Number 292 (Story #2), October 23, 1996 by Phillip F. Schewe and Ben Stein WAVEPACKET TECHNOLOGY . Femtosecond laser pulses have a truncated spatial extent, and according to classical wave mechanics they must be represented not as a wave at a single wavelength but instead as a superposition of waves at different wavelengths (or colors). When such a pulse is absorbed by an atom or molecule, the resultant quantum state is also a superposition of different energy quantum levels. Putting atoms or molecules into such "wavepacket" states can be exploited for practical ends. Physicists at the Weizmann Institute in Israel and the National Research Council of Canada (Albert Stolow, NRC, 613-993-7388) have separated isotopes of bromine molecules in a gas by using femtosecond laser pulses to create wavepacket states. The evolution of these packets is different for the two isotopes, and if a second laser pulse is applied at just the right moment, one isotope can be ionized (and extracted) while the other isotope remains behind. The researchers believe that wavepacket techniques can be used to control chemical reactions and that creating wavepackets in semiconductor materials can potentially lead to the development of ultrafast terahertz switches, a thousand times quicker than the fastest existing switches. (I.Sh. Averbukh et al., Phys Rev Lett, 21 Oct; see also /physnews/preview)