Quantum metrology - standards of measurement based on atomic and quantum phenomena

Abstract

The past quarter of a century has seen a great change in ideas about units and standards of measurement. The system based on mechanical standards such as the metre bar is being replaced by one based on atomic and quantum phenomena such as a wavelength of light. The new system is now, in part, internationally accepted through definitions of the metre based on a wavelength of light of krypton and of the second based on a hyperfine transition in caesium. The advantages of the new system are in part conceptual, but principally they are practical since the new standards are more generally available and allow more accurate and more convenient measurements in science and engineering. The possibilities of the new approach to metrology go much further than length and time measurements. Ways of relating the electrical units, in particular, to measurements of frequency are being explored and it can be foreseen that in the near future it will be possible to relate most measurements to a measurement of frequency, with the great practical advantage that standards of frequency are immediately available from radio broadcasts. Emphasis is placed in this review on the importance of precise measurements in gaining new knowledge of the physical world and in applying it in engineering and it is argued that the most important property of a system of measurement is that it should be widely and easily available. The physical principles of atomic standards of frequency and length are explained, the ways in which they are realized at present are described and the possibility of a unified system of units of length and time based on frequency is discussed. The present basis of the electrical units is explained and an account is given of work in progress whereby it may be replaced by a system related to frequency through such phenomena as the Josephson effect between superconductors and the precession of protons in a magnetic field. The relation of the fundamental constants of physics to metrology is discussed, examples of current work on their determination being given. Finally, the possible structure of a system of units and standards based on frequency is considered and some examples of modern demands on metrology are given.

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