LZ11793
The drunkard returns, coherently.
A group of physicists from the Weizmann Institute of Science and the
Technion - Israel Institute of Technology have shown that one can "see"
the effective dimension of a random walk by looking at its spectrum. In
an experiment exploiting electromagnetically induced transparency, the
random walkers are Rb atoms diffusing in a buffer gas that kicks them
randomly. The atoms are prepared in a coherent superposition of their
two ground states and starts to oscillates like tiny clocks. The atoms
are probed whenever they return to their starting point. Interestingly,
it was proven by G. Polya back in 1932 that a random walker always
returns to the origin in dimensions d=1,2. As the authors explain, by
measuring the average time of return to the origin, one can learn about
the dimensionality of the diffusion. Surprisingly enough, this average
diverges in dimension one and two – a consequence of Polya's theorem.
The authors have measured a related quantity called the "critical
exponent" of the spectrum, which determines this divergency. Using this
spectroscopic probing technique it is possible to learn about the
underlying dynamics of other systems such as quantum billiards or
spintronic devices.
***
LX11147AR
Quantum information processing in optical fibers
Quantum information science aims to harness uniquely quantum effects
to gain advantages in information technologies; photons with their
high-speed transmission and low noise properties appear destined for a
central role. A group of physicists and engineers from the University
of Bristol report a two-photon quantum logic gate implemented entirely
in optical fiber. This all-fiber implementation is of crucial
importance to future applications since it allows the logic gate to be
miniaturized and admits high performance operation. Crucially, such a
fiber approach is compatible with optical fiber communication, which
is expected to form the backbone of future quantum networks that offer
security based on the laws of physics. The controlled-NOT logic gate
reported is also the fundamental building block for many quantum
technologies, including quantum information processing. It was
constructed from specially fabricated fiber couplers (in which two
optical fibers are joined in a short section, enabling photons to hop
from one fiber to the other) that affect the different polarizations
of light in different ways.
***
LA11792
Entanglement, at the heart of quantum computers, can also be a
curse
Entanglement is the key property which enables quantum
mechanical particles to perform some tasks - for example
unconditionally secure cryptography or certain otherwise
intractable computations - which are impossible in the everyday
classical world. Our recent work points to a new aspect of
entanglement: it must come in the right dose to be of use.
In quantum mechanics, two particles are said to be "entangled"
if their behavior is correlated more strongly than classical
physics would allow. While entanglement is a prerequisite for
powerful quantum computers, it also gives rise to the intrinsic
randomness of quantum experiments ("God playing dice").
Any computational scheme must find ways for compensating for
this probabilistic nature. While it is known how to efficiently
cope with the quantum mechanical uncertainty in some specific
cases, no scheme has so far been discovered which could utilize
any quantum system with sufficient entanglement for
computational purposes. Such a universal scheme would have been
very desirable: physicists would like to take advantage of the
states they naturally find in their laboratories, instead of
having to carefully engineer one of the few states known to be
computationally powerful.
Our recent work shows that such a general scheme cannot exist.
We establish that, if the amount of entanglement in a system
grows too large, the intrinsic quantum randomness becomes so
pronounced that no useful information can be extracted.
So while it remains an important and fruitful task to identify
new quantum systems with computational power, researchers must
bear in mind that this property is more elusive than previously
thought.
***
LY11340
NSCL researchers constrain symmetry energy at low density
By analyzing data from several combinations of collisions of tin nuclei,
researchers at Michigan State University National Superconducting Cyclotron
Laboratory (NSCL) have refined understanding of symmetry energy. Their work
marks the first successful theoretical explanation of the common symmetry
energy-related observables – including isospin diffusion and differences in
neutron and proton emitted spectra – in heavy-ion experiments. The result
should help in discerning the properties of neutron stars, particularly in
the crust region
***
BB11168
Thermodynamics goes nano!
A recent study has shown that nineteenth
century thermodynamics can still provide useful insights into
twenty-first century nanosciences; and all this is done with pencil and
paper rather than an expensive super-computer! When the size of
materials becomes smaller than one thousandth the width of a human hair,
matter begins to behave highly exotically. By shrinking the size of
materials, the “surface-to-volume ratio” increases; considering this, we
can study size effects on material properties from macroscopic laws, the
so-called “top-down approach”. In thermodynamics, the Gibb’s energy
concept is particularly adapted to describe the liquid-solid phase
transition, what we mortals call the melting temperature. Interestingly,
Dr Guisbiers, working at IEMN in Northern France, has expressed the
size-dependent relation determining the melting temperature of
nanoparticles. Combining the nano-scale melting temperature with the
bulk phase diagram equations, it has been possible to predict the phase
diagrams of perfectly miscible nano-alloys. Moreover, as the
surface-to-volume ratio governs the size-dependent materials properties;
size effects on the energy bandgap and the melting temperature are
linked. In terms of applications, Dr Guisbiers says that his model can
be used to predict the energy bandgap of nano-semiconductors and to tune
the thermo-optical properties of semiconductor nano-alloys especially
those used in nano-optoelectronics applications.
***
BA11323
RELIEVING THE STRESS OF BEING SMALL
The surface of any material is different from its interior. The atoms in
the surface are half-exposed, so they tend to get closer to and bond
more strongly with their nearest neighbours. This makes surfaces harder,
a phenomenon known as surface tension and familiar to anyone who has
seen an insect walk over water. Small objects have a large surface to
volume ratio, so they can experience considerable stress due to surface
tension.
New research shows that certain materials known as
ferroelastics (a group that includes many ferroelectrics and also
martensitic steels and shape memory alloys) can help relieve the stress
of surface tension by dividing into small regions with different
crystallographic orientations (“domains”). While domain formation (also
known as “twinning”) is common in ferroelastics subject to external
stress (e.g., thin films clamped to rigid substrates), it was not known
that it could be self-imposed in the absence of external forces. Surface
tension twinning is expected to affect not just individual nanodevices,
but also macroscopic samples made of fine-grained powders, including
ceramics.
***
CX10076
“Perfect fluid” observed in RHIC collisions may be more hadronic than
partonic.
Many researchers who study relativistic heavy-ion collisions at the BNL
Relativistic Heavy Ion Collider (RHIC) have concluded that the particles
which take part in these collisions behave as though they make up a
“perfect fluid” since fluid-flow-like effects are seen in measured
observable quantities. The key question about this fluid relates to the
type of particles which mainly composes it: are they partons (i.e.
quarks and gluons) which are fundamental particles, or hadrons (e.g.
pions and nucleons) which are themselves composed of quarks and gluons?
In recent work which is scheduled to be published in Physical Review C,
the author shows results from a simple model which strongly suggests
that the “perfect fluid” may be composed primarily of hadrons rather
than partons. By assuming that the early stage of relativistic gold on
gold collisions is a system of hadrons in a simple geometry and allowing
the hadrons to scattering with each other, the author is able to
qualitatively describe many of the experimental features of RHIC
observables thought to be of fluid flow origin. The author also
extrapolates the model to higher energies to make similar predictions
for lead on lead collisions at the CERN Large Hadron Collider (LHC),
which is scheduled to begin running this Autumn.
