PTB claims 'most accurate' optical single-ion clock

11 Feb 2016

Optical atomic clocks are "more stable and more accurate" than current time-setting standard cesium clocks.

Atomic clock experts from the Physikalisch-Technische Bundesanstalt (“the National Metrology Institute of Germany”), based in Braunschweig, have constructed an optical single-ion clock which offers an accuracy which had only been predicted theoretically, so far.

As early as 1981, Hans Dehmelt, who was later to be awarded a Nobel prize, had developed the principles of how to use an ion kept in a high-frequency trap to build a clock which could attain the – then unbelievably low – relative measurement uncertainty in the range of 10^-18.

Now The PTB scientists are the first group to have reached the long-predicted accuracy using a single-ion clock. Their optical ytterbium clock has achieved a relative systematic measurement uncertainty of 3 x 10^–18. The results have been published in Physical Review Letters.

The definition and realization of the SI unit of time, the second, is currently based on cesium atomic clocks. Their “pendulum” consists of atoms, which are excited into resonance by microwave radiation (at 10¹⁰ Hz). The PTB team commented that a future redefinition of the SI second will likely be based on an optical atomic clock. These have a considerably higher excitation frequency (between 10¹⁰ to 10¹⁵ Hz), which makes them much more stable and more accurate than cesium clocks.

PTB’s announcement of the achievement stated, “The accuracy now achieved with the ytterbium clock is approximately a hundred times better than that of the best cesium clocks. To develop our clock, we have exploited particular physical properties of the Yb⁺ ion. This ion has two reference transitions that can be used for an optical clock.

One of these transitions is based on the excitation into the ion’s so-called "F state" which, due to its extremely long natural lifetime (approx. 6 years), provides exceptionally narrow resonance. In addition, due to the electronic structure of the F state, the shifts of the resonance frequency caused by electric and magnetic fields are exceptionally small. The other reference transition (into the so-called D_3/2 state) exhibits higher frequency shifts and is therefore used as a sensitive "sensor" to optimize and control the operating conditions.

Double procedure

The decisive factor for PTB’s last leap in accuracy was the combination of two measures: firstly, a special procedure was conceived for the excitation of the reference transition. With this procedure, the “light shift” of the resonance frequency caused by the exciting laser is measured separately. This information is then used to immunize the excitation of the reference transition against the light shift and its possible variation.

Secondly, the frequency shift caused by the thermal infrared radiation of the environment (which is relatively small for the F state of Yb⁺ anyway) was 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.

Another particular property of the F state of Yb⁺ is the strong dependence of the state energy on the value of the fine-structure constant (the elementary fundamental constant of electromagnetic interaction) and on the anisotropy effects in the interaction between electrons and certain potential forms of the so-called dark matter which plays an important part in the present cosmologic standard model. The PTB announcement concluded, “Comparisons between Yb⁺ clocks and other highly accurate optical clocks are currently probably the most promising way of verifying theories from this area of relatively new physics in the lab.