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World record for two optical clocks


Optical clocks are deemed the clocks of the future. In optical clocks, the atoms, which act as a “pendulum”, are resonantly excited by means of optical radiation. Compared to cesium atomic clocks – on which the SI base unit, the second, is currently based – their excitation frequency is much higher. Therefore, the resonance quality is much better. This implies considerably increased clock accuracy (i.e. lower deviation from the true frequency) and higher stability (i.e. shorter averaging period for one measurement). In both fields, PTB‘s optical atomic clocks are currently one step ahead.

With a relative systematic measurement uncertainty of only 3< · 10–18, PTB‘s single-ion ytterbium clock is approximately a hundred times more accurate than the best cesium clocks and is currently the world‘s most accurate single-ion clock. With this clock, a research group has, for the first time, attained an accuracy which had been predicted for an optical ion clock as early as 1981 based on theoretical considerations. The decisive aspect was the combination of two measures: firstly, a newly developed procedure which is able to immunize the excitation of the reference transition against the light shift and its possible variations. Secondly, the frequency shift induced by the thermal infrared radiation of the environment was determined with an uncertainty of only 3 % and corrected.

Contrary to ion clocks, a strontium clock uses laser cooling to slow a gas of neutral atoms down to temperatures near absolute zero. Then, an extremely narrow transition between long-lived eigenstates of the atoms is excited in order to stabilize the frequency of the excitation laser to that of the atoms. The simultaneous interrogation of numerous atoms leads to a particularly high signal-to-noise ratio and, thus, to higher stability. By means of a newly constructed resonator, whose frequency is among the most stable worldwide, a fractional instability in normal operation amounting to 1.6 · 10–161/2 was derived as a function of the averaging time τ in seconds. This is the best published value for an atomic clock so far.

Apart from testing the „big issues“ of fundamental physics, possible applications of highest-precision clocks arise in geodesy where they enable – together with fiber connections – direct measurement of height differences in the gravitational potential of the Earth.