A new approach transporting frequencies via commercially available fiber makes it possible to use optical atomic clocks to precisely measure height differences between locations thousands of kilometers apart, a development that could have implications for quantum mechanics, geodesy and radio astronomy. Because the tick rate of a clock is influenced by gravity, time runs slightly faster on clocks depending on their distance from Earth’s surface — an effect called gravitational red shift. For a height difference of one meter, the rate (frequency) of a clock changes by one part in 1016. The new method is sensitive enough to measure frequency deviations caused by height differences of just a few centimeters. The Earth represented as a geoid; i.e., when the actual distribution of the gravitational potential is shown (a geoid is the surface of an ideal global ocean in the absence of tides and currents, shaped only by gravity). Here, the heights of the “bumps,” which are caused by the gravitational field of the Earth, have been determined by the Goce satellite. Although the data are precise as far as the respective height of the bumps, the lateral resolution amounts to a few kilometers. In the future, frequency comparisons between optical atomic clocks can be used to determine the height of the smaller structures and places on Earth. Now it will be possible to transport the frequency over a distance of approximately 2000 km with a resolution that corresponds to a height difference of only 4 mm once the remote clocks can support such accuracy. Courtesy of the European Space Agency. “But how can I measure the height difference, i.e., this frequency difference, if the two clocks are not standing side by side?” said Gesine Grosche, a physicist at PTB. “That is to say, how can I establish the connection to a second clock which is standing where a height must be measured with such accuracy?” Researchers from the Max Planck Institute of Quantum Optics (MPQ) in Garching and the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig used commercial optical fibers and a sophisticated amplifier technique to transport the frequency of one atomic clock to another over a distance of 2000 km. The frequencies at both ends of the fiber link were compared using a highly sensitive interferometry method and found to be accurate to 19 digits. The group first realized a frequency comparison over the 920 km between the MPQ and PTB last year, and have since doubled this range while achieving even better stabilities. According to Stefan Droste of MPQ, the total measurement uncertainty is just four parts in 1019, allowing a height difference of 4 mm between clocks to be resolved within 100 seconds of measurement time once the clocks support such accuracy. This method could help geodesists in standardizing sea-level measurements, possibly surpassing the accuracy of GPS technologies, the researchers say. Since different countries use different points of reference in determining sea level, measurements worldwide are inconsistent. Geodesists potentially could re-compute sea level on the basis of the Earth’s gravitational force to determine the so-called geoid of the Earth to within a few centimeters. Researchers from the MPQ and PTB as well as French researchers have also used this method of frequency transport via fiber to carry out spectroscopic investigations on hydrogen, which are important for understanding quantum mechanics. The work on optical-frequency transfer appeared in Physical Review Letters (10.1103/PhysRevLett.111.110801).