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Metrologia

Cooled Atomic Beams for Frequency Standards

Wolfgang A. Ertmer, Siegfried Penselin

Published Jan 1, 1986• 13 citations
Publisher

Abstract

During the recent past much effort has been spent on cooling of neutral atomic beams. As a result, two successful experimental methods for laser cooling of atomic beams were developed with post-cooling temperatures in the mK regime. We discuss in which ways these cooled atomic beams and new methods for further cooling and handling of these slow atomic beams could improve present and future frequency standards.

DOI
10.1088/0026-1394/22/3/015

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During the recent past much effort has been spent on cooling of neutral atomic beams. As a result, two successful experimental methods for laser cooling of atomic beams were developed with post-cooling temperatures in the mK regime. We discuss in which ways these cooled atomic beams and new methods for further cooling and handling of these slow atomic beams could improve present and future frequency standards.
In 1988, before the beginning of the current contract period, our group discovered that sodium atoms could be laser cooled well below the lower limit (the 'Doppler limit') predicted by the then generally accepted theory. Two main consequences of that discovery were a revamping of the theory of laser cooling and a renewed interest in the applications of laser cooling because of the far lower temperatures. Both of these effects are continuing in full force today, and this report is in large part a record of our group's continuing involvement in the new developments in laser cooling during the past three years. The work of the NIST laser cooling group falls functionally into the categories of: investigating laser cooling mechanisms; studies of collisions between laser cooled atoms; development of new trapping techniques for neutral atoms; study of the fluorescent spectrum of laser cooled atoms; manipulation of atoms and atom optics; development of coherent sources of vacuum ultraviolet light for laser cooling and high resolution spectroscopy; application of laser cooling and trapping technology to atomic frequency standards.
Experimental techniques of laser cooling and trapping, along with other cooling techniques, have produced gaseous samples of atoms so cold that they are, for many practical purposes, in the quantum ground state of their centre-of-mass motion. Such low velocities have virtually eliminated effects such as Doppler shifts, relativistic time dilation and observation-time broadening that previously limited the performance of atomic frequency standards. Today, the best laser-cooled, caesium atomic fountain, microwave frequency standards realize the International System of Units (SI) definition of the second to a relative accuracy of ≈3×10(-16). Optical frequency standards, which do not realize the SI second, have even better performance: cold neutral atoms trapped in optical lattices now yield relative systematic uncertainties of ≈1×10(-16), whereas cold-trapped ions have systematic uncertainties of 9×10(-18). We will discuss the current limitations in the performance of neutral atom atomic frequency standards and prospects for the future.