The most accurate optical single-ion clock worldwide

February 10, 2016
Measuring the influence of thermal ambient radiation on the frequency of the trapped ion: the "clock laser" (blue beam) excites the trapped ion (yellow) with a special pulse sequence. The resonance frequency of the ion is influenced by infrared radiation (here by an infrared laser, red beam). This can be measured by means of the clock laser. Credit: PTB

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 which had only been predicted theoretically so far. As early as 1981, Hans Dehmelt, who was to be awarded a Nobel prize later, had already 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 10-18.

Ever since, an increasing number of research groups worldwide have been trying to achieve this with (either based on single trapped ions or on many neutral atoms). The PTB scientists are the first to have reached the finishing line using a single-ion . Their optical ytterbium clock achieved a relative systematic measurement uncertainty of 3 10-18. The results have been published in the current issue of the scientific journal 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 (1010 Hz). It is regarded as certain that a future redefinition of the SI second will be based on an optical atomic clock. These have a considerably higher excitation frequency (1014 to 1015 Hz), which makes them much more stable and more 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 their clock, the researchers from PTB exploited particular physical properties of Yb+. This ion has two reference transitions which can be used for an optical clock. One of these transitions 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. In addition, due to the particular electronic structure of the F state, the shifts of the caused by electric and magnetic fields are exceptionally small. The other reference 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 factor for the 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 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 interac-tion) 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. Comparisons between Yb+ clocks and other highly accurate optical clocks are currently probably the most promising way of verifying theories from this area of "new physics" in the lab.

Explore further: Optical strontium atomic clock sets new stability record

More information: N. Huntemann et al. Single-Ion Atomic Clock with Systematic Uncertainty , Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.116.063001

Related Stories

Optical strontium atomic clock sets new stability record

February 10, 2016

Researchers from the Physikalisch-Technische Bundesanstalt (PTB) have thoroughly analyzed the noise processes in their optical lattice clock with neutral strontium atoms. This analysis proves that their optical atomic clock ...

New 'pendulum' for the ytterbium clock

March 9, 2012

The faster a clock ticks, the more precise it can be. Due to the fact that lightwaves vibrate faster than microwaves, optical clocks can be more precise than the caesium atomic clocks which presently determine time. The Physikalisch-Technische ...

The tick-tock of the optical clock

March 29, 2012

( -- UK's National Physical Laboratory time scientists have made an accurate measurement of the highly forbidden octupole transition frequency in an ytterbium ion, which could be used as the basis for the next ...

Optical Atomic Clock: A long look at the captured atoms

February 5, 2008

Optical clocks might become the atomic clocks of the future. Their "pendulum", i.e. the regular oscillation process which each clock needs, is an oscillation in the range of the visible light. As its frequency is higher than ...

New technique excites atoms and molecules using pulsed laser

November 22, 2012

The best method to obtain the most precise information on the inner structure of atoms and molecules is to excite them by means of resonant laser light. Unfortunately, just this laser light (above a certain intensity) can ...

Recommended for you

Researchers discover new rules for quasicrystals

October 25, 2016

Crystals are defined by their repeating, symmetrical patterns and long-range order. Unlike amorphous materials, in which atoms are randomly packed together, the atoms in a crystal are arranged in a predictable way. Quasicrystals ...

Making silicon-germanium core fibers a reality

October 25, 2016

Glass fibres do everything from connecting us to the internet to enabling keyhole surgery by delivering light through medical devices such as endoscopes. But as versatile as today's fiber optics are, scientists around the ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.