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Optical frequency metrology

Abstract

Extremely narrow optical resonances in cold atoms or single trapped ions can be measured with high resolution. A laser locked to such a narrow optical resonance could serve as a highly stable oscillator for an all-optical atomic clock. However, until recently there was no reliable clockwork mechanism that could count optical frequencies of hundreds of terahertz. Techniques using femtosecond-laser frequency combs, developed within the past few years, have solved this problem. The ability to count optical oscillations of more than 1015 cycles per second facilitates high-precision optical spectroscopy, and has led to the construction of an all-optical atomic clock that is expected eventually to outperform today's state-of-the-art caesium clocks.

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Figure 1: Consecutive pulses of the pulse train emitted by a mode-locked laser and the corresponding spectrum.
Figure 2: The first direct radio frequency–optical frequency conversion using a femtosecond laser.
Figure 3: The principle of the single-laser optical synthesizer.
Figure 4: An optical synthesizer in a box.

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Acknowledgements

We thank our collaborators P. Lemonde, G. Santarelli, M. Abgrall, P. Laurent and A. Clairon (BNM-LPTF, Paris), C. Salomon (ENS, Paris), J. Knight, W. Wadsworth, T. Birks and P. St J. Russell (University of Bath), and J. Hall, S. Diddams, J. Ye and L. Hollberg (NIST, Boulder, Colorado) for the excellent teamwork and stimulating discussions.

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Udem, T., Holzwarth, R. & Hänsch, T. Optical frequency metrology. Nature 416, 233–237 (2002). https://doi.org/10.1038/416233a

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