The fourth Helmholtz Prize, which was awarded in 1980, was a special prize. It was awarded on the occasion of the Conference on Precision Electromagnetic Measurements (CPEM 80) which took place in Braunschweig in that year and was to go to the best work presented at this event. This prize, which was worth 5,000 German marks, was awarded to four English physicists Dr. David J. E. Knight, Dr. Gordon J. Edwards, Peter R. Pearce and Nigel R. Cross, who worked at the National Physical Laboratory (NPL) in Teddington, England. In their lecture titled “A ± 3 parts in 1011 measurement of the frequency of the methane-stabilized, helium-neon laser at 88 THz”, they reported on how they had determined the frequency of an He-Ne laser, which had been stabilized using an absorption line of methane at a wavelength of 3.39 µm, with an accuracy of 2 kHz that had not been attained before.
At the National Bureau of Standards in Boulder, USA, the speed of light had been determined with an accuracy of 1 m/s in 1972 by measuring the wavelength and the frequency of a methane-stabilized He-Ne laser at 3.39 µm. The redefinition of the metre in 1983 was based on this; it stated that a metre was the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. The work of Knight and his colleagues was part of the effort put into preparing for the later redefinition of the metre by measuring the frequency and wavelength of the He-Ne laser as precisely as possible.
Knight et al. determined this frequency using a complex frequency chain which contained four different oscillators. First, the frequency of an optically pumped methyl alcohol laser, at 4.2 THz, was compared with the 43rd harmonic of the microwave frequency of a 99 GHz klystron and measured by electronically counting the beat frequency. In the next step, the frequency of a CO2 laser, which was at 29 THz, was compared to the 7th harmonic of the methyl alcohol laser. The difference here was however so large that the 3rd harmonic of a microwave frequency of 95 GHz still had to be subtracted to attain an electronically countable beat frequency in the MHz range.
The 3rd harmonic of the CO2 laser frequency was then compared to the frequency of an He-Ne laser, whereby here, a 55 GHz frequency had to be subtracted. Finally, the frequency of this He-Ne laser was compared to the frequency of the methane-stabilized He-Ne laser that was to be measured. It consequently came to (88 376 181 616 ± 3) kHz. To achieve this high measurement accuracy, Knight and his colleagues made sure that the temperature of their lab changed by less than 0.1 °C per hour. This was made less difficult for them, by the weather that summer having been unusually cold.