In an advancement that will simplify the comparison of newly developed optical clocks, a team of researchers from Physikalisch-Technische Bundesanstalt (PTB) has succeeded in transferring ultrastable frequency across a 480-km-long optical fiber link. The PTB researchers have employed so-called fiber Brillouin amplification, which is closely related to stimulated Brillouin scattering. They injected pump light with a well-defined frequency into the far end of the fiber so that the pump light travels in the direction opposite to the signal light, generating sound waves (acoustic phonons) in the glass fiber. The sound waves in turn scatter the pump light, enabling the existing signal photons to stimulate the emission of many more signal photons. Thus a photon avalanche is created, which is kept going by the sound waves and brings the frequency information to the remote end of the optical fiber with extremely small losses and very high precision. Not only data but also extremely precise optical frequencies can be transmitted across optical fiber links. (Image: GasLINE) The PTB researchers demonstrated this technique on a 480-km optical fiber link. The relative measurement uncertainty they achieved is equivalent to a deviation of one second in 16 billion years. Now they plan to span even larger distances. The new method simplifies the comparison of newly developed optical clocks, which possess such high frequency stability that traditional methods for frequency and time comparison via satellite are no longer sufficient. The technique is likely to have applications in other areas where precise synchronization is needed, for example in radio astronomy. Experts in geodesy have already approached the PTB researchers with suggestions for joint projects. The PTB physicists Harald Schnatz and Gesine Grosche are internationally leading experts in the precise measurement and transmission of frequencies via optical fibers. They use the optical frequency of the light with some 195 • 1012 cycles per second as the information. A first application of this new method was the remote measurement, conducted last year, of the so-called optical clock transition in a magnesium clock at Leibniz University of Hannover. The scientists determined the characteristic frequency with which very cold magnesium atoms can be excited to a particular long-lived state, by a measurement from PTB via 73-km optical fiber. It is important to measure such frequencies accurately, as they can in principle be used to “generate” seconds. “For such measurements, there are femtosecond frequency comb generators at both ends, which produce a fixed phase relationship between the transmitted light and the frequency standards on site,” said Schnatz. The frequency standards on site are the new magnesium clock in Hannover and an optical clock at PTB. The different frequencies of the two are synchronized with the aid of femtosecond frequency comb generators, which can be compared to a gear mechanism. “At first we were astonished how well this complete system works,” he added. Now the researchers wanted to bridge larger distances and build a connection for joint experiments to the Max-Planck Institute for Quantum Optics (MPQ) in Garching — a link of 900-km fiber, which attenuates the light by the almost inconceivable factor of 1020 if it is not amplified. Moreover, the fiber has to be passed through twice, because it is part of a huge interferometer; this is how the optical length of the entire link, which otherwise varies due to temperature fluctuations, is stabilized. Here, conventional amplification techniques reach their limits. “Our PhD student, Osama Terra, hit on the brilliant idea of using the Brillouin amplification in the fiber itself,” said Grosche. “This gives us several advantages at the same time: First, even very weak signals are still amplified; the signal power is multiplied by a factor of up to one million. Thus, we need considerably fewer amplifier stations. Moreover, it is possible to selectively amplify very narrow-band light signals.” This is very advantageous for the testing of the narrow-band clock transitions of optical clocks. The group immediately tested this concept on a deployed underground fiber link in cooperation with the Deutsches Forschungsnetz (DFN) (German National Research and Education Network) and the GasLINE company, which together operate a German-wide fiber network. With only one intermediate amplification station, the ultrastable frequency was transmitted over a record 480-km-long fiber link — with a relative transmission uncertainty of only two parts in 1018, which is equivalent to a deviation of about one second in 16 billion years. As a result, even a connection with the French partner institute of PTB in Paris now looks realistic — with a vision of working together on the best optical clocks in the future. At the technical conference, European Frequency and Time Forum (EFTF), the results also received recognition: Osama Terra won the Student Award of the EFTF for his contribution in the field of “Timekeeping, Time and Frequency Transfer.” The results have been submitted for publication and are available on the “arXiv” preprint server. Currently, the three researchers and their colleagues at MPQ Garching are continuing to work frenetically on establishing a connection between their institutes. They want to deliver the ultrastable reference frequency of PTB to the laboratory of the working group of professor Theodor Hänsch, where elementary properties of the hydrogen atom are measured spectroscopically with very high accuracy. For the future, Grosche and Schnatz are seeking to strengthen their team with postdocs and doctoral candidates. Prospective candidates can apply online through mid-July with the keyword “Faserlink” at www.halostar.de. For more information, visit: www.ptb.de