PTB News 1/2006 (138 kB) PTB News 1/2006, English edition, Issue May 2006With a single Ytterbium ion trapped in an ion trap and a femtosecond comb generator, PTB has made a great step towards the realisation of an optical atomic clock. The frequency of an optical Yb+ transition of (688 358 979 309 307.7 ± 2.2) Hz was compared over a period of several days with the frequency of a primary Cs fountain clock
For more than half a century, caesium clocks have successfully defended their reputation as the best clocks in the world. It is difficult to enhance their accuracy any further. The most promising candidates for even more accurate and, at the same time, stable time standards are the so-called optical clocks whose transition frequencies lie in the range of visible light. Due to a frequency which is about 75,000 times higher than the Cs frequency, optical clocks can subdivide time intervals much more accurately.
In order to exploit the advantage of the higher frequency, an optical "clockwork" is needed which is able to generate the "second" from the optical frequency in a stable manner over a long period of time. Nowadays, femtosecond comb generators on the basis of ultra-short pulse lasers serve as clockworks. These lasers link the microwave region (108 Hz) with the optical spectral region (1014 Hz) via the pulse repetition rate. Similar to a ruler, the modes of a frequency comb represent an absolute measure that extends up to the optical frequencies.
PTB has now advanced a great step towards an optical atomic clock. The frequency of an optical frequency standard on the basis of a single Yb+-ion in a Paul trap was compared over several days with that of the Cs fountain clock CSF1 and of a short-time stable hydrogen maser.
These measurements lay the foundation for the operation of an optical clock with a surpassing accuracy. They show that, nowadays, optical frequency standards with trapped ions achieve an uncertainty which is comparable to that of the most accurate caesium clock. Due to a short-term stability which is better by two orders of magnitude, for instance relative frequency variations of 1 · 10–17 of a stabilised fiber laser could be detected within a measuring period of only one hour. Even with the worldwide best Cs clock, it would take an averaging period of many months.
Thus, the time is ripe to prepare for secondary realisations of the second, which, later, could possibly lead to a new definition of the second.