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Number 557, September 20, 2001 by Phil Schewe, James Riordon, and Ben Stein

The Greening of North Latitudes

A new study shows that over the past 20 years vegetation in the 40-70 north latitude zone has in general been increasing and that the vegetal enhancement, as measured from NOAA polar-orbiting satellites, seems to be correlated with temperature increases, as measured on the ground at thousands of stations. This region of the Earth has warmed about 0.8 degrees C since the early 1970s. The greening is not uniform around the world but occurs more in a band across Eurasia and much less in North America. Scientists at Boston University and the Goddard Space Flight Center report that the northern Eurasian growing season grew to be an average of 18 days longer (spring arriving a week earlier, fall arriving 10 days late) over the period 1981-1999, while the northern Western Hemisphere season became about 12 days longer. (Zhou et al., Journal of Geophysical Research (Atmospheres), 16 Sept 2001; a press release contains images and additional information.)

An Anomalous Acoustoelectric Effect

An anomalous acoustoelectric effect has been discovered by a Russia-Poland-Ukraine collaboration (A.V. Goltsev, Ioffe Physical Technical Institute, St. Petersburg, goltsev@gav.ioffe.rssi.ru).

When an acoustic wave propagates through an electrically conducting surface, it can drag electric charge along with it, just as wind drags autumn leaves along a street. This "acoustic wind" is known more formally as the acoustoelectric (AE) effect.

Studying the electric current produced by the AE effect can provide important information on how electrically charged particles interact with the crystal lattice of a conducting material. Such materials include "manganites," manganese-based compounds that can exhibit "colossal magnetoresistance," in which electrical conductivity becomes tremendously sensitive to external pressure and applied magnetic fields.

Towards these ends, the researchers investigated the AE effect in a manganite thin film atop a lithium-niobium-oxygen (LNO) substrate. They observed an unusual effect: sending an acoustic wave in a certain direction produced a much weaker electric current than expected in that direction.

The researchers discovered why: in addition to the ordinary acoustic wind, a countervailing wind was flowing in a direction opposite to the acoustic wave. The countervailing wind arose from the fact that the substrate was "piezodielectric," in which electric fields were generated in response to pressure. When the acoustic wave created an alternating pattern of compression and expansion in the substrate, the compressed regions produced electric fields pointing in the direction of the countervailing wind. These fields interacted with the electrons on the thin film. Since the manganites increase their conductivity dramatically when compressed, this encouraged a flow of electrons in the countervailing direction.

While this anomalous AE effect is probably too weak for technological applications, measuring it could provide a new method for studying the effects of applied pressure on a conducting material. This could be useful in those cases when employing conventional methods for those measurements is difficult, as is the case for thin films or quantum wells, wires, or dots. (Ilisavskii et al., Physical Review Letters, 1 October 2001.)

Multiplayer Quantum Games

Played with atoms and photons rather than dice and coins, quantum games are contests whose outcomes are governed by the unusual logic of the submicroscopic world. The basic token in a quantum game is a "qubit," a bit of data which is stored in an object such as an atomic nucleus. While a classical coin can only be heads (data value 0) or tails (data value 1), a qubit can effectively be both heads (0) and tails (1) at the same time, since the nucleus can be in a combination or superposition of spin-up (0) and spin-down (1). What's more, one can interlink or "entangle" qubits held by separate players so that manipulating one qubit strongly affects the others.

More than a diversion, playing quantum games can reveal new information-processing tasks (possibly even certain types of financial transactions) that quantum computers could perform more efficiently than classical computers. Towards these ends, theorists have been taking traditional games, adapting them for the quantum realm, and checking if new or better strategies emerge for winning.

While past quantum games have focused on two players (Update 411), Oxford researchers (Patrick Hayden, patrick.hayden@qubit.org) have now identified multiplayer games in which the player's optimal strategy differs from that of the classical version of the game. The researchers discovered unique strategies in a three-player quantum version of the Dilemma game, in which three partners engaged in a venture (such as getting the best seats at a concert) each decide whether or not to betray the others in efforts to maximize personal gain.

In the quantum version, the qubits are entangled, then each person uses his qubit to choose between the following strategies: try for good seat (0), settle for poor seat (1) or some superposition of the two. Entanglement actually destroys the incentive for a player to contradict and thereby betray his opponents and it removes the classical dilemma entirely. Although quantum games are mostly played on paper at this point, a Chinese group has just reported the experimental realization of a quantum Prisoner's Dilemma (Los Alamos preprint). (Benjamin and Hayden, Physical Review A, September 2001.)