Optical clocks, like the strontium clock at Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, are already 10 times more precise and stable than the best primary cesium atomic clock and may soon become portable, too. PTB scientists already have a practical application in mind: determining geographical heights even more precisely than before. This is a view into the ultrahigh-vacuum chamber in which strontium atoms are cooled and stored. In the upper third of the window, the blue fluorescent light of a cloud of cold strontium atoms can be seen. (Image: Physikalisch-Technische Bundesanstalt)An optical clock is so exact because its “pendulum” swings so quickly. The microwave radiation that can bring about a spin change in each cesium electron in a cesium atomic clock ultimately defines the second. An optical atomic clock works with the still higher frequency of optical radiation, resulting in an even faster pendulum. As the movement of the atoms leads to very large frequency shifts through the Doppler effect, in the best of these clocks, the atoms are slowed down to a hundredth the speed of a pedestrian in a first preparation step with the aid of laser cooling. In a lattice clock, a further step then follows in which the atoms are held in potential wells. These are created through the intensive light field of a laser. Several tens of thousands of strontium atoms are trapped in this so-called optical lattice. The movement of the atoms is thus limited to the fraction of an optical wavelength, so that shifts through the Doppler effect can be ignored. A few hundred atoms that can disturb each other are trapped in each potential well. If the isotope strontium-87, a fermion, is used, two of these particles do not come close to each other at very low temperatures, due to the Pauli principle. That is why this isotope is used to construct optical clocks. However, because it can be cooled only in a relatively complicated method involving laser light and, moreover, has a natural abundance of only 7 percent, it is, in principle, not so well-suited for simple, transportable clocks or even for clocks suitable for outer space. The isotope strontium-88, which has more than 80 percent natural abundance and is easier to cool, is a boson. This means that, even at the lowest temperatures, many collisions between the atoms occur, leading to losses and a shift and broadening of the reference line. How strongly these collisions influence the accuracy of the clock was, however, not known previously. In an experiment at PTB, these influences have now been measured in detail for the first time. The results of the investigation have shown how the optical lattice has to be dimensioned and how many atoms may be stored in it to operate a very accurate lattice clock also with strontium-88. A clock is now being built on this basis, which is more compact and transportable than the previous lattice clocks. The gravitational red shift of the earth, amounting to a height difference of 10-16 per meter on its surface, is being discussed as a possible first use for the precise determination of the height over the geoid. So the clock could be used to improve, for example, gravitation maps. The paper appears in the current edition of the journal Physical Review Letters. For more information, visit: www.ptb.de