Setup for the Ionic Lifetime Measurement of the ^229mTh³⁺ Nuclear Clock Isomer

Round 1

Reviewer 1 Report

The authors present the experimental setup for measuring the ionic lifetime of the isomeric state of 229Thorium, which is a crucial part of the extensive ongoing investigations into the prospects for developing a nuclear clock that operates on the nuclear rather than the electronic transitions. Compared to the best state-of-the-art atomic clocks, such a clock promises superior accuracy and stability. The manuscript is a comprehensive presentation of the experimental setup for the trapping and cooling of Th and Sr ions that is under development at the LMU Munich. The emphasis is on the technical aspects of the optical parts of the optical setup for laser cooling and detection of the ions. The paper is logically structured and well written. There is a good introduction and presentation of background information on the topic at hand. However, it would be desirable for the authors to give a more concrete and quantitative overview of the prospects for commissioning the experimental setup and to discuss the steps needed to perform initial ion loading tests. I also regret that the promised reference 20 in the current manuscripts with more details about the ion trap is not yet available to the readers. In the context of the planned use of the wavelength meter to stabilise multiple lasers, I would like to draw the authors' attention to the solution described in the paper by Ghandimi et. al. "Multichannel optomechanical switch and locking system for wavemeters" (https://doi.org/10.1364/AO.390881), as it may provide some useful information for their current work. I would also suggest that the authors should be more specific and provide quantitative values for the levels of vacuum (rather than "excellent vacuum conditions") that are necessary to achieve the intended storage times.  Otherwise, I consider the manuscript as it stands to be suitable for acceptance by Atoms.

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Reviewer 2 Report

The authors have provided detailed description of the experimental setup for measurement the lifetime of the 229mTh^3+ nuclear clock isomer. This yet unknown property of the thorium isomer is extremely important for the future nuclear clock. This manuscript presents the principle and the functionality of the optical experimental section and gives an outlook on the ionic lifetime measurement strategy, therefore, the experimental setup being constructed and commissioned in this manuscript should be published as an important step towards lifetime measurement with their planned method.

   The background is clear, organization is also good, lot of detailed technique design and construction as well as some preliminary testing results and estimation are presented. One minor suggestion, please add some more contents of summary and also outlook in the conclusions section.

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Reviewer 3 Report

This manuscript gives a detailed description of the experimental setup to measure the radiative lifetime of 229mTh3+, with special emphasis on the laser systems for spectroscopy and sympathetic cooling.

The measurement is certainly of great importance for the realization of the 229Thorium nuclear clock, and the trapping and cooling of 229mTh3+ would be an important milestone. I believe that this manuscript is of great interest to the 229Thorium nuclear clock community and I recommend publication of this manuscript.

I suggest that you consider the following points to further improve the manuscript.

1: I would be interested to know what the limiting systematics is for this lifetime measurement and what uncertainty you are aiming for.

2: A few ions out of a few hundred total ions will be isomeric. It was not clear to me how you aim the lasers at these targets. Are the lasers loosely focused to address all the tapped Th ions? Or are the lasers tightly focused to illuminate only one isomeric ion?

3: Related to 2, you plan to ensure that the isomer loss is due to radiative decay by confirming the existence of the ground state ion after decay. I did not understand how you could keep track of a single isomeric ion out of a few hundred total ions. 

3: What electronics are used to apply three or two modulation frequencies to the EOM simultaneously?

4: In Table 2 it was not clear to me how to get the nearly identical beam waist for four different wavelengths with one focusing lens. Is the mode field diameter nearly identical in the broadband fiber used? What is the effect of the chromatic dispersion of the lens?

5: Is the fluorescence imaging setup optimized for 422 nm? What is the difference in imaging quality between 422 and 690 nm?

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Reviewer 4 Report

The article “Setup for the Ionic Lifetime Measurement of the ^229mTh³⁺ Nuclear Clock Isomer” presents the principle and the functionality of the optical experimental setup for the measurement of the ionic lifetime of the isomer. It also gives an outlook on the ionic lifetime measurement strategy.

Authors detailly describes the experimental setup currently being commissioned at LMU Munich is based on a cryogenic Paul trap providing long enough storage times of ^229mTh ions that shall be sympathetically cooled with ⁸⁸Sr⁺.

 My questions and suggestions:

1. The authors expect to catch in the RFQ from 10 to 100 ions of ^229mTh³⁺ in the isomer state, which is very sufficient number for laser spectroscopy experiment. The problematic thing is the total efficiency of photon collection for the 690 nm light at level of 1.3%. I was also wandering about the procedure for the continuous observation of ions in the isomeric state that can be transfer for the electronic dark state. The level 5F7/2 (metastable electronic state) has a calculated lifetime more than 1 second [21, R1].

 I guess that main HFS measurement will be done by 690 nm laser excitation and its detection at the same lambda or will be detect at 984 nm?

The laser 984 nm will be used from time to time to prevent dark electronic state and will be used as repumper only or will be the second options to measure the isomeric lifetime vie resonances in the electronic structure of ion?

I was not able to conclude how will be looks like the one cycle of operation the 229mTh3+ Hyperfine Structure Spectroscopy to measure the ionic lifetime.

 

2. Even after reading the cited paper [19], I still don't know how the authors want to conduct the experiment. Will lasers for thorium ion spectroscopy (690 nm and 984 nm) work in continuous or pulsed mode? The complete description lacks the procedure of carrying out the experiment - indication of the sequence of cooling and spectroscopic signals. I will expect some well prepare protocol of useage the lasers and detectors in future experiment. I hope the authors will make this very clear in the next paper.

 

3. The authors decide to store the lasers and their optical arrangements in five 19''-rack drawers. It is done due to get compact system and damping vibrations above 100 Hz reason or authors planned in future to use the experimental setup as transportable setup?

 

In my opinion, the work very well and factually describes the experimental setup for future studies of the radiative lifetime of the ^229mTh ³⁺ isomeric state in the laboratory of the Faculty of Physics, Ludwig-Maximilians-Universität München, Munich in Germany.

 

 This work is worthy of publication in Atoms (publ. MDPI) with minor corrections listed as a comments/questions in attached pdf files and placed on the MDPI server in Report Review Form.

 [R1] U. I. Safronova, W. R. Johnson, and M. S. Safronova, “Excitation energies, polarizabilities, multipole transition rates, and lifetimes in ThIV,” Physical Review A 74, 042511 (2006).

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