Frequency transfer through long-distance optical fiber links for relativistic geodesy
30 years ago, Vermeer and Bjerhammar suggested a new definition of the geoid, using the gravitational red-shift predicted by Einstein theory of General Relativity (Bjerhammar, 1985): “The relativistic geoid is the surface nearest to mean sea level on which precise clocks run with the same speed. The geoid is directly observable anywhere it is accessible.” Bjerhammar focused on clocks running continuously (mobile clocks), while early measurements of gravitational red-shift were reported for frequency comparisons using a frequency link between the clock locations (Pound and Snider, 1964; Vessot et al., 1980). Two problems, however, have delayed the scientific application of these ideas for geodesy:
- To achieve 1 m height resolution on earth, the clock rate (or frequency) needs to be realized with a relative fractional inaccuracy below 10^−16 at any location where the geoid is to be determined.
- The local clock rates (or frequencies) need to be transported and measured between the clock sites with significantly better stability compared to the performance of the clock. Ultimately aiming for 1 cm resolution, this requires clock rate intercomparison at or below the 10−18 level.
Now, laboratory clocks with frequency inaccuracy below 10−17 have been demonstrated, both for clocks based on single ions (Chou et al., 2010b) and clocks using neutral atoms (Bloom et al., 2013). In fact, a frequency instability near 10−18 (7 hours averaging) was reported (Hinkley et al., 2013). Chou et al. demonstrated the idea of relative height measurement with a resolution of 0.3 m, using two adjacent laboratory clocks (Chou et al., 2010b). The tremendous challenge of making clocks transportable is addressed in project A03, and further details are given there. This project A04 addresses the second point (ii): reliably transporting and measuring the clock rate, while aiming for fractional inaccuracy below 10−18, for distances of up to 1000 km, ideally reaching many locations simultaneously. We will achieve this by establishing phase-coherent optical fiber links between remote clocks, and measuring the clock frequencies, based on our previous experience (Grosche et al., 2008, 2009; Pape et al., 2010; Predehl et al., 2012; Matveev et al., 2013). Through close cooperation between projects A03 and A04, relativistic geodetic data spanning regional and later national distances with sub-meter height resolution will become available for the first time. Additional experiments via satellite microwave links, including, when available, the ground station of ACES (Seidel et al., 2007), complement this work. Specifically, within A04, the following four challenges to adapt and use optical links for geodesy will be addressed:
- refine signal amplification methods to provide continuous remote measurements (> 3 hours) over up to 1000 km distance
- assess link resolution and accuracy at the 10−18 level, including clock frequency measurements with frequency comb generators,
- connect many remote locations, thus realizing a geodesy clock network; and as a long-term vision
- transport and compare timing signals to enable time-scale (rather than just frequency) comparisons using mobile clocks.
We plan several field missions within this project to compare remote, stationary or portable clocks (project A03) via a fiber link to a local clock to determine the gravitational potential difference with a resolution at the cm-level. We will also support the characterization of improved GNSS (global navigation satellite system)-links, which are developed in project B03 for future gravity field measurements.
Scientists working on this project
Dr. Sebastian Koke
phone: +49 531 592-4344
Dr. Alexander Kuhl
phone: +49 531 592-4342
Dr. Thomas Waterholter
phone: +49 531 592-4345
Delva P., Lodewyck J., Bilicki S., Bookjans E., Vallet G., Le Targat R., Pottie P.-E., Guerlin C., Meynadier F., Le Poncin-Lafitte C., Lopez O., Amy-Klein A., Lee W.-K., Quintin N., Lisdat C., Al-Masoudi A., Dörscher S., Gerbing C., Grosche G., Kuhl A., Raupach S., Sterr U., Hill I. R., Hobson R., Bowden W., Kronjäger J., Marra G., Rolland A., Baynes F. N., Margolis H. S. and Gill P. (2017):
