Atomic clock experts from the Physikalisch-Technische Bundesanstalt (PTB) are the first research group in the world to have built an optical single-ion clock which attains an accuracy that is 100 times better than predicted theoretically so far.
Eve3r since Hans Dehmelt in 1981, who was to be awarded a Nobel prize later, had developed the basic notions of how to use an ion kept in a high-frequency trap to build a clock which could attain the — then unbelievably low — relative measure-ment uncertainty in the range of 1E-18, several research groups worldwide have been trying to achieve this with optical atomic clocks either based on single trapped ions or on many neutral atoms.
Breaking the impasse, the PTB scientists have reached the finishing line using a single-ion clock. Their optical ytterbium clock achieved a relative systematic measurement uncertainty of 3 E-18, said their paper published in the scientific journal Physical Review Letters.
The realization of the SI unit of time, the second, is currently based on cesium atomic clocks and their “pendulum” consists of atoms which are excited into resonance by microwave radiation (1E10 Hz). It is expected that a future redefinition of the SI second will be based on an optical atomic clock. These have a considerably higher excitation frequency (1E14 to 1E15 Hz), which makes them more stable and accurate than cesium clocks.
The accuracy now achieved with the ytterbium clock is approximately a hundred times better than that of the best cesium clocks. To develop it, PTB researchers exploited particular physical properties of Yb+. This ion has two reference transitions –one is based on the excitation into the so-called “F state” which, due to its extremely long natural lifetime (approx. 6 years), provides exceptionally narrow resonance.
The second transition (into the D3/2 state) exhibits higher frequency shifts and is therefore used as a sensitive “sensor” to optimize and control the operating conditions. Another advantage is that the wavelengths of the lasers required to prepare and excite Yb+ are in a range in which reliable and affordable semiconductor lasers can be used.
The decisive leap in accuracy was the combination of — a special procedure was conceived for the excitation of the reference transition and the frequency shift caused by the thermal infrared radiation of the environment, determined with a measurement uncertainty of only 3 %. For this purpose, the frequency shift caused by laser light and its intensity distribution at the ion’s location were measured at four different wavelengths in the infrared range.