My watch list


The most accurate optical single-ion clock worldwide


Radio-frequency trap of PTB's optical ytterbium single-ion clock


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.

12-Feb-2016: 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 optical atomic clocks (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 clock. Their optical ytterbium clock achieved a relative systematic measurement uncertainty of 3 x 10-18.

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 resonance frequency 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 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. 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.

Original publication:
N. Huntemann, C. Sanner, B. Lipphardt, Chr. Tamm, E. Peik; "Single ion atomic clock with 3 E-18 uncertainty"; Phys. Rev. Lett.; 2016

Facts, background information, dossiers

  • measurement uncertainty
  • cesium
  • ytterbium
  • atomic clocks
  • infrared radiation
  • Physikalisch-Techni…

More about Physikalisch-Technische Bundesanstalt

  • News

    Don't Give the Slightest Chance to Toxic Elements in Medicinal Products

    Lead, cadmium, mercury, and arsenic do not belong in medicinal products. International requirements for the quality of medicines have therefore become more stringent and introduced new control requirements. Hence, it makes sense to use high-accuracy reference solutions with defined contents ... more

    Measurement of the dynamic mechanical properties of viscous materials

    In microsystems metallic components are increasingly being replaced by those from low-cost polymers. For the thickness measurement of polymers, there is now the DIN standard 32567 available, which describes both, optical and tactile surface measuring methods for the precise measurement of t ... more

    A deep look into a single molecule

    A German research group has demonstrated the first implementation of a non-destructive state detection technique for molecular ions. Piet Schmidt and his colleagues from the QUEST-Institute at the Physikalisch-Technische Bundesanstalt (PTB) observed changes in the rotational state of a trap ... more

  • q&more articles

    Natural constants take center stage

    The 20th of May 2019 is a special day. From that day on, the old definitions of what a kilogram, a mole, an ampere and a kelvin represent, are history. The future of the International System of Units will see natural constants play the star roles. more

    How the Avogadro constant was measured for the last time

    Since the 20th of May 2019, the mass unit kilogram is no longer defined by the international kilogram prototype but by the numerical value of the Planck constant, the most important fundamental constant in quantum physics. For this definition, the Planck constant needed to be measured with ... more

    The Best Measurement

    It is now more than 30 years since attempts started to find a way of defining the kilogram based on an atomic constant or a fundamental physical constant. Since then, two methods have advanced sufficiently far to make it likely that there will be a new definition within the coming years, na ... more

  • Authors

    Dr. Jens Simon

    Jens Simon, born in 1962, has followed two academic paths as he moved between Braunschweig, Jülich and Cologne. One path led to a doctorate at the Technical University of Braunschweig in German linguistics on the language of the author Arno Schmidt. The other path led to a doctorate at the ... more

    Prof. Dr. Joachim H. Ullrich

    Joachim Ullrich, born in 1956, studied geophysics and physics at the University of Frankfurt where, after graduating in 1983, he also completed his doctorate in 1987 and qualified as a professor in 1994. From 1989 to 1997 he worked as a research assistant at the Gesellschaft für Schwerionen ... more

    Dr. Horst Bettin

    Horst Bettin, born in 1955, studied physics at the Technical University of Braunschweig where he completed his PhD at the Institute for Semiconductor Physics and Optics. In 1990 he joined the Physikalisch-Technische Bundesanstalt and focused his research mainly on density measurements. He h ... more

q&more – the networking platform for quality excellence in lab and process

The q&more concept is to increase the visibility of recent research and innovative solutions, and support the exchange of knowledge. In the broad spectrum of subjects covered, the focus is on achieving maximum quality in highly innovative sectors. As a modern knowledge platform, q&more offers market participants one-of-a-kind networking opportunities. Cutting-edge research is presented by authors of international repute. Attractively presented in a high-quality context, and published in German and English, the original articles introduce new concepts and highlight unconventional solution strategies.

> more about q&more

q&more is supported by:


Your browser is not current. Microsoft Internet Explorer 6.0 does not support some functions on Chemie.DE