Test of Special Relativity Using a Fiber Network of Optical Clocks, Phys. Rev. Lett. American Physical Society, 118, 221102 (2017) more
Guéna J., Weyers S., Abgrall M., Grebing C., Gerginov V., Rosenbusch P., Bize S., Lipphardt B., Denker H., Quintin N., Raupach S. M. F., Nicolodi D., Stefani F., Chiodo N., Koke S., Kuhl A., Wiotte F., Meynadier F., Camisard E., Chardonnet C., Le Coq Y., Lours M., Santarelli G., Amy-Klein A., Le Targat R., Lopez O., Pottie P. E. and Grosche G. (2017):
First international comparison of fountain primary frequency standards via a long distance optical fiber link, Metrologia 54, 348 (2017) more
arXiv: 1703.02892v2 [physics.atom-ph]
Lisdat C., Grosche G., Quintin N., Shi C., Raupach S.M.F., Grebing C., Nicolodi D., Stefani F., Al-Masoudi A., Dörscher S., Häfner S., Robyr J.-L., Chiodo N., Bilicki S., Bookjans E., Koczwara A., Koke S., Kuhl A., Wiotte F., Meynadier F., Camisard E., Abgrall M., Lours M., Legero T., Schnatz H., Sterr U., Denker H., Chardonnet C., Le Coq Y., Santarelli G., Amy-Klein A., Le Targat R., Lodewyck J., Lopez O. and Pottie P.-E. (2016):
A clock network for geodesy and fundamental science, Nature communications 7 12443
Droste S., Grebing G., Leute J., Raupach S., Matveev A., Hänsch T., Bauch A., Holzwarth R. and Grosche G. (2015):
Characterization of a 450 km baseline GPS carrier-phase link using an optical fiber link, New Journal of Physics 17, 083044
Raupach S., Koczwara A. and Grosche G. (2015):
Brillouin amplification supports 1x10-20 uncertainty in optical frequency transfer over 1400 km of underground fiber, Physical Review A: 92, 2, 021801(R)
Non Peer-Reviewed Literature
Vogt S., Grotti J., Koller S., Häfner S., Herbers S., Al-Masoudi A., Grosche G., Denker H., Sterr U. and Lisdat C. (2016): Using a transportable optical clock for chronometric levelling, Geophysical Research Abstracts, Vol. 18: EGU 2016-16061, EGU General Assembly 2016, Vienna, Austria, 17–22 April 2016 more
Presentations, Talks and Posters
Grosche G. (2016): Frequency transfer through long-distance optical fiber links, 609. WE-Heraeus-Seminar: Relativistic Geodesy: Foundations and Applications, Bad Honnef, Germany, 14 - 18 March, 2016
Grosche G. (2016): Frequency and time transfer using optical fibers , 30th European Frequency and Time Forum, York, UK, 04 - 07 April, 2016
Grosche G. (2016): Interferometric optical fibre links for long-distance frequency transfer with 10-18 resolution, RTG: Models of Gravity, Colloquium, ZARM, Bremen, Germany, 06 July
Koke S., Grebing C., Kuhl A. and Grosche G. (2016): Validating frequency transfer via a stabilised fibre link for optical clock comparisons, 30th European Frequency and Time Forum, York, UK, 04 - 07 April, 2016
Kuhl A., Waterholter T., Froh J. and Grosche G. (2016): Amplification schemes and multiple frequency dissemination for the optical fiber link PTB-LUH, 609. WE-Heraeus-Seminar: Relativistic Geodesy: Foundations and Applications, Bad Honnef, Germany, 14 - 18 March, 2016
Kuhl A., Waterholter T., Froh J. and Grosche G. (2016): Comparison of Different Amplification Concepts for Multiple Point Frequency Dissemination, 30th European Frequency and Time Forum, York, UK, 04 - 07 April, 2016
Lisdat C., Koller S., Grotti J., Vogt S., Al-Masoudi A., Dörscher S., Herbers S., Häfner S. and Sterr U. (2016): Using a transportable optical clock for chronometric levelling, Fall Meeting, AGU, San Francisco, Calif., 12-16 Dec.
Koke S., Raupach R., Koczwara A. and Grosche G. (2015): Developing fiber link instrumentation for long-distance frequency transfer with 10-20 frequency uncertainty , 8th Symposium on Frequency Standards and Metrology 2015, Potsdam, Germany, 12 - 16 October, 2015
Schnatz H. and Grosche G. (2014): Towards international optical clock comparisons using optical fibers: current status and prospects, Third International VLBI Technology Workshop, Groningen/Dwingeloo, the Netherlands, 10 - 13 November, 2014
Schnatz H., Bolognini G., Calonico D., Dierikx E., Grosche G., Hedekvist P. O., Kuna A., Marra G., Merimaa M., Niessner A., Pottie P.E., Śliwczyński Ł., Slavik R., Smotlacha V. and Spahic F. (2014): NEAT-FT: The European Fiber Link Collaboration, EFTF 2014 (European Frequency and Time Forum), Neuchâtel, Switzerland, 23 - 26 June, 2014