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Optical Lattice Clocks

Working Group 4.32

Strontium Lattice Clock


Blue fluorescing cloud of strontium atoms (arrow) that have been laser-cooled to milli-Kelvin temperatures.

Optical clocks could be the atomic clocks of the future. Their “pendulum”, i.e. the periodic oscillation required in every clock, is an electro-magnetic oscillation in the spectral range of visible light. Since these frequency is several 10 000 fold higher than the microwave oscillation in cesium atomic clocks, we expect a considerable improvement of the clocks’ stability and accuracy.

Energy level scheme of the strontium atom with transition wavelength and transition rates. The 698 nm transition serves as “pendulum” for the clock.

In optical lattice clocks strontium atoms are trapped in the interference pattern of two laser beams. In this so called “optical lattice”, the atomic “pendulum”, i.e. the absorption frequency of the atoms, can then be determined very precisely – currently with an accuracy of 17 digits.

Schematic representation of an optical lattice. The atoms are captured in the intensity maxima of the standing wave of the optical lattice. At the “magic wavelength”, both states of the clock transition are shifted exactly the same amount by the trap light and the line can be observed undisturbed.

Recently, we have precisely determined the frequency response of the clock transition to the ambient thermal radiation. This lead to reduction of the fractional inaccuracy of PTB’s strontium lattice clock to few parts in 1017. Our result is also used by other groups in the world to improve the accuracy of their lattice clocks.

Currently, we are further improving the accuracy of our clock by detailed investigation and control of systematic frequency shifts.

By comparisons of the lattice clock with the primary clocks at PTB or other optical clocks, we can obtain better insight into e.g. questions regarding temporal variations of fundamental constants.


Head of Working Group Dr. Christian Lisdat
Phone: +49-531 592-4320
Fax: +49-531 592-694320
E-mail: Christian Lisdat