ELECTRON HOLOGRAPHY can supply an atomic-resolution image both of the atoms in a surface and (unlike STM) some of the atoms in layers underneath. Physicists at the University of Erlangen-Nurnberg in Germany have converted a popular surface- imaging technique---low energy electron diffraction (LEED)--- into a form of holography. In conventional holography, part of a laser beam (the object beam) is scattered from an object and part (the reference beam) left unscattered. The scattered and unscattered waves meet in a piece of film where they inscribe an interference pattern which, when reconstituted, renders a three-dimensional image of the object. In the Erlangen experiment all of this happens on a nanoscopic level, with electron waves instead of light waves. When an electron beam strikes a surface, any prominent atom can be thought of as a beam splitter creating a reference electron wave and---after subsequent scattering by neighboring atoms---an object wave. From the measured electron diffraction pattern a 3- dimensional image of the local environment of the beam-splitting atom can be reconstructed. In this way, the surface structure of the crystal SiC (a potentially important material for electronics applications) was determined. (K. Reuter et al., Physical Review Letters, 15 Dec.; contact Klaus Heinz, kheinz@fkp.physik.uni- erlangen.de, 011-49-913-185-8403; or Ulrich Starke, ustarke@fkp.physik.uni-erlangen.de)

THE MOST PRECISE FREQUENCY MEASUREMENT ever made in the visible or ultraviolet portion of the electromagnetic spectrum has been carried out at the Max Planck Institute for Quantum Optics near Munich. Measuring the frequency (or, equivalently, the energy) of a light wave is easy in the microwave region (around 10⁹ Hz), where one can directly count oscillations in an electronic circuit. This does not work for visible or ultraviolet light, so Theodor Hansch (011-49-892-180-3212) steps down UV waves by mixing them with light at lower frequencies, producing an average or "beat" signal. After numerous stages the resultant signal is amenable to high-precision counting methods (Physics Today, Dec. 1997). In this way the frequency corresponding to the important (for the study of quantum mechanics) interval between the 1S and 2S quantum states in hydrogen has been determined to be 2.466 061 413 187 34 x 10¹⁵ Hz, with an uncertainty of only 3 parts in 10¹³, an improvement by a factor of almost 100 over previous work. (Udem et al., Phys. Rev. Lett., 6 Oct.) A new article, upcoming in Phys. Rev. Lett., reports on comparable measurements for deuterium, allowing the best calculation of the difference in the mean square charge radii for the proton and the deuteron.

TRANSISTORS WERE INVENTED 50 YEARS AGO next week. This simple three-terminal electronic device can act as amplifier or switch by allowing a tiny electrical (gate) signal to control a much bigger current. (For the history of the transistor, see the December Physics Today and the book "Crystal Fire: the Birth of the Information Age," by Michael Riordan and Lillian Hoddeson.) Examples of ongoing research include the development of all- polymer transistors (Update 196); spin transistors, in which the spin as well as the charge of electrons is important (Physics Today, July 1995); room-temperature, single-electron transistors (Update 308); silicon-carbide transistors for high-temperature applications (Update 327); 10-nm metal transistors (Update 322); the development of molecular-scale transistors (New Scientist, 2 Aug 1997); and neuron transistors, in which gate signals are supplied by ions from leech neurons (Jenkner and Fromherz, Phys. Rev. Lett., 8 Dec.)