MEASURING THE FREQUENCY OF LIGHT TO NEW LEVELS OF PRECISION is now possible, opening a new chapter in metrology which may lead to greatly improved determinations of fundamental constants and one way of making powerful optical versions of atomic clocks. Even the most advanced electronic equipment cannot directly measure electromagnetic frequencies higher than roughly 100 GHz (in the microwave range, where frequencies can be counted in terms of the number of oscillations induced in an electrical circuit). Now, researchers at the Max Planck Institute for Quantum Optics (Thomas Udem, 011-49-89-32905-257,Thomas.Udem@mpq.mpg.de) have shown that a femtosecond laser pulse can be used as a "ruler" for precisely determining the frequencies of visible light (which goes up to roughly a million GHz). A femtosecond pulse does not contain a single frequency; rather, its spectrum consists of many frequency peaks which give the appearance of a comb with the tips pointing upwards. The researchers have now shown that the very regular spacing of these peaks can potentially be used to measure differences of at least 20 THz (20,000 GHz) between two electromagnetic waves with a precision as high as 3 parts in 10¹⁷ (Udem et al., Optics Letters, 1 July 1999). For comparison, the best atomic clocks today, based on measuring radio-frequency atomic transitions, have accuracies of 2 parts in 10¹⁵. Locking the wave of interest to the low-frequency end of the femtosecond comb and locking a reference wave to the high-frequency end can determine the frequency difference between the two waves and ultimately allow one to reconstruct the frequency of the visible-light wave. Using femtosecond lasers, the researchers have already measured the frequency of visible light emitted by a cesium atom undergoing a specific transition (specifically, its "D1 line") to a precision of 120 parts per billion, almost 1000 times more precise than previous measurements of that light. (Udem et al, Phys. Rev. Lett., 3 May 1999). The D1 frequency can be plugged into a formula for precisely calculating the fine structure constant, which dictates the strength of the electromagnetic force.