Physics tip sheet #6 - March 27, 2002

March 27, 2002

1) Evacuating buildings
Ansgar Kirchner, Andreas Schadschneider
arXiv preprint/To appear in Physica A

Designing buildings that can be easily evacuated in times of emergency depends on computer simulations. This paper discusses new techniques that make possible the simulation of large crowds in complicated geometries. Different types of behaviors can be considered and indicates that the fastest time for a room evacuation depends on the right combination of knowledge of exit paths and appropriate herding behavior. The model uses cellular automata techniques and ideas inspired by chemotaxis, a process in which cells follow chemical gradients as a movement guide.


2) Plug'n'play quantum cryptography
D. Stucki, N. Gisin, O. Guinnard, G. Ribordy, H. Zbinden
ArXiv preprint/Submitted for publication

A commercially available system for quantum key distribution and cryptography has been released by a spin-off company from the University of Geneva. The system has been tested over distances up to 70km (from Geneva to Lausanne) through standard optical fiber cables and connects to PCs via USB ports. Transmission rates of about 60 bits per seconds were achieved, sufficient for key distribution.


3) Molecular Bose-Einstein condensation C. Wieman, et al.
APS March meeting

At the American Physical Society March meeting in Indianapolis last week, Carl Wieman announced the creation of a molecular BEC. Wieman reported the observation of a quantum superposition of diatomic molecules and disassociated atoms in a trap. In this experiment, Wieman used Rubidium-85 rather than his usual Rubidium-87 so as to take advantage of Feshbach resonances, which allow the interactions between atoms to be tuned from attractive to repulsive at will. This interaction tuning also creates new and interesting effects and dynamics, including the "Bosenova", an atomic equivalent of a supernova explosion.

Physics News Update:
Conference abstract:

4) Granular mixtures: shaken not stirred
N. Burtally, P. J. King, and M. R. Swift
To appear in Physical Review Letters

"Shake contents to mix" often appears on cereal packets and other granular products. A new experimental study shows that shaking can lead to the separation of components rather than good mixing. If the particles are of equal size, it is the heaviest particles that rise to the top. At high shaking frequencies, the heavier particles can end up as a layer sandwiched between upper and lower regions of the lighter particles. Air damping appears to be responsible for these effects, which disappear completely in the absence of air.


5) Why does lightning fork?
M. Arrayas, U. Ebert, W. Hundsdorfer
To appear in Physical Review Letters

The common forking nature of lightning strikes also occurs in upper atmospheric sprites and blue jets. The branching of the electrical currents occurs on all scales from kilometers to centimeters. A theoretical analysis of these phenomena indicates that a discharge channel builds up becoming nearly ideally conducting over time. At that stage, the channel becomes unstable and branches. The discharge channel can be thought of as having two phases - ionized and non-ionized - separated by a moving boundary. The dynamics of the process is analogous to other problems that involve two phases such as motion of fluids of different viscosities in which fingers of one fluid protrude into another.


6) Intense X-rays from a plasma wiggler
S. Wang, et al. Physical Review Letters (Print issue: April 1, 2002)

Current high intensity x-rays usually come from synchrotron light sources and are used for basic and applied research in physical, chemical and biological sciences and engineering. X-rays are created when high-speed electrons beams are bent. In the case of a synchrotron, the bending is done with magnets. A new method for creating a similar x-ray source has been created by passing the beam of electrons through a plasma. The electron beam carves out its own channel through the plasma but suffers from instabilities that cause the beam to wiggle back and forth. This wiggling of the electron beam creates a collimated beam of x-rays. The new technique may impact on the next generation of x-ray light sources.

Journal article:

7) First rigorous proof that Bose-Einstein condensation should exist
E. Lieb and R. Seiringer
To appear in Physical Review Letters

Bose-Einstein condensation was first predicted over 75 years ago and experimentally created in gases of alkali atoms in 1995. However, there had not been any rigorous proof that the phenomenon should occur based solely on the basic laws of quantum physics. A new analysis rigorously shows that BEC is indeed predicted by quantum physics thereby mathematically confirming the intuition of Einstein and Bose so long ago.

Journal article: On request

American Physical Society

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