The effective electron neutrino mass can be determined by analyzing the end-point region of the ^{163}Ho electron capture spectrum, provided a measurement with high-energy resolution and high statistics using calorimetric techniques. Here, the Electron Capture in ^{163}Ho Collaboration (ECHo) presents an analysis of the most precise ^{163}Ho spectrum currently available, obtained with the ECHo-1k experiment and comprising about 200 million events. A very low background rate of b_{const}=9.1(1.3)×10^{-6} eV/pixel/day was achieved allowing for a reliable analysis of the end-point region. The derived end-point energy Q=2862(4)  eV is in excellent agreement with the one independently determined via Penning-trap mass spectrometry of Q=2863.2(6)  eV [Ch. Schweiger et al., Nat. Phys. 226, 921 (2024)NPAHAX1745-247310.1038/s41567-024-02461-9]. The upper limit of the effective electron neutrino mass is improved by almost a factor of 2 compared to the lowest current value [B. K. Alpert et al., Phys. Rev. Lett. 135, 141801 (2025)PRLTAO0031-900710.1103/s9vl-7n24], reaching m_{ν_{e}}<15  eV/c^{2} (90% credible interval).

In this Letter, we present mass-ratio measurements on highly charged Yb$^{42+}$ ions with a precision of $4\times 10^{-12}$ and isotope-shift measurements on Yb$^{+}$ on the $^{2}$S$_{1/2}$ $\to$ $^{2}$D$_{5/2}$ and $^{2}$S$_{1/2}$ $\to$ $^{2}$F$_{7/2}$ transitions with a precision of $4\times 10^{-9}$ for the isotopes $^{168,170,172,174,176}$Yb. We present a new method that allows us to extract higher-order changes in the nuclear charge distribution along the Yb isotope chain, benchmarking ab-initio 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.

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 ^{3}P_{0}→^{3}P_{1} in Ca^{14+} and ^{2}S_{1/2}→^{2}D_{5/2} 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 (^{40,42,44,46,48}Ca). 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}.

The absolute atomic mass of ^208 Pb has been determined with a fractional uncertainty of 7× 10^-11 by measuring the cyclotron-frequency ratio R of ^208 Pb ^41+ to ^132 Xe ^26+ with the high-precision Penning-trap mass spectrometer Pentatrap and computing the binding energies E_Pb and E_Xe of the missing 41 and 26 atomic electrons, respectively, with the ab initio fully relativistic multi-configuration Dirac–Hartree–Fock (MCDHF) method. R has been measured with a relative precision of 9× 10^-12 . E_Pb and E_Xe have been computed with an uncertainty of 9.1 eV and 2.1 eV, respectively, yielding 207.976 650 571(14) u ( u=9.314 941 024 2(28)× 10^8 eV/c ^2 ) for the ^208 Pb neutral atomic mass. This result agrees within 1.2σ with that from the Atomic-Mass Evaluation (AME) 2020, while improving the precision by almost two orders of magnitude. The new mass value directly improves the mass precision of 14 nuclides in the region of Z = 81–84 and is the most precise mass value with A>200 . Thus, the measurement establishes a new region of reference mass values which can be used e.g. for precision mass determination of transuranium nuclides, including the superheavies.

The investigation of the absolute scale of the effective neutrino mass remains challenging due to the exclusively weak interaction of neutrinos with all known particles in the standard model of particle physics. At present, the most precise and least-model-dependent upper limit on the electron antineutrino mass is set by the Karlsruhe Tritium Neutrino Experiment (KATRIN) from the analysis of the tritium beta-decay. Another promising approach is the electron capture in Ho-163, which is under investigation using microcalorimetry by the Electron Capture in Holmium (ECHo) and HOLMES collaborations. An independently measured Q value for this process is vital for the assessment of systematic uncertainties in the neutrino mass determination. Here we report a direct, independent determination of this Q value by measuring the free-space cyclotron frequency ratio of highly charged ions of Ho-163 and Dy-163 in the Penning-trap experiment PENTATRAP. Combining this ratio with atomic physics calculations of the electronic binding energies yields a Q value of 2,863.2 +/- 0.6 eV c(-2), which represents a more than 50-fold improvement over the state of the art. This will enable the determination of the electron neutrino mass on a sub-electronvolt level from the analysis of the electron capture in Ho-163.

