Physics tip sheet #20 - July 3, 2002

July 03, 2002

1) Universal veins
S. Bohn, B. Andreotti, S. Douady, J. Munzinger, Y. Couder
Physical Review E (Print issue: June 2002)

Despite the stunning diversity of leaf shapes from one plant variety to another, a universal formula may guide the vein patterns in all leaves. An analysis of leaf vein networks revealed simple relationships between the angles that veins form when they intersect and the thickness of the veins at the intersections. The universal similarity of vein structure from one plant to another suggests that the patterns provide insight to leaf mechanics, but are of little help in distinguishing or cataloging plants, thereby eliminating one proposed botanical cataloging scheme.

Physics News Update: http://www.aip.org/enews/physnews/2002/split/596-3.html
Journal article: http://link.aps.org/abstract/PRE/v65/e061914
Graphics: http://www.aip.org/mgr/png/2002/156.htm

2) Quantum entanglement in carbon nanotubes
C. Bena, S. Vishveshwara, L. Balents, M. P. A. Fisher
Physical Review Letters (Print issue: July 15, 2002)

Entangled pairs of particles, in which measuring the state of one simultaneously determines the state of the other, are a central part of proposed schemes for quantum cryptography and teleportation. One potential source of such particles is the stream of conjoined electrons that exist in superconductors, but no one has been able to drive a wedge between these so-called Cooper pairs. A new proposal suggests one possible separation method: send them down a pair of carbon nanotubes.

Physical Review Focus: http://focus.aps.org/v9/st33.html
Journal article: http://link.aps.org/abstract/PRL/v89/e037901

3) Atoms take negative time to cross a BEC
U. V. Poulsen, K. Mølmer
arXiv preprint server

Experiments that involve passing a light wave through various materials can show the somewhat surprising property of light appearing to leave the material before it enters. This doesn't imply a breaking of the light-speed barrier, time travel or anything overly exotic but is a property of waves being broken down into component parts and being reassembled slightly differently. This phenomenon has now been predicted in Bose-Einstein condensates where atoms that are fired at a BEC seem to leave before they enter. As an atom hits the BEC, it is absorbed into the collective state but still exists as a vibration. The vibration travels through the BEC but can escape as an atom once more. The study reinforces the similarity between atoms as waves and light as waves. The results also show the conditions for which incident atoms are reflected or pass through unaffected.

Preprint: http://www.arxiv.org/abs/cond-mat/0207029

4) Quantum virtual reality
E. Jané, G. Vidal, W. Dür, P. Zoller, J.I. Cirac
arXiv preprint server

Simulating reality is hard task. The combined computer power of the planet still couldn't run a full simulation of simple quantum systems. However, quantum computers could efficiently simulate other quantum systems. This, in fact, was the original motivation for Richard Feynman to propose such a device as a quantum computer in the 1980s. A quantum device that runs a perfect virtual reality simulation is just a form of a quantum computer but specific things must be taken into account in the design, different from a quantum computer to factor large numbers or run other applications. The authors propose some designs for quantum simulators and point out that such a tool would be invaluable in studying new material designs, especially for new superconductors and magnets.

Preprint: http://www.arxiv.org/abs/quant-ph/0207011

5) A closer look at gravitational lensing
M. C. Smith, S. Mao, P. Wozniak, A. Udalski, M. Szymanski, M. Kubiak, G. Pietrzynski, I. Soszynski, K. Zebrun
To appear in Monthly Notices of the Royal Astronomical Society

Massive astronomical bodies can bend light from stars behind them and act as a type of lens. Many lensing events have been observed but a new event is of a different nature. The relative speed of the distant body and lensing body is so small that the Earth's motion around the Sun is relatively large. This means that the distant body appears twice in images taken from Earth. This rare combination of circumstances means that astronomers may be able to get a better handle on exactly how massive bodies act as lenses. It is usually difficult to obtain this information as the lens is close to transparent, making optical measurements near impossible.

Preprint: http://www.arxiv.org/abs/astro-ph/0206503
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American Physical Society

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