Number 439, July 16, 1999 by Phillip F. Schewe and Ben Stein
HAVING YOUR PHOTON AND SEEING IT TOO. Measuring a photon repeatedly without destroying it has been achieved for the first time, enabling researchers to study an individual quantum object with a new level of non-invasiveness. Physicists have long realized that it is possible to perform non-destructive observations of a photon with a difficult-to-execute technique known as a "quantum non-demolition" (QND) measurement. After many years of experimental effort, researchers in France (Serge Haroche, Ecole Normale Superieure, 011-33-1-4432-3420, email@example.com) have demonstrated the first QND measurement of a single quantum object, namely a photon bouncing back and forth between a pair of mirrors (a "cavity"). A conventional photodetector measures photons in a destructive manner, by absorbing the photons and converting them into electrical signals. "Eating up" or absorbing photons to study them is not required by fundamental quantum mechanics laws and can be avoided with the QND technique demonstrated by the French researchers. In their technique, a photon in a cavity is probed without absorbing any net energy from it. (Of course, Heisenberg's Uncertainty Principle ensures that counting a photon still disturbs the "phase" associated with its electric and magnetic fields.) In the experiment, a rubidium atom passes through a cavity. If a photon is present, the atom acquires a phase shift which can easily be detected. Sending additional rubidium atoms through the cavity allowed the researchers to measure the photon repeatedly without destroying it or, as the French would say, "Avoir le beurre et l'argent du beurre" ("Getting the butter and money out of it at the same time"). This technique can allow physicists to study the behavior of a photon during its natural lifespan; it can potentially allow researchers to entangle (Update 414) an arbitrary number of atoms and build quantum logic gates (Update 250). (Nogues et al., Nature, 15 July; see also Scientific American, April 1993; figure at Physics News Graphics.)
IMPLEMENTATION OF MOLECULAR SWITCHES. In order to plan for integrated circuits of ever greater complexity and compactness, computer engineers would like "grow" components and interconnections with chemical self-assembly instead of building them with lithography. The next step toward creating such nano-scale computer circuits, once the discrete molecular units have been assembled, is to wire them up and configure them into logic gates (OR, AND, etc.). This has now been done by a Hewlett Packard/UCLA/Berkeley team, which has set itself the task of producing a working 16-bit memory cell, no larger than a square 100 nm on a side, within two years. In the 16 July issue of Science, the team reports on an experiment in which an array of rotaxane molecules, grown on a substrate, are controlled by a grid of wires which, through a system of applied voltages trigger local chemical reactions at each rotaxane. These reactions serve to configure the rotaxanes which become in effect molecular-scale switches whose resistivity in the "on" state is 80-100 times less than in the "off" position. Furthermore, addressing the rotaxanes and reading out their condition will require only two wires per molecule rather than the four wires typically needed in conventional integrated circuits based on the complementary metal oxide semiconductor (CMOS) design.
VERSATILE CARBON NANOTUBES are (1) now observed to be superconducting. A group at the Universite Paris-Sud has detected the flow of supercurrents through single nm-wide nanotubes and through bundles of 100 nanotubes at temperatures below 1 K (Kasumov et al., Science, 28 May). (2) Nanotubes have been used to produce muscle-like actuators. A cantilever consisting of two sheets of nanotubes separated by a layer of Scotch tape could, when a voltage was applied across the sandwich, produce stresses higher than natural muscle (Baughman et al., Science, 21 May). (3) Nanotubes, which can be only nm in width but microns or longer in length, are expected to be an ideal strengthening agent in composite materials (Nature, 20 May.) Finally, (4) alkali-doped nanotubes are expected to be great for storing hydrogen, perhaps for use as fuel (Science, 2 July 1999.)