On June 1, 2008 Prof. Theodor W. Hänsch, Ludwig Maximilian University, Munich (Germany) and Director of the Max Planck Institute of Quantum Optics was accepted as a new member in the order "Orden Pour le mérite for Sciences and Arts". In Germany this order is regarded as one of the highest honors that a scientist or artist can be awarded. The new time measurement has wide-ranging consequences – it raises the issue of redefining the kilogram.

Prof. Theodor Hänsch has earned an outstanding reputation worldwide. He demonstrated a new way for developing a technology for extremely precise optical time measurement which will lead to the creation of a new standard for the unit of time and a uniform global measurement of time: the "optical clock".

Optical clocks could be the clocks of the future. No matter whether it is a sundial, hour glass, pendulum clock, quartz watch or cesium atomic clocks: a clock always comprises two components, the oscillating "pendulum" and a counter that counts its oscillations. Currently the 9,192,631,779th oscillation of the cesium atoms of the atomic clock is the official definition of the second. The faster the pendulum oscillates, the more exact the clock is. An atom that emits light at a specific frequency is a much more precise "optical" pendulum.

However, light oscillates so fast that the oscillations cannot be counted with conventional methods. Green light, for example, has around 600 trillion oscillations per second. Computers and atomic clocks that have to be used for counting work with only around 0.01 trillion oscillations per second. Prof. Theodor Hänsch developed a method with which extremely fast oscillations can be measured: the frequency comb. For this Professor Hänsch along with John L. Hall from the University of Colorado and Roy Glauber, Harvard University received the Nobel Prize for Physics in 2005.

In Prof. Hänsch's laboratory in the Max Planck Institute of Quantum Optics in Garching, Germany a titanium: sapphire pulsed laser with a constant frequency, whose short light pulses circulate between deflection mirrors, is used to generate the optical frequency comb. By overlapping the oscillations it is possible to measure the frequency of the light with a previously impossible accuracy of 15 decimal points, which forms the basis for the optical clock. The frequency comb is used in many laboratories throughout the world as a basis for optical frequency measurements. This value could be used to redefine the SI basic unit the second if further investigations and international comparisons show that this frequency measurement is accurate enough. Scientists are still working on a "tiny" inaccuracy in the optical clock. If they had measured the time since the origin of the universe 13.7 billion years ago, today they would be five minutes out.

As optical atomic clocks divide the time in hundred thousand times smaller intervals than conventional cesium clocks, a dramatic increase in accuracy is expected in the future. Worldwide there is a big interest in more accurate clocks, as in future extremely precise comparisons of clocks will enable a worldwide telecommunications network to be developed, improved gravitation potentials to be formulated with far-reaching consequences for geodesy, environmental monitoring and mineral resource prospecting and the question as to the constancy of natural constants to be asked.

Redefinition of the kilogram
This fundamental new research and the precise time measurement that is now available also raises the issue of redefining the kilogram. The original kilogram, which was manufactured in 1889 from a platinum alloy and is today kept in a safe in Paris, is losing weight for some unexplained reason; the loss is hardly noticeable but measurable. Under the auspices of Physikalisch-Technischen Bundesanstalt (PTB) in Brunswick, Germany a way of creating a new standard, based on an extremely accurate measurement of the Avogadro constants, is being sought.

These constants give the number of atoms in one mol of a substance and combine the microscopic and macroscopic variables of a silicon crystal. To do this, the latest measuring methods to determine the density and number of atoms in a sphere consisting to 99.99% of the silicon isotope 28 with an almost perfect crystal structure are used The diameter of the sphere is 9.36 cm with a radius deviation of <30 nm. In the numerous, time-consuming measurements it is always necessary to ensure that the sphere is aligned correctly in the measuring instrument. The marking with no loss of mass is done with a femtosecond laser.

The aim of the project is to achieve a measuring uncertainty of 10-8. This project is due to run for around 6 years and is sponsored by well-known metrological institutes. If the project is successful, the mass embodiment of the kilogram based on the international SI unit system (Le Système International d'Unités) could be based on a much more precise definition of the mass. The SI was created in 1960 by the 11th General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM). The SI is today's form of the metric system as used throughout the world. Two paths were taken to redefine the units of time and mass extremely precisely. The success is expected to generate a global innovation boost in almost every area of human society.
Prof. Theodor Hänsch beside a test set-up
Source: Max Planck Society

Avogadro sphere made from the silicon isotope 28, diameter 9.36 cm
Source: PTB