An international team of scientists has demonstrated that femtosecond-laser-based frequency synthesizers, which are key components of next-generation optical atomic clocks, offer fractional uncertainties approaching one part in 1019. The work suggests that the frequency synthesizers are suitable for experimental tests of Einstein's Equivalence Principle, such as by setting lower bounds on time-dependent changes in the fine structure constant, which defines the strength of the electromagnetic force.An outcome of current string theory is the prediction that fundamental physical constants may vary with time and space, in violation of Einstein's prediction that the physical laws in a uniformly accelerated reference frame are indistinguishable from those in a uniform gravitational field. Variations in the fine structure constant, for example, would indicate that the speed of light has changed over the lifetime of the universe.A comparison of the "optical clockworks" in next-generation atomic clocks based on optical transitions reveals that they display the necessary stability for use in experiments that probe violations of Einstein's Equivalence Principle. ©2004 Bruce Erik Steffine. One way to probe whether this is so involves the comparison of ultraprecise frequency standards based on different atomic species or on different atomic transitions to determine whether their relative rates deviate over time. This requires a high degree of confidence in the components of these atomic frequency standards, said Scott A. Diddams of the National Institute of Standards and Technology (NIST) in Boulder, Colo. He took part in the recent demonstration with researchers from the Bureau International des Poids et Mesures in Sèvres, France, from East China Normal University in Shanghai and from OFS Laboratories in Murray Hill, N.J.Specifically, the group investigated the reproducibility over a period of months in the performance of four femtosecond laser synthesizers: two built at NIST, and one each in France and in China. Such synthesizers act as the "optical clockworks" for new frequency standards based on a resonant optical transition in a cooled atom, offering a stable chain of ticks that are locked to the much higher frequency transition so that each countable pulse of the synthesizer represents a fractional number of optical cycles from the atom. The synthesizers compared in the experiment incorporate 800- to 1000-MHz Ti:sapphire lasers that generate either a 620- to 1000-nm broadband spectrum directly or a 530- to 1100-nm spectrum after broadening in a 30-cm-long nonlinear microstructure fiber.The scientists connected the synthesizers to the same 456-THz optical oscillator and used heterodyning techniques to compare the positions of corresponding output modes. Diddams said they had anticipated that the systems would perform well but that the calculated uncertainty of 1.4 X 10–19 was an order of magnitude better than they had expected."We were particularly pleased to learn that researchers from three different labs could construct different devices and then bring them together and have them agree so favorably," he said. "It was a strong affirmation that the femtosecond-laser-based optical frequency synthesizer will be a reliable metrology tool for the emerging optical clocks we are constructing in addition to its use in other precision measurements."In a separate round of experiments, Diddams and fellow scientists at NIST have used a femtosecond laser synthesizer to investigate any variation in the relative rates of a 9-GHz cesium atomic fountain clock and a 1-PHz mercury single-ion atomic clock over a period of two years. The upper bound of ±7 X 10–15 per year yields a maximum variation in the fine structure constant of 1.2 X 10–15 per year.