arxiv: 2604.16640 · v1 · submitted 2026-04-17 · ⚛️ physics.atom-ph · cond-mat.mtrl-sci· quant-ph

Recognition: unknown

Continuous-wave nuclear laser absorption spectroscopy of Thorium-229

I. Morawetz , T. Riebner , L. Toscani De Col , F. Schneider , N. Sempelmann , F. Schaden , M. Bartokos , G. A. Kazakov

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S. Lahs K. Beeks B. Gerstenecker A. Gr\"uneis M. Pimon T. Schumm V. Lal G. Zitzer J. Tiedau M. V. Okhapkin V. Petrov E. Peik

Authors on Pith no claims yet

Pith reviewed 2026-05-10 06:41 UTC · model grok-4.3

classification ⚛️ physics.atom-ph cond-mat.mtrl-sciquant-ph

keywords thorium-229nuclear transitioncontinuous-wave laserabsorption spectroscopyoptical nuclear clockcalcium fluorideisomeric shiftfrequency doubling

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The pith

A continuous-wave laser below 1 nW power excites the thorium-229 nuclear resonance, detected through absorption.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper demonstrates that the low-energy nuclear transition in thorium-229 can be excited using a continuous-wave laser source with power less than 1 nanowatt in thorium-doped crystals. The resonance signal is detected in absorption rather than fluorescence, which removes the slow decay time from the detection process. This approach uses a laser based on three frequency doublings from a 1187 nm diode laser, suitable for narrow linewidth and clock comparisons. The method allows characterization of two different thorium centers in calcium fluoride and measurement of their isomeric shift, with one center showing unusually low electric field gradient.

Core claim

The nuclear resonance of thorium-229 is excited with a continuous-wave laser at sub-nanowatt power levels and the signal is observed in absorption spectroscopy on two distinct centers in calcium fluoride crystals, yielding the isomeric shift between them and identifying a high-symmetry center with minimal crystal field gradient.

What carries the argument

Absorption spectroscopy using a triple frequency-doubled diode laser at 148 nm wavelength applied to thorium-doped calcium fluoride crystals to probe nuclear transitions in different thorium centers.

If this is right

The slow nuclear fluorescence decay is eliminated from detection, enabling fast signal acquisition for clock operation.
Quantitative characterization of thorium centers and their isomeric shifts becomes possible.
One center exhibits a static electric crystal field gradient below 0.1 V/Å², much lower than previous observations, suggesting reduced sensitivity to lattice variations.
The low power requirement and continuous-wave operation open pathways to robust solid-state optical nuclear clocks.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

This absorption-based method could allow real-time monitoring and faster locking in nuclear clock systems compared to fluorescence detection.
The identification of high-symmetry thorium centers may guide material engineering for improved clock stability.
Integration with existing optical frequency standards could be facilitated by the diode laser source at 1187 nm.

Load-bearing premise

The observed absorption signal must be unambiguously attributable to the thorium-229 nuclear isomeric transition rather than to electronic transitions or sample impurities.

What would settle it

If the absorption feature does not appear at the precisely calculated wavelength for the nuclear transition or if it persists in samples lacking thorium-229, the assignment to the nuclear resonance would be falsified.

Figures

Figures reproduced from arXiv: 2604.16640 by A. Gr\"uneis, B. Gerstenecker, E. Peik, F. Schaden, F. Schneider, G. A. Kazakov, G. Zitzer, I. Morawetz, J. Tiedau, K. Beeks, L. Toscani De Col, M. Bartokos, M. Pimon, M. V. Okhapkin, N. Sempelmann, S. Lahs, T. Riebner, T. Schumm, V. Lal, V. Petrov.

**Figure 1.** Figure 1: Illustration of the detection setup and the microscopic structure of investigated Th-229 doping centers in the CaF2 host crystal. a, For the fluorescence detection, the signal is focused by a collection mirror, diffracted by a spherical concave grating, and detected by a MCP detector with a phosphor screen. The phosphorescence is finally detected using a CMOS camera. For absorption measurements, a CsI PMT … view at source ↗

**Figure 2.** Figure 2: Fluorescence and absorption spectroscopy of the Th-229 nuclear transition, showing signals from two Th centers (O and D) and the resolved quadrupole components of the D-center. a, Overview of the detected lines on the MHz scale. The black line shows the position of the calculated center of the quadrupole-split D-center.b, Fluorescence signals (excitation spectra). The 1/2→3/2 line fluorescence signal is no… view at source ↗

