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From physnews@aip.org Wed May 14 13:21:46 1997 Date: Wed, 14 May 97 10:24:01 EDT From: physnews@aip.org (AIP listserver) Message-Id: <9705141424.AA09540@aip.org> To: physnews-mailing@aip.org Subject: update.321
PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 321 May 13, 1997 by Phillip F. Schewe and Ben Stein A PHOTON CONVEYOR BELT has been created using sound waves and lasers, bringing about a new method for processing and storing light signals on a chip. In some opto-electronic devices it is desirable to delay or store an optically-encoded message by dispatching it down kilometer-long fiber cul-de-sacs. In a device developed at the University of Munich, the delay can be accomplished more compactly by first converting the light into a splash of excitons (electron-hole pairs) which propagate at a more leisurely pace, the electrons and holes surfing along on different parts of a guiding acoustic wave. Later the electron-hole pairs recombine into photons, which are read out at the other end of the sample. In effect the signal has been converted from a speed-of- light wave into a speed-of-sound wave, and back again. This technique is also a way of prolonging the lifetime of excitons, which typically live for mere nanoseconds before recombining; in this experiment they have now been preserved for microseconds. (C. Rocke et al., Physical Review Letters, 19 May 1997; contact Achim Wixforth, Achim.Wixforth @physik.uni-muenchen.de; animation at www.aip.org/physnews/graphics) THE QUANTUM WAVEFUNCTION OF A MATTER WAVE, the complete mathematical description of a quantum system, has been experimentally reconstructed for the first time. Trapping a single beryllium ion in electric fields, Dietrich Leibfried and his colleagues at NIST created a state in which the ion has exactly one quantum of vibrational energy. Determining the wavefunction, which contains all the knowable information about this system, is difficult because the uncertainty principle says that measuring its position alters its momentum and vice versa. But by preparing the same quantum state 500,000 times and making a different measurement each time, the researchers sidestepped this limitation and reconstructed piecemeal the probability for the ion to have certain values of position and momentum. Known as the Wigner function, this "quasiprobability" distribution can be mathematically transformed into an average quantum wavefunction for the system which, the researchers argue, is nearly identical to the actual wavefunction. The NIST researchers were the first to measure negative Wigner function values for certain coordinates of position and momentum--something that can only happen for quantum systems; this reflects the fact that the system can exist in many states simultaneously. (Physical Review Letters, 18 November 1996.) Subsequently, physicists at the University of Konstanz in Germany measured the Wigner function of a matter wave traveling in free space--a helium atom traversing a pair of slits. (Nature, 13 March; also Science News, March 15.) PHYSICISTS ARE 46 YEARS OLD AND MAKE $65,000 A YEAR. These are median values for a PhD physicist in the U.S. in 1996. Those who work at federal labs made the most (median $78,500), even more than in industry (median $77,000); those at 4-year colleges made the least, with a median of $49,200. Geographically, median salaries ranged from $70,000 (Pacific states) to $56,200 (East South Central). New PhD's earn $31,000 at universties and $39,600 at federal labs. Salaries for female physicists who have earned the PhD in the past 10 years are comparable to salaries for male physicists with similar experience ("Society Membership Survey: Salaries 1996," a report issued in April by the AIP Education and Employment Statistics Division; contact Ray Chu, rchu@aip.org)