Physics tip sheet #42

In recent years, researchers have found that they can improve acoustic signal transmission in complex environments with a technique known as time reversal. In effect, they broadcast a signal to determine how it propagates through an arrangement of obstructions, and then send a reversed version of the signal back from the detectors to the original source. Now researchers at the Laboratoire Ondes et Acoustique in Paris have managed to apply the same technique to electromagnetic signals at 2.45 GHz, a frequency close to the operating range of cell phones. The experiment heralds the possibility of improving cell phone communications in cities and other environments where obstructions, such as skyscrapers, may eventually aid cellular performance rather than hindering it.

2) Exploiting Noise to Increase Profits
A. Traulsen et al.
Physical Review Letters (to appear)

The interactions between populations with different ambitions (shoppers and sellers, attackers and defenders, men and women) can lead to intricate and complicated dynamics as competitors struggle to adapt and gain an advantage. Researchers at the Christian-Albrechts University in Germany now find that it's not only beneficial for competitors to adapt, but it can also be helpful for them to adjust the rate at which they adapt. The researchers studied competition in numerical models that included random fluctuations, i.e. statistical noise. They found that flexible competitors who adjust their adaptation rate could improve their prosperity in noisy systems. The researchers suggest that their model may have important economic implications, potentially showing ways that competitors can increase their odds of success in noisy systems such as stock markets

3) Searching for Supersymmetry with the Celestial Accelerator
I. F. M. Albuquerque et al.
Physical Review Letters (to appear)

Particle accelerators are invaluable tools for physics research, but they have their limits. Researchers at the University of California at Berkeley propose that we could move beyond the limitations of manmade particle accelerators by studying the interactions of cosmic rays, which are particles accelerated by natural sources. The precise astrophysical origin of high-energy cosmic rays is unknown, but it's possible that they could interact with matter in the Earth to produce exotic particles that might be observed with enormous detectors currently used to study neutrinos. In particular, the researchers propose, cosmic rays might produce particles predicted by supersymmetric theories that go beyond the accepted Standard Model of physics.

4) DAMOP Meeting Papers

The APS Division of Atomic, Molecular and Optical Physics (DAMOP) holds its 35th annual meeting this week in Tucson Arizona. A few of the papers being presented are highlighted below.

(View all the meeting details and abstracts online at http://www.aps.org/meet/DAMOP04/)

i) Slow Light, Fast Information

Information encoded on a light pulse has been believed to travel at the same speed as the peak of the pulse (known as the group velocity), at least in media where light propagates more slowly than the speed of light in a vacuum, c. But now researchers have found that in some cases information can in fact travel much faster than the group velocity. They will demonstrate that in a "slow-light" medium in which light travels at about one percent of c, the information travels 60 times faster than the group velocity. This result complements results reported last year by the same group, in which they showed that in a "fast-light" medium where a light pulse propagates faster than c, the information travels slower. (Paper S3 11, www.aps.org/meet/DAMOP04/baps/abs/S420011.html)

ii) Entangling a Photon with an Atom

A group from the University of Michigan will report on the first observation of quantum entanglement of a single photon and a single atom, a development that could eventually be used for a variety of quantum communication protocols or for quantum computing. Until recently quantum entanglement had only been achieved between two photons or two atoms, not one of each. In this experiment, a trapped cadmium ion in an excited state decays by one of several possible decay paths, emitting a photon that carries off information about the state of the de-excited atom. This is the first observation of entanglement between stationary and "flying' qubits. (Paper G2 1, www.aps.org/meet/DAMOP04/baps/abs/S190001.html)

iii) One Million Years of Groundwater History Revealed by Krypton Dating

Analysis of levels of krypton has revealed water samples from the Nubian Aquifer in Egypt to be between 200,000 and 1,000,000 years old. Researchers will present results of the first use of the Atom Trap Trace Analysis (ATTA) system, which uses laser manipulation to count atoms. The team compared levels of krypton in the groundwater to atmospheric levels in order to date the samples. The information can be useful for analysis of geological processes in this huge reservoir beneath the Sahara desert. (Paper F3 9, www.aps.org/meet/DAMOP04/baps/abs/S160009.html)

iv) Frontiers in Cold Quantum Gases

Several leading researchers will present the latest from the strange realm of ultracold quantum gases. Wolfgang Ketterle of MIT will report on a new record-low temperature of less than one nanokelvin, the conversion of ultracold sodium atoms into molecules, and experiments with atom chips where the magnetic field of miniature wires is used to manipulate ultracold atoms (H1 2). Tin-Lun (Jason) Ho of Ohio State will discuss surprising developments in fast rotating Bose gases (G2 2). Deborah Jin of NIST/JILA will describe her group's recent experiments with degenerate gases of cold potassium atoms in the "crossover region," in which the interactions between atoms can be tuned by varying the external magnetic field (G2 3). And Anthony Leggett of the University of Illinois will offer some reflections on the theory of this crossover region, pointing out that much of what we know about the system relies on approximations (H1 1).

v) Atom Chips

Recent advances in atom interferometry and quantum optics may lead to a whole new class of components – atom chips. Researchers from the University of Heidelberg describe their experiments with the control and manipulation of ultra-cold rubidium atoms in nanostructured traps and guides. The development is an important step on the way to developing devices that control the flow of atoms in much the same way that microelectronic chips control the flow of electrons. (Paper B2 4, www.aps.org/meet/DAMOP04/baps/abs/S50004.html)

vi) Processing Quantum Information with Photons

The first demonstration of a controlled-NOT logic gate for photons, an experimental source of single photons, and a prototype quantum memory device are among the highlights of a talk focusing on photon-based quantum information processing. Devices such as these are vital in the development of optical quantum computers, which could dramatically outperform conventional computer architecture. (Paper C2 3, www.aps.org/meet/DAMOP04/baps/abs/S90003.html)

Journal articles available to journalists on request.