Costa 2 . 1 Millimeter - Wave Frequency Measurement of the Rydberg Constant

The Rydberg constant, R., relates the wavelengths of the spectrum of atomic hydrogen to practical laboratory units. As such, R. is the natural unit for measurements of many atomic properties. Recent advances in optical wavelength metrology have enabled experiments using laser spectroscopy to measure R. with accuracy approaching 2 parts in 1011.4 While R. remains the most accurately measured fundamental constant, other high-precision experiments, which depend on R. as an auxiliary constant, demand even more accurate measurement. The accuracy of optical wavelength measurements is approaching the practical limitations of wavelength metrology. Further progress in the measurement of R. will require frequency measurements making use of the modern definition of length in terms of time intervals and the defined speed of light.