Number 96 (Story #2), September 29, 1992 by Phillip F. Schewe and Ben Stein A LASER-BASED MAGNETOMETER has been demonstrated by scientists at the University of Rennes in France (F. Bretenaker et al., Phys. Rev. Lett., 10 Aug. 1992). Their laser consists of a 60-cm long cylinder of the non-magnetic material Zerodur, filled with Ne-20, and capped with "quasi-mirrors" with a reflectivity of only 3%. In this setup, a weak external magnetic field along the laser axis causes a splitting in the neon energy levels (via the Zeeman effect); radiation at the two frequencies interferes in a characteristic way that depends on the size of the magnetic field. With this device, the French scientists have measured variations in the mean magnetic field as small as 10 microgauss. They expect that their magnetometer will be useful in prospecting, volcanology, and particularly in seismology, where the 100-picosecond response time might help in sensing earthquakes. (Nature, 17 Sept. 1992.)