electron pumps, molecular switches, and transistors.
This beautiful effect takes place in presence of lattice asymmetry, external driving, thermal noise and friction.
The fascinating question then arises: what about a deterministic world in which only dynamical chaos is present? Can directed transport emerge in a purely deterministic system?
In our paper, we show that dynamical chaos alone is of little help to generate directed motion. But, here is the surprise: periodic orbits come into play. Indeed we show that large currents can be generated thanks to the presence of stable islands located around periodic orbits and embedded in the chaotic sea. Chaos, friction and external driving are still needed but periodic orbits play the decisive role.
Once again the old sentence of Poincare’ turns out to be true!
(*) English translation: …what makes these periodic
solutions so invaluable is that they are, so to say, the only breach throughout
which we can try to penetrate inside a region so far considered unapproachable.
P.S. We can provide some nice colour figure of a mixed phase space structure
embedded in a chaotic sea. Here is an example in black and white.
***
LE11272
Electron Billiard with a Laser Cue
For centuries, the classical three-body problem has fascinated scientists. The motion of a system of three planets has no closed form solution, and can exhibit chaotic behavior. The quantum mechanical three-body problem is of even more importance, since all molecules and virtually all atoms are many-body systems. The simplest such system, the helium atom, provides a test bed for understanding multielectron systems. We report a study of electron correlation in Helium using a strong laser field. The laser field serves two purposes -- it drives an electron to collide with the atom, and it provides a clock with which to observe the double ionization dynamics. We concentrate on a unique feature in the correlated electron spectrum. We employ a classical model and a quantum simulation to relate the field driven three-body system to its field-free analogue.
***
LB11532
NO-CLONING, NO-BROADCASTING ARE NOT PECULIARLY QUANTUM
Copying restrictions are generic in non-classical theories
***************************************************************
One of the fundamental results of quantum information theory, the
"no-cloning" theorem (due to Wootters and Zurek and to Dieks), states
that no physical process can produce two independent copies of either
of two pure quantum states, unless those states can be distinguished
by a single measurement. A generalization, the ``no broadcasting"
theorem, extends this result to pairs of mixed states: it says that
even correlated copies are forbidden, unless the states
commute---behaving, essentially, like a set of classical states. In
the present work, researchers Howard Barnum, Jonathan Barrett, Matthew
Leifer and Alexander Wilce show that these results generalize to
essentially any probabilistic theory. A broadcastable set of states
must behave as an effectively classical set of states, and clonable
sets of states must be distinguishable---that is, behave effectively
as a set of classical pure states. So, all nonclassical theories in a
broad probabilistic framework must contain sets of states---for
example, the set of all states---that cannot be broadcast, and sets of
pure states that cannot be cloned.
No-cloning and no-broadcasting have often been considered "peculiarly
quantum" features of a theory, closely related to the possibility, in
quantum theory, of classically impossible feats such as distribution
of secret cryptographic key in such a way that any eavesdropping on
the key will be guaranteed to be detected. If all states could be
copied, an eavesdropper could make a perfect copy of the state of all
systems being used to distribute the key, without making her presence
known through any disturbance to the original state. The present work
shows that no-broadcasting and no-cloning are, in fact, generic
features present in any non-classical theory within a very broad
framework, suggesting that all such theories may permit
eavesdropping-evident key distribution. The program to characterize
quantum theory in terms of information-processing properties, then,
must bring to bear properties beyond no-cloning and
no-broadcasting---such as, perhaps, the impossibility of bit
commitment, the possibility of teleportation, or the nontriviality of
communication complexity, to name three issues under active
investigation by researchers engaged in this program.
***
LHK1043
COMMENT TO PRL: CONTROVERSIAL CONSTANT OF NATURE MAY VARY AFTER ALL
The laws of physics are supposed to be remain solid, unchanging throughout the Universe. But astronomical observations peering back through its 14 billion year history have suggested the opposite, that electromagnetism may have changed subtly with time. Although this surprising effect wasn't seen in more recent observations, our new paper demonstrates faults in the analysis, leaving wide open the possibility of varying laws of physics. The observations focus on galaxies seen in silhouette in front of bright background beacons of light called quasars. As the quasar light travels to Earth, the galaxies absorb some in a barcode-like pattern (see Figure 1 attached) which, when decoded by astronomers, reveals the strength of electromagnetism in the galaxies 10 billion light years away. Our new work shows that recent observations from the Very Large Telescope in Chile were decoded incorrectly, giving physicists too much confidence in electromagnetism's solidity. In fact, when we correct the decoding errors and replicate the analysis, there is broad agreement with previous observations from the Keck Telescope in Hawaii: one of physicists' cherished constants of Nature may actually be varying after all.
Figure 1 caption: The experiment. Extremely distant quasars are used as bright background beacons to probe galaxies which happen to lie along the line of sight to Earth. Metallic gas in the galaxies absorbs light in a pattern of very specific wavelengths. Much like reading a barcode, the spacing between the different metal absorption lines can be decoded to measure the strength of electromagnetism at distances up to 12 billion light years away.
