Researchers develop transportable optical atomic clock

January 23, 2018, Physikalisch-Technische Bundesanstalt
The laser setups of the optical atomic clock being developed at the QUEST Institute of PTB. Credit: PTB

Atomic clocks are no longer based on a microwave transition in cesium, instead operating with other atoms that are excited using optical frequencies. Some of these new clocks are portable. At its QUEST Institute, PTB is currently developing a transportable optical aluminum clock in order to measure physical phenomena outside a laboratory. A prerequisite for this is that the required lasers are able to endure transportation to other locations. PTB physicists have therefore developed a frequency-doubling unit that will continue to operate when it has been shaken at three times the Earth's gravitational acceleration. The results have been published in the current issue of the Review of Scientific Instruments.

It was Einstein who determined that two clocks located at two different positions in the gravitational field of the Earth operate at different speeds. What initially sounds bizarre has quite practical effects: Two optical with an extremely small relative measurement uncertainty of 10-18 can measure the difference in height between arbitrary points on the Earth at an accuracy of just one centimeter. This so-called chronometric levelling represents an important application of clocks in geodesy. One of the prerequisites for this is that the of the two clocks can be compared.

PTB is currently developing several types of atomic clock that can each be transported in a trailer or in a container. Their operation outside a protected laboratory, however, involves many challenges: The ambient temperature, for example, is much less stable. Furthermore, significant shocks may occur during transportation. This is why optical structures that have worked perfectly well in the laboratory may initially be unusable at the destination. They must painstakingly be readjusted—which leads to a loss of valuable research time.

This problem concerns the transportable aluminum clock being developed at the QUEST Institute. This clock requires two UV lasers at 267 nm. For this wavelength, researchers developed a long-wave infrared laser that can be frequency-doubled twice in succession. During this process, the is coupled into a closed ring of four mirrors so that a high optical power is circulating within the ring. A non-linear crystal placed in this ring transforms the circulating light into light of half the wavelength.

Due to the dichroic coating of the mirror, the circulating light passes out of the resonator and is then used for reading the clock. The QUEST Institute has developed a design for this so-called frequency-doubling cavity, which is based on a monolithic, highly stable frame onto which all mirrors and the crystal are mounted. This sealed,setup is gas-tight to the outside in order to protect the crystal, which is highly sensitive even to the slightest contamination.

The developers of the cavity were able to demonstrate on a prototype that it also doubles the while it is exposed to accelerations of 1 g. Furthermore, they demonstrated that the frequency doubling efficiency is not impaired after being subjected to accelerations of up to 3 g for 30 minutes. This corresponds to five times the value stated in Standard ISO 13355:2016 about road transportation on trucks. The cavity is, however, not only mechanically robust, but it is just as efficient as comparable systems that have been developed by research groups of other institutes. Moreover, 130 hours of uninterrupted continuous operation was demonstrated.

In view of these properties, the QUEST Institute has made several of these doubling cavities for different wavelengths (not only for UV) which became integral components of various quantum-optical experiments, with the aim of providing these experiments reliably with laser light. Moreover, a German optomechanics company has licensed the design in order to use it as a basis for a commercial product.

Explore further: NIST's next-generation atomic clocks may support official timekeeping

More information: S. Hannig et al, A highly stable monolithic enhancement cavity for second harmonic generation in the ultraviolet, Review of Scientific Instruments (2018). DOI: 10.1063/1.5005515

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 ...

Converting optical frequencies with 10^(-21) uncertainty

October 21, 2016

Frequency synthesizers from audio frequency to the microwave region have been widely used in daily life, high technology and scientific research. Those frequency synthesizers can output a signal with frequency related to ...

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 ...

Recommended for you

Zirconium isotope a master at neutron capture

January 17, 2019

The probability that a nucleus will absorb a neutron is important to many areas of nuclear science, including the production of elements in the cosmos, reactor performance, nuclear medicine and defense applications.

Understanding insulators with conducting edges

January 16, 2019

Insulators that are conducting at their edges hold promise for interesting technological applications. However, until now their characteristics have not been fully understood. Physicists at Goethe University have now modelled ...


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.