Right on time: Strontium clock

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  • Published: Feb 15, 2016
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Right on time: Strontium clock


Noise in a strontium lattice clock as a function of the number of atoms. The predicted total noise at suppressed frequency noise of the interrogation laser (green line) is confirmed by experimental data (green circles). The quantum projection noise (blue line) already dominates with few atoms. Credit: PTB

An analysis by researchers in Germany of the processes that generate noise in their optical lattice clock demonstrates that this optical strontium atomic clock has the greatest stability of any such device thanks to a newly developed laser system.

Researchers - Ali Al-Masoudi, Sören Dörscher, Sebastian Häfner, Uwe Sterr, and Christian Lisdat - from the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany, suggest that the stability of this clock will allow high-precision measurements to be carried out quickly with fractional frequency uncertainty as low as a few parts in 1018.

Atomic clocks have been available to researchers for decades and incremental improvements have led to their use in defining the SI base unit of time, the second, and allowing experiments in physics to be carried out whether ever increasing accuracy. The validation of the PTB clock could usurp the current definition of the second, which is based on the interaction between microwave radiation and caesium atoms. This might then allow geodesy experiments to determine directly the gravitational potential of the Earth to be carried out as well as facilitating the search for a unified theory of the fundamental interactions by revealing variations in the fundamental constants, such as the fine-structure constant, and in verifying the existence of the gravitational waves predicted by Einstein in his General Theory of Relativity.


The accuracy and the stability of this more recent class of optical clock relies on the fact that the frequency of the optical radiation used is higher (by several orders of magnitude) than that of the microwave radiation used to drive the caesium atomic clocks. In the strontium clock, laser cooling is used to slow an atomic gas down to temperatures close to 0 Kelvin, absolute zero. 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 greater precision.

There is a drawback to this approach in that the atomic cloud must be freshly prepared after each interrogation and so there are interruptions to the observation of the laser frequency. The laser can serve as a "flywheel" to pre-stabilize an optical resonator which keeps the laser frequency stable over short periods of time. The scientists from PTB have therefore developed a resonator whose frequency is among the most stable worldwide: with a length of 480 millimetres and isolated thermally and mechanically from its environment, it reaches fractional frequency instability of 8 x 10-17.The clock reaches the quantum projection noise limit with as few as 130 atoms.


To analyze the clock's instability, the model derived from this was supplemented by the known influence of the laser frequency noise, and its prediction was experimentally verified by a self-comparison of the clock, the team explains. From this, the PTB researchers could derive a fractional instability under normal operating conditions of 1.6 x 10-16/τ1/2 as a function of the averaging time τ in seconds. This is the best published value for an atomic clock so far.

"We are working to improve further the accuracy and stability of the strontium clock to reach the fundamental limit," Al-Masoudi told spectroscopyNOW.

Related Links

Phys Rev A 2015, 92, 063814: "Noise and instability of an optical lattice clock"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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