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Optical strontium clock to become much more accurate

Especially interesting for
  • developers of optical atomic clocks
  • fundamental research

Optical clocks with neutral strontium atoms are being successfully developed worldwide at a number of institutes. From now on, it will probably be possible to determine their frequency more accurately, by one order of magnitude, since the influence of the most important uncertainty factor, namely the ambient temperature, has been measured at PTB for the first time. To date, its influence could only be derived theoretically.

View of the ultra-high vacuum chamber where strontium atoms are cooled and stored. There, we can see the parallel-plate capacitor in front of which a blueish cloud of some millions of strontium atoms is fluorescing (arrow). Prior to the excitation of the transition, the atoms are transported into the capacitor.

Optical clocks are deemed the clocks of the future. There are several reasons for this: they could allow the SI base unit the second to be realized even more accurately. Its definition would then no longer be based on the interaction between microwave radiation and caesium atoms, but on the interaction of optical radiation with strontium (or other) atoms or ions. But also beyond the definition issue, high-precision optical clocks are useful: for example in geodesy, where they can contribute to determining the gravitational potential of the Earth more accurately. In fundamental research, they can be used to improve the search for changes in fundamental constants such as the finestructure constant. The reason why optical clocks are so accurate is that optical radiation oscillates considerably faster than microwave radiation – the latter being currently used in caesium atomic clocks to “produce” the second. The faster the oscillation, the finer the scale, which is advantageous when it comes to the accuracy and stability of the clock. In an optical strontium clock, a cloud of neutral strontium atoms is cooled using laser radiation. In these atoms, a transition between two energy levels is excited by means of a laser. This is used to stabilize the frequency of this laser. Unfortunately, strontium atoms react relatively strongly to changes in the ambient temperature; their atomic levels are then shifted energetically, which causes the clock to become inaccurate. This is the highest contribution to the uncertainty of this clock, and PTB scientists have now succeeded in measuring it for the first time. This, however, required an auxiliary construction: the effect was considerably increased when using a static electric field rather than the alternating electromagnetic field of thermal radiation. To generate this electric field, a parallel-plate capacitor was conceived whose electric field is known with an accuracy of a few hundredths of per mille.

This capacitor was used to measure, for the first time, the influence of electromagnetic fields on the two decisive (for the clock) eigenstates in the strontium atom. In this way, the PTB scientists determined its uncertainty contribution to the total measurement uncertainty as being 5 ∙ 10–18. This is an accuracy increase by one order of magnitude – compared to the previously known value. And as just this influence had, to date, been the most restrictive influence on the total measurement uncertainty, one can expect the next frequency measurements to lie well below the previously attained 1 ∙ 10−16 with regard to their relative measurement uncertainty.

Scientific publication:

Th. Middelmann, St. Falke, Chr. Lisdat, U. Sterr: High accuracy correction of blackbody radiation shift in an optical lattice clock. Phys. Rev. Lett. 109, 263004 (2012)