Phys. Rev. Lett. 105, 063001 (2010)

Modern atomic clocks and our knowledge of fundamental physical constants both depend on precise measurements of the radiation emitted by atoms or molecules. Such high-resolution spectroscopy is made possible by the frequency comb laser, which generates an array — or comb — of photons at regular, finely spaced and well defined frequencies.

Kjeld Eikema and his colleagues at the Free University of Amsterdam have, for the first time, performed spectroscopy experiments with a frequency comb laser in the extreme ultraviolet region of the spectrum, down to wavelengths of 51 nanometres — beating the previous record of 125 nanometres. The results might be extended to create X-ray combs, and could also be used to make precision tests of the theory of quantum electrodynamics, the researchers say.