Nils Huntemann

Optical clocks have achieved remarkable estimated fractional frequency uncertainties reaching the 10 − 18 level and below, enabling applications in fundamental physics, general relativity, and geodesy. However, the challenge of verifying the international consistency of optical clocks remains critical as efforts intensify toward redefining the SI second based on an optical transition or transitions. We report on a two-month international clock comparison campaign involving seven optical clocks in four national metrology institutes (INRIM, LNE-OP, NPL, and PTB) connected via the optical fiber network established in Europe. The campaign resulted in optical frequency ratios with uncertainties ranging from 7.7 × 10 − 18 to 6.1 × 10 − 17 . Among the results, the Yb + 171 ( E 3 ) clocks at NPL and PTB demonstrated agreement within an uncertainty of 7.7 × 10 − 18 , marking the first international verification of two independently developed optical clocks below one part in 10 17 . The operation of the Hg 199 clock at LNE-OP (formerly LNE-SYRTE) resulted in frequency ratios with improved uncertainties with Yb + 171 ( E 3 ) , Yb 171 , and Sr 87 optical clocks. These results provide input for the redefinition of the second and underscore how fiber-linked clock networks can advance metrology and scientific applications.

This corrects the article DOI: 10.1103/PhysRevLett.127.213001.

Compared to other stable isotopes of Yb + , Yb + 173 has a richer hyperfine structure, which leads to more favorable clock transitions, spectroscopic techniques for probing new physics, and more sophisticated quantum computing architectures. However, to date, its electronic spectrum remains poorly characterized. Here, we report on efficient laser cooling, state preparation, and detection of a single trapped Yb 173 + ion. The previously unobserved S 2 1 / 2 → D 2 3 / 2 electric quadrupole transition at 436 nm is coherently excited, and the isotope shift between Yb 171 + on this transition is determined with an uncertainty of 1.4 Hz. Using microwave spectroscopy, we resolve the hyperfine structure (HFS) of the D 2 3 / 2 state with a relative uncertainty below 10 − 8 . From the HFS measurement data, we infer for Yb 173 a nuclear magnetic octupole moment Ω = − 0.062 ( 8 ) ( b × μ N ) with uncertainty reduced by more than 2 orders of magnitude compared to previous studies. The data also allow us to determine hyperfine anomalies for the S 2 1 / 2 and D 2 3 / 2 states.

We show that coupling of ultralight dark matter (UDM) to quarks and gluons would lead to an oscillation of the nuclear charge radius for both the quantum chromodynamic (QCD) axion and scalar dark matter, an effect which is of particular importance for heavy elements. Consequently, the resulting oscillation of electronic energy levels could be resolved with optical atomic clocks, and their comparisons can be used to investigate UDM nuclear couplings, which were previously only accessible with other platforms. We demonstrate this idea using the S 2 1 / 2 ( F = 0 ) ↔ F 2 7 / 2 ( F = 3 ) electric octupole and S 2 1 / 2 ( F = 0 ) ↔ D 2 3 / 2 ( F = 2 ) electric quadrupole transitions in Yb + 171 . Based on the derived sensitivity coefficients for these two transitions and a long-term comparison of their frequencies using a single trapped Yb + 171 ion, we find bounds on the scalar UDM-nuclear couplings and the QCD axion decay constant. These results are at a similar level compared to the tightest spectroscopic limits, and future investigations, also with other optical clocks, promise significant improvements.

Ion traps are used for a wide range of applications from metrology to quantum simulations and quantum information processing. Microfabricated chip-based 3D ion traps are scalable to store many ions for the realization of a large number of qubits, provide deep trapping potentials compared to surface traps, and very good shielding from external electric fields. In this work, we give an overview of our recent developments on chip-based 3D ion traps. Different types of chip materials, the integration of electronic filter components on-chip and compact electrical connections in vacuum are discussed. Further, based on finite element method (FEM) simulations, we discuss how integrating micro-optics in 3D ion traps is possible without disturbing the trapped ions.

We measured the g -factor of the excited-state P 1 3 in Ca 14 + ion to be g = 1.499032 ( 6 ) with a relative uncertainty of 4 × 10 − 6 . The magnetic field magnitude is derived from the Zeeman splitting of a Be + ion, cotrapped in the same linear Paul trap as the highly charged Ca 14 + ion. Furthermore, we experimentally determined the second-order Zeeman coefficient C 2 of the P 0 3 − P 1 3 clock transition. For the m J = 0 → m J ′ = 0 transition, we obtained C 2 = 0.39 ± 0.04 Hz mT − 2 , which is to our knowledge the smallest reported for any atomic transition to date. This confirms the predicted low sensitivity of highly charged ions to higher-order Zeeman effects, making them ideal candidates for high-precision optical clocks. Comparison of the experimental results with our state-of-the art electronic structure calculations shows good agreement and demonstrates the significance of the frequency-dependent Breit contribution, negative energy states, and QED effects on magnetic moments.

Nonlinearities in King plots (KP) of isotope shifts (IS) can reveal the existence of beyond-standard-model (BSM) interactions that couple electrons and neutrons. However, it is crucial to distinguish higher-order standard model (SM) effects from BSM physics. We measure the IS of the transitions P 3 0 → P 3 1 in Ca 14 + and S 2 1 / 2 → D 5 / 2 2 in Ca + with sub-Hz precision as well as the nuclear mass ratios with relative uncertainties below 4 × 10 − 11 for the five stable, even isotopes of calcium ( Ca 40 , 42 , 44 , 46 , 48 ). Combined, these measurements yield a calcium KP nonlinearity with a significance of ∼ 10 3 σ . Precision calculations show that the nonlinearity cannot be fully accounted for by the expected largest higher-order SM effect, the second-order mass shift, and identify the little-studied nuclear polarization as the only remaining SM contribution that may be large enough to explain it. Despite the observed nonlinearity, we improve existing KP-based constraints on a hypothetical Yukawa interaction for most of the new boson masses between 10 eV / c 2 and 10 7 eV / c 2 .

In this Letter, we present mass-ratio measurements on highly charged Yb 42 + ions with a precision of 4 × 10 − 12 and isotope-shift measurements on Yb + on the S 2 1 / 2 → D 5 / 2 2 and S 2 1 / 2 → F 7 / 2 2 transitions with a precision of 4 × 10 − 9 for the isotopes Yb 168 , 170 , 172 , 174 , 176 . We present a new method that allows us to extract higher-order changes in the nuclear charge distribution along the Yb isotope chain, benchmarking nuclear structure calculations. Additionally, we perform a King plot analysis to set bounds on a fifth force in the keV / c 2 to MeV / c 2 range coupling to electrons and neutrons.

We devise and demonstrate a method to search for nongravitational couplings of ultralight dark matter to standard model particles using space-time separated atomic clocks and cavity-stabilized lasers. By making use of space-time separated sensors, which probe different values of an oscillating dark matter field, we can search for couplings that cancel in typical local experiments. This provides sensitivity to both the temporal and spatial fluctuations of the field. We demonstrate this method using existing data from a frequency comparison of lasers stabilized to two optical cavities connected via a 2220 km fiber link [Schioppo , ], and from the atomic clocks on board the global positioning system satellites. Our analysis results in constraints on the coupling of scalar dark matter to electrons, d m e , for masses between 10 − 19 and 2 × 10 − 15 eV / c 2 . These are the first constraints on d m e alone in this mass range.