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Collisions in optical lattice clocks under control

Optical atomic clocks have attained an accuracy and stability which is already superior to the primary atomic clocks today. However, these clocks are very elaborate laboratory set-ups which are not yet suitable for special applications, for example in space. In PTB it has now been shown that such a high accuracy (one order of magnitude better than with current fountain clocks) can also be attained by means of an optical lattice clock with a much simpler design.

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 (Sr) is to be seen.

Optical atomic clocks have attained an accuracy and stability which is already superior to the primary atomic clocks today. However, these clocks are very elaborate laboratory set-ups which are not yet suitable for special applications, for example in space. In PTB it has now been shown that such a high accuracy (one order of magnitude better than with current fountain clocks) can also be attained by means of an optical lattice clock with a much simpler design.

A few hundred atoms which 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. As this isotope can only be cooled relatively complicatedly with laser light and, moreover, only has a natural abundance of 7 %, it is not so well suited for simple, transportable clocks or even for clocks suitable for space.

The isotope strontium-88 with over 80 % natural abundance, which is also easier to cool, is, however, a boson. That means that even at the lowest temperatures many collisions between the atoms occur. They can lead to losses and to 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 more 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.

Contact at PTB:

Phone: +49-531-592-0