A high-precision Penning-trap experiment was operated with a programmable 20 V Josephson voltage source, providing a significantly higher stability of the electrostatic trapping potential. This increased the motional frequency stability of a trapped ion by about a factor of two with respect to state-of-the-art voltage sources. An absolute axial frequency stability of 7.8(6) mHz corresponding to a relative precision of 9.7(8)×10−9 at 795 kHz was demonstrated using a trapped 9Be3+ ion as a measurement probe. The enhanced stability in the trapping potential opens up the possibility for improved determinations of the proton and antiproton magnetic moments and direct high-precision measurements of the nuclear magnetic moments of 2H, 3He, and 7Li. In the field of mass spectrometry, the developments will enable measurements of cyclotron frequency ratios and, thus, mass ratio measurements with unprecedented relative statistical uncertainties below the 10−12 level.

The atomic mass of uranium-238 has been determined to be $238.050\,787\,618(15)\,\text{u}$, improving the literature uncertainty by two orders of magnitude. It is obtained from a measurement of the mass ratio of $^{238}$U$^{47+}$ and $^{132}$Xe$^{26+}$ ions with an uncertainty of $3.5\times 10^{-12}$. The measurement was carried out with the Penning-trap mass spectrometer \textsc{Pentatrap} and was accompanied by a calculation of the binding energies $E_{\text{U}}$ and $E_{\text{Xe}}$ of the 47 and 26 missing electrons of the two highly charged ions, respectively. These binding energies were determined using an \textit{ab initio} multiconfiguration Dirac-Hartree-Fock (MCDHF) method to be $E_{\text{U}} = 39\,927(10)\,\text{eV}$ and $E_{\text{Xe}} = 8\,971.2(21)\,\text{eV}$. The new mass value will serve as a reference for high-precision mass measurements in the heavy mass region of the nuclear chart up to transuranium nuclides.

We present the measurements of individual bound electron g factors of ^{20}Ne^{9+} and ^{22}Ne^{9+} on the relative level of 0.1 parts per billion. The comparison with theory represents the most stringent test of bound-state QED in strong electric fields. A dedicated mass measurement results in m(^{20}Ne)=19.992 440 168 77(9)  u, which improves the current literature value by a factor of 18, disagrees by 4 standard deviations, and represents the most precisely measured mass value in atomic mass units. Together, these measurements yield an electron mass on the relative level of 0.1 ppb with m_{e}=5.485 799 090 99(59)×10^{-4}  u as well as a factor of seven improved m(^{22}Ne)=21.991 385 098 2(26)  u.

We present the measurements of individual bound electron g factors of Ne-20(9+) and Ne-22(9+) on the relative level of 0.1 parts per billion. The comparison with theory represents the most stringent test of bound-state QED in strong electric fields. A dedicated mass measurement results in m(Ne-20) = 19.992 440 168 77(9) u, which improves the current literature value by a factor of 18, disagrees by 4 standard deviations, and represents the most precisely measured mass value in atomic mass units. Together, these measurements yield an electron mass on the relative level of 0.1 ppb with m(e) = 5.485 799 090 99(59) x 10(-4) u as well as a factor of seven improved m(22) = 21.991 385 098 2(26) u.

We present the measurements of individual bound electron $g$ factors of $^{20}\text{Ne}^{9+}$ and $^{22}\text{Ne}^{9+}$ on the relative level of $0.1\,\text{parts}$ per billion. The comparison with theory represents the most stringent test of bound-state QED in strong electric fields. A dedicated mass measurement results in $m\left(^{20}\text{Ne}\right)=19.992\,440\,168\,77\,(9)\,\text{u}$, which improves the current literature value by a factor of nineteen, disagrees by $4$ standard deviations and represents the most precisely measured mass value in atomic mass units. Together, these measurements yield an electron mass on the relative level of $0.1\,\text{ppb}$ with $m_{\text{e}}=5.485\,799\,090\,99\,(59) \times 10^{-4}\,\text{u}$ as well as a factor of eight improved $m\left(^{22}\text{Ne}\right)=21.991\,385\,098\,2\,(26)\,\text{u}$.

