Number 121, March 31, 1993 by Phillip F. Schewe and Ben Stein OPTICAL AMPLIFICATION WITHOUT POPULATION INVERSION has been achieved in a vapor of samarium, a rare-earth element. Optical amplification is a first step in the laser process in which a single photon, emitted by an excited atom, stimulates other atoms to emit photons having identical attributes, resulting in a buildup of light intensity. Optical amplification normally requires a population inversion whereby a majority of atoms must be excited into a higher energy state; otherwise the ground state atoms, which readily absorb photons, would sabotage this process. W. Lange and colleagues at the University of Munster in Germany use lasers to prepare a sample of samarium atoms in a superposition of two ground states which interfere in such a way as to prevent the atoms from absorbing light. As a result, only a small fraction of atoms needs to be in an excited state to induce optical amplification. This technique may one day be exploited in a working laser, particularly in an x-ray or gamma-ray laser, where population inversion is difficult to achieve because excited states are so short-lived. (A. Nottelman et al., 22 March 1993 Phys. Rev. Lett.; contact Lorenzo Narducci, Drexel University, 215-895-2711, for background information.) Amplification without inversion in sodium vapor was reported from China several months ago, but certain critical verification tests were not performed in that instance. A MERCURY-BARIUM COPPER OXIDE SUPERCONDUCTOR , developed by a Moscow State-Grenoble-AT&T team of scientists, has a transition temperature of 94 K. This does not improve upon thallium compounds, but because the new mercury material's crystallography is simpler---it has only a single intermediate HgO layer and only a single superconducting CuO layer per unit cell---it may be more useful technologically (and possible better able to thrive in the presence of large magnetic fields) than the other copper oxide superconductors. (S. N. Putilin et al., Nature 18 March 1993.) NEURAL NOISE , the random fluctuations present in the electrical signals that flow through biological nervous systems, may aid rather than hinder the transmission of sensory information, new experiments suggest. At the APS March Meeting, Frank Moss and John Douglass of the University of Missouri at St. Louis presented measurements in a crayfish of a hair mechanoreceptor cell, a nerve cell that detects water motion. The external signal they applied--a weak water wave--was better picked up by the nerve cell when its internal noise levels were increased. (This was done by increasing the temperature of the crayfish's ambient water environment, which heightened the rate of random firings of the nerve cell.) Although the researchers observed an increase in the signal-to-noise ratio (SNR) with temperature, they did not isolate an expected phenomenon known as "stochastic resonance," in which a maximum in the SNR would be observed. The researchers speculated that internal effects in the crayfish, unidentified as of yet, may be obscuring or altering the phenomenon. Also at the meeting, John Milton of the University of Chicago Medical Center showed that fluctuations in human pupil size can be stabilized by noise arriving from the ascending reticular activating system, a part of the brainstem that controls consciousness.