ForschungTopical Groups
Optical Clock Networks

Optical Clock Networks

Establish fiber-based optical clock networks, improve optical clocks and frequency transfer techniques, exploit chronometric levelling for geodesy.


  • Optical clocks:
    • Bring the inaccuracy and instability stationary optical clocks below the level of 1 part in 1018
    • Bring today’s optical clocks from prototype status to more reliable, rugged, transportable and miniaturized devices
    • Investigate compound clocks
    • Improve transportable Sr clock
    • Test new Al+ logic clock
  • Frequency Transfer:
    • Consolidate inaccuracy and instability of interferometric fibre links in the 10-19to 10-20 regime
    • Investigate link non-reciprocality and relativistic effects
    • Investigate coherent free-space frequency transfer
  • CRC 1464 TerraQ:
    • Establish chronometric levelling with cm resolution as routine tool for geodesy
    • Transfer lab-based link performance to robust in-field systems
    • Identify locations and applications for chronometric levelling
  • Other:
    • Coordinate measurement campaigns and results
    • Investigate new network schemes


  • Grosche: Frequency transfer techniques
  • Lisdat: Stationary and transportable Sr optical lattice clocks
  • Schmidt: Transportable Al+ clocks
  • Müller: Relativistic geodesy
  • Lämmerzahl: Relativistic geodesy


Dörscher S, Huntemann N, Schwarz R, Lange R, Benkler E, Lipphardt B et al. Optical frequency ratio of a 171Yb+ single-ion clock and a 87Sr lattice clock. METROLOGIA. 2021 Feb;58(1). 015005. doi.org/10.1088/1681-7575/abc86f

Micke P, Leopold T, King SA, Benkler E, Spieß LJ, Schmöger L et al. Coherent laser spectroscopy of highly charged ions using quantum logic. NATURE. 2020 Jan 29;578:60-65. doi.org/10.1038/s41586-020-1959-8

Porsev SG, Safronova UI, Safronova MS, Schmidt PO, Bondarev AI, Kozlov MG et al. Optical clocks based on the Cf15+ and Cf17+ ions. Physical Review A. 2020 Jul;102(1). 012802. doi.org/10.1103/PhysRevA.102.012802

Schulte M, Lisdat C, Schmidt PO, Sterr U, Hammerer K. Prospects and challenges for squeezing-enhanced optical atomic clocks. Nature Communications. 2020 Nov 24;11(1). 5955. doi.org/10.1038/s41467-020-19403-7

Wu H, Müller J, Lämmerzahl C. Clock networks for height system unification: A simulation study. Geophysical journal international. 2018 Nov 28;216(3):1594-1607. doi.org/10.1093/gji/ggy508

Involved QF Members
Members Institution Relevant Expertise
Gesine Grosche, LeaderPTBFree-Space Frequency Transfer; Frequency Transfer Techniques
Alexander KuhlPTBFree-Space Frequency Transfer
Thomas WaterholterPTBFrequency Transfer Techniques
Christian LisdatPTBSr Optical Lattice Clock
Chetan VishwakarmaPTBSr Optical Lattice Clock
Jürgen MüllerLUHRelativistic Geodesy; LLR Relativity Test; Application of Quantum Gravimetry
Hu WuLUHRelativistic Geodesy
Piet O. SchmidtPTB / LUHQuantum Logic Spectroscopy of Highly Charged Ions; Transportable Al+ Clock
Stephan HannigPTBTransportable Al+ Clock
Claus LämmerzahlZARMQuantum Sensors in Free Fall; Relativistic Geodesy; Quantum Objects in Gravity
Steffen SauerLUHUltra-stable cavities for optical clocks
Tara LiebischPTBClock Network Schemes
Ernst M. RaselLUHQuantum Gravimeters; Atom-Chip Based Gravimeters and Inertial Sensors
Stefanie KrokerPTB / LUHComplex Coupled High Index Waveguide Arrays; Photonic Nanomaterials in the Strong Optomechanical Coupling Regime
Dennis PhilippZARMGeneral Relativity, relativistic geodesy
Eva HackmannZARMGeneral Relativity, relativistic geodesy
Steffen SchönIfEGNSS Frequency Transfer
Sebastian KokePTBFrequency Transfer Techniques
Jingxian JiPTBFree-Space Frequency Transfer
Jaffar KadumPTBFrequency Transfer, Fibre Brillouin Amplifiers