Highly charged ions (HCIs) offer many opportunities for next-generation clock research due to the vast landscape of available electronic transitions in different charge states. The development of extreme ultraviolet frequency combs has enabled the search for clock transitions based on shorter wavelengths in HCIs. However, without initial knowledge of the energy of the clock states, these narrow transitions are difficult to be probed by lasers. In this Letter, we provide experimental observation and theoretical calculation of a long-lived electronic state in Nb-like Pb^{41+} that could be used as a clock state. With the mass spectrometer PENTATRAP, the excitation energy of this metastable state is directly determined as a mass difference at an energy of 31.2(8) eV, corresponding to one of the most precise relative mass determinations to date with a fractional uncertainty of 4×10^{-12}. This experimental result agrees within 1σ with two partially different ab initio multiconfiguration Dirac-Hartree-Fock calculations of 31.68(13) eV and 31.76(35) eV, respectively. With a calculated lifetime of 26.5(5.3) days, the transition from this metastable state to the ground state bears a quality factor of 1.1×10^{23} and allows for the construction of a HCI clock with a fractional frequency instability of <10^{-19}/sqrt[τ].

We present the measurements of individual bound electron $g$ factors of ${^{20}\mathrm{Ne}}^{9+}$ and ${^{22}\mathrm{Ne}}^{9+}$ on the relative level of 0.1 parts per billion. The comparison with theory represents the most stringent test of bound-state QED in strong electric fields. A dedicated mass measurement results in $m(^{20}\mathrm{Ne})=\phantom{\rule{0ex}{0ex}}19.992\text{ }440\text{ }168\text{ }77(9)\text{ }\text{ }\mathrm{u}$, which improves the current literature value by a factor of 18, disagrees by 4 standard deviations, and represents the most precisely measured mass value in atomic mass units. Together, these measurements yield an electron mass on the relative level of 0.1 ppb with ${m}_{\mathrm{e}}=5.485\text{ }799\text{ }090\text{ }99(59)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\text{ }\text{ }\mathrm{u}$ as well as a factor of seven improved $m(^{22}\mathrm{Ne})=21.991\text{ }385\text{ }098\text{ }2(26)\text{ }\text{ }\mathrm{u}$.

We present the measurements of individual bound electron g factors of ^20Ne^9+ and ^22Ne^9+ on the relative level of 0.1 parts per billion. The comparison with theory represents the most stringent test of bound-state QED in strong electric fields. A dedicated mass measurement results in m(^20Ne)=19.992 440 168 77 (9) u, which improves the current literature value by a factor of nineteen, disagrees by 4 standard deviations and represents the most precisely measured mass value in atomic mass units. Together, these measurements yield an electron mass on the relative level of 0.1 ppb with m_e=5.485 799 090 99 (59) × 10^-4 u as well as a factor of eight improved m(^22Ne)=21.991 385 098 2 (26) u.

In this paper, we report on the development of a fast high-voltage switch, which is based on two enhancement mode N-channel silicon carbide metal-oxide-semiconductor field-effect transistors in push-pull configuration. The switch is capable of switching high voltages up to 600 V on capacitive loads with rise and fall times on the order of 10 ns and pulse widths ≥20 ns. Using this switch, it was demonstrated that, from the charge state distribution of bunches of highly charged ions ejected from an electron beam ion trap with a specific kinetic energy, single charge states can be separated by fast switching of the high voltage applied to a Bradbury-Nielsen Gate with a resolving power of about 100.

The definition of the absolute neutrino mass scale is one of the main goals of the Particle Physics today. The study of the end-point regions of the $\beta$- and electron capture (EC) spectrum offers a possibility to determine the effective electron (anti-)neutrino mass in a completely model independent way, as it only relies on the energy and momentum conservation. The ECHo (Electron Capture in $^{163}$Ho) experiment has been designed in the attempt to measure the effective mass of the electron neutrino by performing high statistics and high energy resolution measurements of the $^{163}$Ho electron capture spectrum. To achieve this goal, large arrays of low temperature metallic magnetic calorimeters (MMCs) implanted with with $^{163}$Ho are used. Here we report on the structure and the status of the experiment.