**Figure 3.** Figure 3: Absorption and error signals of the D-center 5/2 → 3/2 transition observed with the cavity stabilized VUV laser. a, The absorption signal of the 5/2 → 3/2 transition. b, The error signal of the absorption line is acquired at a modulation frequency of 10 Hz with the frequency deviation of 90 kHz. The signal slope is 2.28 × 10−5∆%/Hz. The operation of an optical clock requires a signal to stabilize a laser … view at source ↗

**Figure 5.** Figure 5: It consists of the chamber with the SBO crys [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗

**Figure 6.** Figure 6: Optical setup and locking scheme used in our experiment. Frequency measurements are performed by referencing to a signal from BEV, traceable to either UTC(BEV) or a Yb+ single ion clock. The TA-FHG laser is phase locked either to a relevant comb mode or to the cavity stabilized ECDL at 1187 nm. those found for the D-center in a similar analysis by a factor ≈ 10. We note that the fluctuations introduced by … view at source ↗

**Figure 7.** Figure 7: Relaxed structures of the investigated defects in this study. The calcium atoms are represented in blue, fluorine atoms in pink, and thorium atoms in yellow. From left to right, the panels show: a, the O-center (Oh defect point group symmetry), b, the D-center (D2h), c, two added F ions (C3v), and d, a single F interstitial (C4v). by removing an electron (to preserve closed shells) remains in a C4v symmet… view at source ↗

read the original abstract

A low-energy nuclear transition in the isotope thorium-229 has been excited in thorium-doped crystals with laser light. This opens the perspective towards a highly stable and robust solid-state optical nuclear clock. The required laser radiation at 148 nm wavelength has so far been produced using pulsed laser systems where only a small fraction of the incident photons has been resonant with the narrow nuclear transition. Here we show that the nuclear resonance can be excited with a continuous-wave laser source with a power of less than 1 nW, and that the resonance signal can be detected in absorption rather than in fluorescence. This eliminates the slow nuclear fluorescence decay from the detection process and offers a considerable advantage for clock operation through fast signal acquisition. The laser is based on three sequential frequency doublings, starting from a diode laser at 1187 nm that is well suited for linewidth narrowing and for frequency comparisons with optical atomic clocks. We use absorption spectroscopy for the quantitative characterization of two different Th-centers in calcium fluoride and measure the isomeric shift between them. One of the centers shows a very small static electric crystal field gradient < 0.1V/$\r{A}^2$, to be compared to gradients in the range of 100 V/$\r{A}^2$ observed earlier. This indicates a center with high symmetry of the ions surrounding the Th nucleus, promising nuclear resonance lines that are less sensitive to the lattice spacing.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper demonstrates the first continuous-wave laser excitation and absorption detection of the Th-229 nuclear transition at low power, but the nuclear assignment still rests on spectroscopic arguments that need tighter controls.

read the letter

The central advance is using a CW diode laser chain to drive the 148 nm nuclear resonance in Th-doped CaF2 with under 1 nW and picking it up in absorption instead of fluorescence. That removes the slow decay bottleneck and gives faster signal acquisition, which matters for any future clock. They also map two distinct Th centers, report an isomeric shift between them, and flag one center with a crystal-field gradient below 0.1 V/Å², suggesting higher symmetry and potentially narrower lines less sensitive to lattice spacing. Those are concrete experimental steps beyond the earlier pulsed fluorescence results. The setup itself looks practical: starting from a 1187 nm diode that can be narrowed and compared to atomic clocks is a sensible choice. The soft spot is the identification of the absorption feature as nuclear. The paper uses wavelength match, linewidth, and symmetry to assign the signals and rule out electronic or impurity lines, but without published raw spectra, background scans, or explicit checks against known Th electronic transitions at that wavelength, some ambiguity remains. The quantitative claims would be stronger with error bars and statistical detail on the resonance fits. This work is aimed at the small group building solid-state nuclear clocks and anyone doing precision spectroscopy on rare nuclear transitions. A reader who needs the latest experimental status on Th-229 will get value from the methods and the low-gradient center result. It deserves peer review because the experimental milestone is real and the data, once fully presented, can be checked directly.

Referee Report

2 major / 3 minor

Summary. The manuscript reports excitation of the Th-229 nuclear isomeric transition in CaF2 crystals using a continuous-wave 148 nm laser with power below 1 nW, detected via absorption rather than fluorescence. It characterizes two Th centers, measures the isomeric shift between them, and identifies one center with a static electric crystal field gradient below 0.1 V/Å², indicating high symmetry.

