Physics Tip Sheet #15 - May 29, 2002

Contact: David Harris
harris@aps.org
301-209-3238
American Physical Society

1) Computational capacity of the universe
S. Lloyd
Physical Review Letters (Print issue: June 10, 2002)

All physical systems register and process information. The laws of physics determine the amount of information that a physical system can register (number of bits) and the number of elementary logic operations that a system can perform (number of ops). The amount of information that the Universe can register and the number of elementary operations that it can have performed over its history are calculated. The Universe can have performed 10¹²⁰ ops on 10⁹⁰ bits (10¹²⁰ bits including gravitational degrees of freedom).

Physical Review Focus: http://focus.aps.org/v9/st27.html
Journal article: http://link.aps.org/abstract/PRL/v88/e237901

2) Modeling ozone depletion
A. Wonhas, J. C. Vassilicos
Physical Review E (Print issue: May 2002)

Modeling ozone depletion is difficult because it is hard to resolve the fine scales needed to take into account how molecules diffuse through the atmosphere. This paper introduces new techniques that are able to work at the appropriate scales and show the role of the fractal dimension of the interface between different chemicals in the atmosphere. This allows the authors to model the diffusion of chlorine monoxide, nitrogen oxide and other gases and predict how they deplete ozone. Then, from an experimental observation of conditions, gas mixing and reactions in the atmosphere can be predicted.

Journal article: http://link.aps.org/abstract/PRE/v65/e051111

3) Patterns in food web structures
J. Camacho, R. Guimer`, L. A. Nunes Amaral
Physical Review Letters (Print issue: June 3, 2002)

Natural ecosystems are connected via intricate networks. Past modeling of these "food webs" involving small systems has not allowed discovery of significant universal factors across all webs. Now a larger study of seven sufficiently detailed webs has led researchers to discover that a single parameter, the linkage density, determines many important properties such as distribution of prey and number of predators. The advantage is that measuring the linkage density in real-life ecosystems is relatively easy and may allow a greater understanding of ecosystem interactions.

Journal article: http://link.aps.org/abstract/PRL/v88/e228102

4) Is cosmology consistent?
X. Wang, M. Tegmark, M. Zaldarriaga
Physical Review D (Print issue: June 15, 2002)

The wealth of cosmological data now collected has become almost bewildering. To help make sense of it, the authors have collected the result of over 100 studies and looked at their implications for the standard cosmological model. The data all appear to be consistent and this compilation allows for better constraints on some of the free parameters of the model. This results in better estimates of neutrino and baryon densities, dark matter and dark energy densities and other cosmological values.

Journal article: http://link.aps.org/abstract/PRD/v65/e123001

5) Single photon LED
Physics News Update/CLEO-QELS meeting

A light-emitting diode that fires one photon at a time has been created. It is a potentially inexpensive, easily manufactured component for quantum cryptography and other applications. The researchers believe this is the first electrically driven single-photon source.

Physics News Update: http://www.aip.org/enews/physnews/2002/split/591-3.html

6) Short pulses of entangled light for quantum communication
I. Marcikic, H. de Riedmatten, W. Tittel, V. Scarani, H. Zbinden, N. Gisin
arXiv preprint server

Researchers have created pairs of entangled photons, the basic resource for quantum communication, in a form that is suitable for transmission over long-distance optical fibers. This will allow for realistic complex quantum communication protocols.

Preprint: http://www.arxiv.org/abs/quant-ph/0205144

7) Micromagnets for trapping cold gases
J. Fortagh, H. Ott, G. Schlotterbeck, C. Zimmermann, B. Herzog, D. Wharam
arXiv preprint server

Micrometer scale magnets, suitable for trapping and manipulating cold atomic gases such as Bose-Einstein condensates, have been created by electroplating a sapphire substrate with copper.

Preprint: http://www.arxiv.org/abs/cond-mat/0205586

Papers from the Physical Review series of journals are available pre-publication to journalists on request.

For media assistance with these or other physics stories, contact:

David Harris
Head of Media Relations
American Physical Society
Ph: +1 301 209 3238
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Email: harris@aps.org