Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Atomic clock comparison via data highways

In future, optical fibers could connect all optical atomic clocks within Europe – a milestone for various users of optical frequencies in research and industry
(Joint press release of Physikalisch-Technische Bundesanstalt, PTB, and the Max Planck Institute of Quantum Optics)

Optical atomic clocks measure time with unprecedented accuracy. However, it is the ability to compare clocks with one another that makes them applicable for high-precision tests in fundamental theory, from cosmology all the way to quantum physics. A clock comparison, i.e. a comparison of their optical frequencies, proved to be challenging so far as the few existing optical clocks around the world are not readily portable due to their complex nature.

Atomic clock comparison via data highways (artwork)
(Credit: MPQ + woogie-works Wien)

A team of researchers from the Physikalisch-TechnischeBundesanstalt (PTB) in Braunschweig and from the Laser Spectroscopy Division at the Max Planck Institute of Quantum Optics (MPQ) in Garching have now demonstrated an optical frequency transfer with high stability through a standard telecommunication optical fiber network (Science, April 27, 2012). The optical fiber connecting the two institutes was installed below ground and had a total length of 920 kilometres.

This demonstration enables the ability to compare optical clocks located far apart from each other and to transmit their stability to distant laboratories where the signals can be used for high precision experiments. At first, fundamental research will benefit from this, e.g. in the precise determination of natural constants, tests of the validity of Einstein’s theory of General Relativity or for predictions in quantum electrodynamics.

In an atomic clock the frequency that an atom emits when changing from one energy level to another defines the unit of time. The unit “second” is the interval occupied by 9,192,631,770 cycles of the microwave radiation corresponding to a transition of the cesium-133 isotope. The generation and dissemination of the second is statutory and in Germany the duty of the PTB. Optical atomic clocks use a frequency that is about 100,000 times higher compared to microwave clocks and thus slice time into much finer intervals. The latest generation of optical atomic clocks differs only in the 18th decimal place – that corresponds to one second in a time period of the age of the universe.

The researchers asked themselves the question of how well one can transfer optical frequencies over long distances. Well established techniques with the help of satellites reach a stability of 15 decimal places. While this is sufficient for microwave signals it is inadequate to exploit the full potential of optical atomic clocks. During the last years the MPQ/PTB researchers therefore investigated how to transmit an optical frequency through optical fibers. They were supported by the Cluster of Excellence QUEST at Leibniz Universität Hannover, the European Space Agency (ESA), the Deutsches Forschungsnetzwerk DFN as well as GasLine, a German consortium of gas distribution companies.

In the project described here the researchers feed light from a highly stable laser with a wavelength around 1.5 micrometers (near infrared) in the optical fiber connecting the laboratories at MPQ and PTB. The optical fiber is commonly used in telecommunication and has an attenuation minimum for light in the near infrared. However, to transmit a signal over a distance of almost 1,000 kilometers it has to be refreshed periodically. For this, novel optical amplifier units have been developed and installed along the entire path of the optical fiber.

Another problem to overcome is the falsification of the sent laser frequency due to mechanical, acoustic or thermal disturbances that originate from temperature fluctuations, traffic or construction work close to the optical fiber. New techniques allow for detection and compensation of these disturbances in such a way, that the entire 920 kilometers of optical fiber between Garching and Braunschweig is changing its optical length by less than one atomic diameter in one second. Hence, the delivered frequency of 194,353 Gigahertz at the remote laboratory differs by less than one ten thousandth from the fed frequency. To put the frequency transfer to the test, the researchers of the laser spectroscopy division used the signal from PTB’s primary cesium fountain clock for the spectroscopy of hydrogen at MPQ. By means of a frequency comb, this signal is compared to the optical phase of the transfer laser. They achieved a significantly higher accuracy as would have been possible with satellite-based frequency transfer.

Optical frequencies can now be disseminated with a quality that was only available locally at metrology institutes so far. The use of optical fiber infrastructure that the national research networks provide already today will in future allow for the connection of optical atomic clocks pan-European. In the same way as it is state-of-the-art that conventional alarm or station clocks receive the “correct time” from the PTB via the long-wave transmitter DCF77, the dissemination of an optical reference via optical fibers for the determination of wavelengths or frequencies of optical radiation will find wide application in research and industry.

Original publication:
K. Predehl, G. Grosche, S.M.F. Raupach, S. Droste, O. Terra, J. Alnis, Th. Legero, T.W. Hänsch, Th. Udem, R. Holzwarth, and H. Schnatz: A 920 km Optical Fiber Link for Frequency Metrology at the 19th Decimal Place. Science, April 27, 2012

Dr. Harald Schnatz, Department 4.3 Quantum Optics and Unit of Length, Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany, Phone: +49 531 592-4300, E-mail:

Dr. Gesine Grosche, Department 4.3 Quantum Optics and Unit of Length, Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany, Phone: +49 531 592 -4318, E-Mail:

Katharina Predehl, Max Planck Institute of Quantum Optics, Hans-Kopfermann-Straße 1,85748 Garching, Germany, Phone: +49 89 32905-295, E-mail:

Dr. Ronald Holzwarth, Max Planck Institute of Quantum Optics, Hans-Kopfermann-Straße 1, 85748 Garching, Germany, Phone: +49 89 32905-262, E-mail:

Erika Schow | PTB
Further information:

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: LaserTAB: More efficient and precise contacts thanks to human-robot collaboration

At the productronica trade fair in Munich this November, the Fraunhofer Institute for Laser Technology ILT will be presenting Laser-Based Tape-Automated Bonding, LaserTAB for short. The experts from Aachen will be demonstrating how new battery cells and power electronics can be micro-welded more efficiently and precisely than ever before thanks to new optics and robot support.

Fraunhofer ILT from Aachen relies on a clever combination of robotics and a laser scanner with new optics as well as process monitoring, which it has developed...

Im Focus: The pyrenoid is a carbon-fixing liquid droplet

Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.

A warming planet

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

All Focus news of the innovation-report >>>



Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

Latest News

Fraunhofer ISE Pushes World Record for Multicrystalline Silicon Solar Cells to 22.3 Percent

25.09.2017 | Power and Electrical Engineering

Usher syndrome: Gene therapy restores hearing and balance

25.09.2017 | Health and Medicine

An international team of physicists a coherent amplification effect in laser excited dielectrics

25.09.2017 | Physics and Astronomy

More VideoLinks >>>