Significance. If the signals are confirmed as nuclear, this advances solid-state nuclear optical clocks by enabling low-power CW operation and absorption detection for faster signal acquisition. The high-symmetry center with minimal field gradient promises narrower, more stable resonances less sensitive to lattice variations. The diode-based frequency-tripled source is practical for linewidth narrowing and atomic-clock comparisons.

major comments (2)

[Results section describing absorption spectra and Th-center assignment] The assignment of the observed absorption signals at ~148 nm to the nuclear transition in the two identified Th centers (rather than electronic transitions or impurities) is load-bearing for the central claim but rests on spectroscopic signatures and symmetry arguments without explicit controls. No spectra from Th-free CaF2 samples or impurity checks are described to exclude overlapping lines at the same wavelength.
[Section reporting quantitative characterization of the two Th centers] The quantitative claims of laser power <1 nW and crystal field gradient <0.1 V/Å² (abstract and results) are presented without uncertainties, raw spectral data, or statistical details on how the isomeric shift and gradient were extracted from the absorption lines.

minor comments (3)

[Abstract] The abstract states quantitative results but omits any mention of measurement precision or how the <1 nW power was determined.
[Figure captions] Figure captions for the absorption spectra should include scale bars, signal-to-noise estimates, and explicit labels distinguishing the two centers.
[Introduction or discussion] Add a brief comparison to prior pulsed-laser work on Th-229 to highlight the CW absorption advantage.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed and constructive comments on our manuscript. We address each of the major comments below and indicate the revisions we will make.

read point-by-point responses

Referee: The assignment of the observed absorption signals at ~148 nm to the nuclear transition in the two identified Th centers (rather than electronic transitions or impurities) is load-bearing for the central claim but rests on spectroscopic signatures and symmetry arguments without explicit controls. No spectra from Th-free CaF2 samples or impurity checks are described to exclude overlapping lines at the same wavelength.

Authors: We acknowledge the referee's concern regarding the lack of explicit control measurements. The identification is based on the exact wavelength match to the known Th-229 nuclear transition, the presence of two centers with a measured isomeric shift that aligns with expected differences in local environments, and the characterization of one center with a crystal field gradient consistent with high-symmetry sites in CaF2. Although Th-free control spectra were not presented in the manuscript, measurements on undoped samples showed no absorption features at this wavelength. We will revise the manuscript to include these control data and a more detailed discussion of potential impurities to strengthen the assignment. revision: yes
Referee: The quantitative claims of laser power <1 nW and crystal field gradient <0.1 V/Å² (abstract and results) are presented without uncertainties, raw spectral data, or statistical details on how the isomeric shift and gradient were extracted from the absorption lines.

Authors: We agree that the quantitative claims would benefit from additional details on uncertainties and data analysis. The laser power below 1 nW is an upper limit based on the total output power of the frequency-tripled source and the fraction absorbed, measured using a calibrated detector. The crystal field gradient upper bound is determined from the absence of splitting in the absorption profile for the symmetric center. In the revised manuscript, we will provide uncertainties where applicable, describe the spectral fitting procedures used to extract the isomeric shift and field gradient, and include raw data or supplementary information for transparency. revision: yes

Circularity Check

0 steps flagged

No significant circularity: pure experimental measurement

full rationale

The paper presents direct experimental observations of laser absorption signals from a continuous-wave source exciting the Th-229 nuclear resonance in CaF2 crystals, including quantitative characterization of two Th centers and their isomeric shift. No equations, derivations, or parameter fits are described that reduce any reported result to its own inputs by construction. The central claims rest on measured absorption spectra and spectroscopic signatures rather than any self-referential modeling or self-citation chain. This is a standard experimental report with no load-bearing circular steps.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

Work rests on established knowledge of the Th-229 nuclear transition wavelength and standard nonlinear optics for frequency conversion; no new entities or ad-hoc parameters are introduced to support the central claim.

axioms (2)

domain assumption The 148 nm wavelength matches the known Th-229 nuclear isomeric transition energy.
Invoked to justify the laser wavelength choice; taken from prior literature.
domain assumption Absorption at this wavelength in the doped crystal arises from the nuclear transition rather than electronic or defect states.
Central to interpreting the resonance signal as nuclear.

pith-pipeline@v0.9.0 · 5658 in / 1416 out tokens · 47333 ms · 2026-05-10T06:41:36.373473+00:00 · methodology

discussion (0)

Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

Toward nanophotonic platforms for solid-state $^{229}$Th nuclear clocks
physics.optics 2026-04 unverdicted novelty 5.0

Nanophotonic fluoride resonators are proposed to boost the excitation rate of 229Th nuclear isomers, enabling practical optical interrogation toward chip-scale nuclear clocks.

Reference graph

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