The possibility of applying active feedback to a single ion in a Penning trap using a fully digital system is demonstrated. Previously realized feedback systems rely on analog circuits that are susceptible to environmental fluctuations and long term drifts, as well as being limited to the specific task they were designed for. The presented system is implemented using a field-programmable gate array (FPGA)-based platform (STEMlab), offering greater flexibility, higher temporal stability, and the possibility for highly dynamic variation of feedback parameters. The system's capabilities were demonstrated by applying feedback to the ion detection system primarily consisting of a resonant circuit. This allowed shifts in its resonance frequency of up to several kHz and free modification of its quality factor within two orders of magnitude, which reduces the temperature of a single ion by a factor of 6. Furthermore, a phase-sensitive detection technique for the axial ion oscillation was implemented, which reduces the current measurement time by two orders of magnitude, while simultaneously eliminating model-related systematic uncertainties. The use of FPGA technology allowed the implementation of a fully-featured data acquisition system, making it possible to realize feedback techniques that require constant monitoring of the ion signal. This was successfully used to implement a single-ion self-excited oscillator.

The cyclotron frequency ratio of ^{187}Os^{29+} to ^{187}Re^{29+} ions was measured with the Penning-trap mass spectrometer PENTATRAP. The achieved result of R=1.000 000 013 882(5) is to date the most precise such measurement performed on ions. Furthermore, the total binding-energy difference of the 29 missing electrons in Re and Os was calculated by relativistic multiconfiguration methods, yielding the value of ΔE=53.5(10)  eV. Finally, using the achieved results, the mass difference between neutral ^{187}Re and ^{187}Os, i.e., the Q value of the β^{-} decay of ^{187}Re, is determined to be 2470.9(13) eV.

First ever measurements of the ratios of free cyclotron frequencies of heavy, highly charged ions with Z>50 with relative uncertainties close to 10^{-11} are presented. Such accurate measurements have become realistic due to the construction of the novel cryogenic multi-Penning-trap mass spectrometer PENTATRAP. Based on the measured frequency ratios, the mass differences of five pairs of stable xenon isotopes, ranging from ^{126}Xe to ^{134}Xe, have been determined. Moreover, the first direct measurement of an electron binding energy in a heavy highly charged ion, namely of the 37th atomic electron in xenon, with an uncertainty of a few eV is demonstrated. The obtained value agrees with the calculated one using two independent, different implementations of the multiconfiguration Dirac-Hartree-Fock method. PENTATRAP opens the door to future measurements of electron binding energies in highly charged heavy ions for more stringent tests of bound-state quantum electrodynamics in strong electromagnetic fields and for an investigation of the manifestation of light dark matter in isotopic chains of certain chemical elements.

State-of-the-art optical clocks 1 achieve precisions of 10 −18 or better using ensembles of atoms in optical lattices 2 , 3 or individual ions in radio-frequency traps 4 , 5 . Promising candidates for use in atomic clocks are highly charged ions 6 (HCIs) and nuclear transitions 7 , which are largely insensitive to external perturbations and reach wavelengths beyond the optical range 8 that are accessible to frequency combs 9 . However, insufficiently accurate atomic structure calculations hinder the identification of suitable transitions in HCIs. Here we report the observation of a long-lived metastable electronic state in an HCI by measuring the mass difference between the ground and excited states in rhenium, providing a non-destructive, direct determination of an electronic excitation energy. The result is in agreement with advanced calculations. We use the high-precision Penning trap mass spectrometer PENTATRAP to measure the cyclotron frequency ratio of the ground state to the metastable state of the ion with a precision of 10 −11 —an improvement by a factor of ten compared with previous measurements 10 , 11 . With a lifetime of about 130 days, the potential soft-X-ray frequency reference at 4.96 × 10 16 hertz (corresponding to a transition energy of 202 electronvolts) has a linewidth of only 5 × 10 −8 hertz and one of the highest electronic quality factors (10 24 ) measured experimentally so far. The low uncertainty of our method will enable searches for further soft-X-ray clock transitions 8 , 12 in HCIs, which are required for precision studies of fundamental physics 6 .