Optical strontium clock to refine accuracy of second

The experimental results, which have been published in the latest issue of Physical Review Letters, is expected to be of interest to scientists working in geodesy (measurement of 3D forces around the Earth) and in fundamental physical research – such as whether fundamental constants are really constant.

Optical clocks are deemed “the clocks of the future”, say the PTB team of Christian Lisdat. This is because they could allow the SI base unit the second – already the most accurate of all SI base units – to be realized even more accurately. Its definition would then no longer be based on the interaction of microwave radiation with caesium atoms, but on the interaction of optical radiation with strontium atoms or ions.

But even before the latest work on a new definition of the second, optical clocks have proved their usefulness, such as in geodesy where they can help determine the Earth's “geoid” (exact position of sea level) more accurately than ever. Furthermore, they provide fundamental physicists with the long-awaited instrument to detect possible changes in the fundamental constants, such as “alpha”, the fine-structure coupling constant, which characterizes the strength of electromagnetic interaction.

Optical advantage

The reason why optical clocks are so accurate is that optical radiation oscillates extremely fast; considerably faster than microwave radiation, which is currently used in caesium atomic clocks to create the second. The faster the "pendulum" (the oscillating system) of a clock, the finer the time can, in principle, be broken down and, thus, the more stable and accurate the clock becomes. In an optical strontium clock, a cloud of neutral strontium atoms is cooled down in two steps by means of laser radiation until the atoms finally exhibit a speed of only a few centimeters per second.

A so-called "optical lattice" ensures that the atoms are trapped and can virtually no longer move. 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. To this end, they, however, needed an auxiliary system. In order to carry out measurements with the required accuracy, they considerably amplified the effect by using a static electric field instead of the alternating electromagnetic field of blackbody radiation.

They constructed a special parallel-plate capacitor whose electric field is known with a few-ten-ppm accuracy. For this purpose, the distance between the two plates, which amounted to 0.5 cm, may only vary by a few 100 nm over its length of 7 cm; the same applies to the accuracy of the distance.

Using the parallel-plate capacitor, the PTB scientists measured the influence of electromagnetic fields on the two decisive (for the clock) eigenstates in the strontium atom. They determined its uncertainty contribution to the total measurement uncertainty to 5 x 10^-18. Because just this influence had, to date, been the most restrictive influence on the total measurement uncertainty, one can expect the next frequency measurements of the clock as a whole to be well below the previously attained 1 x 10^-16.

About the Author

Matthew Peach is a contributing editor to optics.org