Physicists Change the Color and Shape of Single Photons

LE13282

- Physicists have simultaneously changed the color and shape of a single photon, the smallest unit of light. The work represents an important step towards implementing communication over long distances with privacy secured by the laws of quantum physics. The photon was extracted from a quantum dot, a semiconductor version of an atom that emits photons one at a time, using a specially-designed optical fiber. Then, the single photon was combined with a much stronger, pulsed laser beam inside a crystal that enables the two light beams to interact efficiently. After exiting the crystal, the color or wavelength of the single photon had been shifted by almost 600 nm, an amount greater than the size of the entire visible spectrum. Because the researchers were using a pulsed laser, its pulse shape became imprinted on the single photon during the color-conversion process. Researchers utilizing different quantum technologies, which often require single photons of a specific wavelength and shape, can now use this approach to link their systems together in a large-scale network for quantum information processing applications.

Why can't we avoid queues in MANHATTAN-like road networks?

LE13030

- As a Mayor of a growing city with traffic issues: would you choose
Manhattan-like road patterns or more “disordered” old European styles?
As a cellular biologist: do you understand how active transport on
biofilament networks organizes inside a cell?

The question of how the structure of a network affects its transport
properties dates back to 19th century Kirchhoff's work on the
conductance of resistor networks. Kirchhoff's well-known linear laws are
nowadays the basis of any electric circuit “current-voltage” analysis.

Network transport characteristics, however, are trickier if the conveyed
species reciprocally interact in narrow channels or on filamentous
structures: non-linear collective phenomena such as queues or jams can
appear.

A paradigmatic model to study traffic phenomena is the Totally
Asymmetric Simple Exclusion Process: particles move stochastically along
one-way lanes and cannot occupy the same position in space.

Via this model, we show that connectivity is very important for traffic
issues on networks. "Regular" connectivities, with junctions having an
equal number of incoming and outgoing segments, produce fluctuating jams
at each junction. Differently and surprisingly, in “irregular”
connectivities traffic jams as such disappear altogether, leaving high
or low dense traffic lanes with small transport fluctuations.

In urban traffic, these results would suggest that rationally designed
Manhattan-like road layouts could lead to traffic jams everywhere,
whereas the anarchy of historically grown cities could help avoiding
this problem. Speculations on the complex layout of cytoskeletal
transport in living cells are tempting…

Beautiful physics in a cup of water

LE13271

- Everyone who has ever observed the surface ripples that form in a simple vibrating cup of water served in an airplane or a train has unconsciously performed the experiment carried out for the first time in 1831 by Faraday. It consists in the rhythmical vertical shaking of a recipient filled with water and ever since Faradays first observations the developing surface waves are known as Faraday Waves. Only a few researchers have paid attention to another phenomenon which seems to be linked to these surface waves: the induced movements of tiny particles added to the fluid. In this paper, we determine for the first time the whole surface velocity field and its statistics using a technique called Particle Image Velocimetry (PIV). Surprisingly the energy spectra of this fluid flow show characteristics typical for 2-dimensional turbulence which is an important approximation for natural systems such as currents in the ocean or circulations in the atmosphere. In contrast to three-dimensional turbulence it includes the possibility of passing energy from small to large spatial scales, a process which explains a seemingly contradiction: the creation of large structures such as eddies when stirring only happens on much smaller scales.

Critical behavior and transmission of information of a swarm of units

LE13424

- This paper shows how a few lookout birds transmit to the whole swarm their reactions to either environmental scares or attractions. Cooperation generates organization as a phase transition from the random motion of single independent individuals to an order condition where all the units fly in the same direction. However, at criticality the flock organization is not permanent and undergoes collapses allowing the single individuals to recover their independence, thereby making the flock select randomly new directions. The danger/opportunity perceiver birds act as nucleation seeds generating a bias turning the free-will condition into wise decisions, namely the choice of the most convenient flying directions, according to the environment state: As a result of criticality the swarm of cooperating units behaves as a single "intelligent" individual. This article shows that criticality turns local interaction into a long-distance communication, with a time delay determined by the distance between two consecutive organizational collapses. The time delay is proportional to the flock size, thereby generating two challenging issues, (a) the choice of the optimal size for the swarm function and (b) the assessment of whether the occurrence of organizational collapses is related or not to the process of information transmission diffusion via diffusion.

Is religion doomed?

LA12632

- Much like animals competing for a limited supply of food or water,
social groups can be thought of as self-perpetuating entities that
compete for members. The group of religiously-unaffiliated individuals
has grown rapidly in recent times. We use research on social
conformity, together with methods from nonlinear physics, as the basis
for a mathematical model of this growth. We come to a startling
conclusion: coexistence of religious and irreligious segments of
society may be unstable! That is, our model suggests that religion
may be driven toward extinction in the long run. We test the model
with historical census data drawn from 85 regions of nine modern
secular democracies, and find that it fits data well. According to
the model, a slight advantage in the perceived utility of religious
non-affiliation should lead to the long-term decline of religion.