Quantum Support

by David Bradley

Professor Christian Bordé of Académie des Sciences and CNRS, in France explained how inextricably linked the SI units are to modern physics. For the past 40 years, he said, a number of discoveries in quantum physics have completely transformed our vision of fundamental metrology. The revolution started with the discovery of a method for stabilising the frequency of lasers using saturation spectroscopy, which allowed scientists to redefine the unit of length, the metre, by fixing the velocity of light in the vacuum, c.

Today, he added, this concept is being extended so that researchers might redefine all the basic SI units using measurements of such fundamental constants. Various strategies to this end are being developed by research groups around the world. Bordé made a clear distinction between the two frameworks within which such work is being carried out.

First, he considered the "kinematical frame", in which fundamental constants with a dimension (or unit), such as the velocity of light (dimension - length divided by time in metres per second), the Planck constant, (dimension - energy multiplied by time in joule seconds), the Boltzmann constant, (dimension - energy divided by temperature in joule per Kelvin), or the electron mass (dimension - mass in kilograms) can be used to connect the basic units. This system uses understanding from the theory of relativity, quantum mechanics, and statistical physics and does not require knowledge of any forces.

The second system invokes forces, which in modern physics are described by interaction fields. These interactions involve quantities that have no units, they are dimensionless, and include the likes of the "fine structure constant" for electromagnetic interactions. Physicists can then introduce these various interactions into a "dynamical" frame which allows them to characterise measurements in terms of dimensionless coupling constants only (such as the fine structure, ?, or its gravitational analogue ?-G).

This is the ultimate goal of modern metrology to provide definitions of all units in terms of the fundamental constants of nature rather than relying on measurements of arbitrary objects and phenomena. As examples, he showed that one should be able to redefine the unit of mass (kilogram) and the unit of temperature (Kelvin) in the near future, thanks to frequency measurements, by fixing respectively the Planck constant and the Boltzmann constant. He stressed that the modern understanding of quantum effects is underpinning the advent of this new quantum metrology.

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