Dr. Fabrizio Giorgetta (NIST)
Frequency-comb based open-path time and frequency transfer and its application to clock networks

The networking of optical clocks using time and frequency transfer is essential to realize their full potential for, e.g., the redefinition of the second, relativistic geodesy, and fundamental physics tests. I will discuss optical frequency comb based open-path time and frequency transfer, and recent improvements that reduced its detection threshold from 10’s of nW to 100’s of fW. I will present a proof-of-concept measurement across a 300 km long freespace link and a three-node clock network with optical iodine clocks connected across 14.5 km free-space links. I will discuss upcoming measurement campaigns.

Dr. Bruno Pelle (EXAIL)
First Accuracy Budget below  for a Cold-Atom-Based Commercial Microwave Clock
Luc Archambault[1], Floriane Sparma[2], Arnaud Landragin[2], Cédric Majek[1], Bruno Desruelle[1], Luca Lorini[2], and Bruno Pelle[1]
[1] Quantum Systems Division, Exail, Gradignan, France
[2] Laboratoire Temps-Espace, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Paris, France
Microwave clocks have reached their best performances in both short- and long-term frequency stabilities, as well as their best accuracy, with cold-atoms-based fountain clocks. We present a more compact commercial microwave clock that demonstrates a preliminary accuracy budget below 5x10^-15, with a turnkey product dedicated to continuous long-term operation. Long-term frequency stabilities of 10^-15 over more than a month have been demonstrated on the first generation of the MuClock [1], allowing to evaluate more finely its accuracy budget [2]. Among other investigated frequency shifts, the Ramsey pulling, the microwave phase transient and the phase gradient shifts will be discussed.Microwave clocks have reached their best performances in both short- and long-term frequency stabilities, as well as their best accuracy, with cold-atoms-based fountain clocks. We present a more compact commercial microwave clock that demonstrates a preliminary accuracy budget below , with a turnkey product dedicated to continuous long-term operation. Long-term frequency stabilities of  over more than a month have been demonstrated on the first generation of the MuClock [1], allowing to evaluate more finely its accuracy budget [2]. Among other investigated frequency shifts, the Ramsey pulling, the microwave phase transient and the phase gradient shifts will be discussed.
[1] B. Pelle, L. Archambault, B. Desruelle and A. Landragin, “Cold-atom-based Commercial Microwave Clocks at 1×10−15 Relative Instability Over More than One Month”, 2022 Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS), 2022.
​[2] F. Sparma, L. Archambault, B. Pelle, L. Lorini, A. Landragin, B. Desruelle, P. Rosenbusch, "Accuracy Assessment of a Commercial Cold-Atom Rb Clock: A Use Case Within the Qu-Test Project," 2024 European Frequency and Time Forum (EFTF), Neuchâtel, Switzerland, 2024.

Dr. Masao Takamoto (Quantum Metrology Laboratory, RIKEN)
Development of transportable optical lattice clocks and their applications
M. Takamoto[1], I. Ushijima[2], H. Katori[1,2]
[1] RIKEN, Saitama, Japan
[2] The University of Tokyo, Tokyo, Japan
 Atomic clocks based on optical transitions have achieved fractional uncertainty of  10^-18, two orders of magnitude better than caesium clocks, and discussions are currently underway to redefine the second using optical clocks. Such high-precision clocks are expected to have applications not only in frequency metrology but also in various fields such as relativistic geodesy and fundamental physics. In this presentation, we will present the development of transportable optical lattice clocks and their applications. This work was supported by the JST-Mirai Program "Space-time information platform with a cloud of optical lattice clocks" (JPMJMI18A1).

Dr. Jonas Keller (PTB)
Accurate and stable optical clocks based on trapped-ion Coulomb crystals
 
Trapped ions are ideally suited for precision spectroscopy and have enabled optical clocks with relative systematic uncertainties below 10^-18, but resolving frequencies at this level with a single ion currently requires multiple weeks of averaging. Parallel operation with multiple clock ions linearly reduces the required time to reach a given frequency resolution. In a common trapping potential, ions arrange into strongly coupled Coulomb crystals (CC), which allow for combining the strengths of different ion species. I will present considerations and techniques for operating a high-accuracy clock with mixed-species CC’s, based on our experience with an In^+ multi-ion clock, sympathetically cooled with Yb^+ ions [1, 2].
[1] Hausser et al., Phys. Rev. Lett. 134, 023201 (2025)
[2] Keller et al., Journal of Physics: Conference Series 2889, 012050 (2024)