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Getting closer to the zero point

An international low-temperature scale allows more accurate measurements close to the absolute zero point

01.12.2000

[es] The ladder of the International Temperature Scale has been extended downwards: With the new low-temperature scale it is possible now to exactly measure temperatures below 0,65 kelvin. The new scale is based, among other things, on measurements performed by the Berlin Institute of the Physikalisch-Technische Bundesanstalt (PTB).

Exactly 100 years ago, on December 14, 1900, Max Planck held a lecture which triggered off a physical revolution. To the amazed public he presented his idea that radiation particles can absorb energy only in very small quantities, so-called "quanta". This was the beginning of the era of quantum physics. As much as they are incredible even today, its theoretical findings now belong to the best-proven areas of physics. And they will soon be applied in practice as well: First successes of quantum cryptography (encoding techniques for the future antitapping computer) invite us to areas which up to now have rather been of interest for basic research. One of these areas is the investigation of so-called Fermi liquids. At very low temperatures, helium (3He) becomes such a quantum liquid whose particles show a most peculiar behaviour.

In order that theoretical models might also be confirmed by experiments, it must be possible to accurately measure the ultra-low temperatures. For such measurements a generally binding temperature scale has not been available up to now. Below 0,65 K - the temperature at which the International Temperature Scale of 1990 (ITS-90) ends -, the situation is confusing: The scientists had to resort to several scales which in the millikelvin range differed by up to six percent. Now a single scale is applicable here. In mid-October, the International Committee for Weights and Measures (CIPM) adopted a new International Temperature Scale which eventually clarified the situation in the areas close to the absolute zero point. It covers the range from 0,9 mK to 1 K, i.e. in the upper range it intersects with the ITS-90 and is founded on the same successful principle: It profits from the property of an element to change from one state of aggregation into another always at exactly the same temperature. This property often is dependent on pressure so that a curve is obtained - in this case, the melting pressure curve of helium or more exactly: the internationally agreed relation between the temperature and the melting pressure of helium. To know the exact temperature, it now is sufficient to measure the pressure.

To establish the temperature scale - and this is one of the tasks of the national metrology institutes such as the PTB -, exact measurements using primary thermometers have to be carried out. These are measuring instruments which do not use the material properties (such as, for example, the mercury thermometer) but exclusively fundamental physical relations. For the new low-temperature scale, this instrument is a combination of various methods of measurement: among others, noise thermometry (at the PTB and at the metrology institute of the US, the NIST) and nuclear orientation thermometry (at the NIST and at the University of Florida). The combination of these measurements offers the advantage that errors are minimized but it also has the disadvantage that in the lower range of measurement the values are not yet in perfect agreement. This is why the new scale is referred to as "provisional low-temperature scale". However, it can already provide good services not only to basic researchers but also to the manufacturers of cryostats - the instruments generating very low temperatures. The manufacturers now can state with greater accuracy which temperatures their instruments really reach.

Further information:
Prof.Dr. Michael Kuehne, tel. 030 - 3481 473
e-mail: michael.kuehne(at)ptb.de
"Temperature and Heat" Section
Physikalisch-Technische Bundesanstalt (PTB)
Abbestrasse 2-12, 10587 Berlin