Femtosecond Comb Technique Vastly Simplifies Optical Frequency Measurements

A recent measurement of the transition frequency between the 1S and 2S states of hydrogen has now reached a precision of 1.8 parts in ${10}^{14}.$ As the precision of optical spectroscopy has developed, further improvements appear to be getting easier instead of harder. So says Theodor Hänsch, of the Max Planck Institute of Quantum Optics in Garching, Germany (near Munich), who has been a leader in the field since the 1970s. At that time, hydrogen spectroscopy could be done at the level of 1 part in ${10}^7$ or ${10}^8.$ But Hänsch dreamt that eventually one would be able to reach a level of 1 part in ${10}^{18}$ by splitting the natural linewidth for the 1S‐2S resonance (which is 5 parts in ${10}^{16}$) to about 1%. Now he’s within four orders of magnitude of his dream. The new hydrogen measurement was done jointly by Hänsch and his collaborators in Munich, and by Christoph Salomon and his collaborators at the Paris Observatory.

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