Self-assembly of a model quasicrystal
Although this year marks the 25th anniversary of the discovery of
quasicrystals, the mechanism of their formation is only poorly understood
yet. One reason is the lack of suitable theoretical models of atomic
interactions that lead to stable quasicrystals. A realistic model would be a
formidable task and simplifications have to be made. Michael Engel and
Hans-Rainer Trebin from Stuttgart University in Germany have now developed
interactions that allow the first direct observation of quasicrystal
growth. Their simple model consists of identical particles moving in two
dimensions favoring two distinct interparticle distances. The competition
between the distances can favor a local particle arrangement with ten-fold
symmetry, which is not compatible with periodicity. A quasicrystal is then
self-assembled in computer simulations at elevated temperatures. Upon
cooling
the quasicrystal undergoes a reversible phase transition into a complex
periodic crystal. A remarkably large variety of other crystals and a
quasicrystal with twelve-fold symmetry have also been found. In the
future the
model system might be experimentally realized with colloidal particles.
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Essential building-block for large-scale quantum communication
In the present paper we have experimentally demonstrated an essential
building-block for large-scale quantum communication. We demonstrate
efficient creation of photonic entanglement using spatially distributed,
narrow-band, and memory built-in single-photon sources. This progress may
pave the way for future global quantum networks.
Currently, the channel length of quantum communication is limited to about
150 km due to photon losses and decoherence. Intuitively, one would expect a
"repeater", widely used in the traditional communication networks, will help
to extend the distance of quantum communication to an arbitrary desired
length. However, the quantum nature requires very special properties for
such a quantum repeater. Efficient schemas require a quantum memory to be
scalable.
In our experiment, atomic ensembles are used as quantum memory (stationary
qubit) while single photons are used as information carrier (flying qubit).
The inherent memory built-in properties of single photon sources make the
generation of entanglement--fundamental resource for quantum
communication--of independent single photons coming from remote sites very
efficient, based on which the quantum network is scalable.
***
Anomalous thermodynamics of Coulomb interacting massless Dirac fermions in two spatial dimensions
Low temperature thermodynamics of metals is a part of standard
condensed matter curriculum. It is well know, yet still remarkable,
that on the basis of the low temperature dependence of specific
heat, an experimentalist would be hard pressed to distinguish a
non-interacting electron gas from Coulomb interacting Fermi liquid.
In both cases the specific heat vanishes linearly with temperature.
This paper considers the effect of Coulomb interactions on the
specific heat of a two dimensional semimetal, which is in some sense
a critical point between a metal and a semiconductor. If the
electrons were free, the specific heat of a semimetal would vanish
quadratically with temperature, the extra power of temperature comes
from the linear depletion of single particle states at low energies.
Remarkably, the Coulomb interactions suppress this non-interacting
result by logarithmic factors, whose strength is given by the
effective fine structure constant, differing from the 1/137 by the
ratio of the speed of light to the Fermi velocity. Experimental
observation of such effect, would firmly place two dimensional
semimetals, such as graphene, into the category of non-Fermi liquid.
