Schematic
representation: 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. (Fig.: PTB)
(February 10, 2016) Scientists
from the Physikalisch-Technische Bundesanstalt (PTB) reduce the measurement
uncertainty of their ytterbium clock down to 3 ∙ 10–18
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 measurement 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 ∙ 10–18.
The results have been published in the current issue of the scientific journal
"Physical Review Letters".
Radio-frequency
trap of PTB's optical ytterbium single-ion clock. (Photo: PTB)
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.