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A two-color magneto-optical trap of Strontium in pursuit of a continuous atomic clock

Milán János Negyedi

Published Mar 27, 2026• 0 citations
Publisher

Abstract

In this thesis we describe our work on the creation and characterization of a two-color magneto-optical trap suitable for continuous extraction of ultracold Sr atoms. Such a trap would be a very important building block of a continuous atomic frequency standard. With interruption-free evaluation of this standard, the accuracy of neutral atomic clocks could be improved significantly, therefore enabling several important applications. In this work we will give a brief introduction into the general topic of atomic clocks, and the aforementioned applications in which a continuous clock could provide an improvement. Then we will provide a summary of the relevant theoretical concepts, followed by a detailed discussion of our measurement results. We finish the work with a description of theoretical calculations and their comparison to our measurement data regarding the achievable minimum temperatures in such a trap.

DOI
10.15496/publikation-119127

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We report a two-color magneto-optical trap that continuously operates on the $\mathrm{(5s^2)\:^1S_0-(5s5p)\:^1P_1}$ and $\mathrm{(5s5p)\:^3P_2-(5s5d)\:^3D_3}$ transitions in $\mathrm{^{88}Sr}$. Owing to the sub-Doppler cooling from the metastable $\mathrm{(5s5p)\:^3P_2}$ state, a bimodal momentum distribution is observed, with the cold part of the ensemble reaching a temperature of 105 $\mathrmμ$K. A detailed simulation is established to explain the data. The absolute frequency of the $\mathrm{(5s5p)\:^3P_2-(5s5d)\:^3D_3}$ transition is measured to be 603 976 473 ${\pm}$ 2 MHz using a frequency comb, improving the literature value. The accompanying magnetic trap is characterized, and is shown to contribute to the sequential production of ensembles with temperatures down to 45 $\mathrmμ$K.

arXiv:2507.02693•Physical Review A•May 12, 2026•1 citation
In this thesis we describe our work on the creation and characterization of a two-color magneto-optical trap suitable for continuous extraction of ultracold Sr atoms. Such a trap would be a very important building block of a continuous atomic frequency standard. With interruption-free evaluation of this standard, the accuracy of neutral atomic clocks could be improved significantly, therefore enabling several important applications. In this work we will give a brief introduction into the general topic of atomic clocks, and the aforementioned applications in which a continuous clock could provide an improvement. Then we will provide a summary of the relevant theoretical concepts, followed by a detailed discussion of our measurement results. We finish the work with a description of theoretical calculations and their comparison to our measurement data regarding the achievable minimum temperatures in such a trap.
Strontium optical lattice clocks can measure the passage of time with extraordinary precision. Capitalising on this precision, we can anticipate the deployment of lattice clocks to search for new physics beyond the standard model, to build new technologies for geodesy and navigation, and potentially to underpin a future redenition of the SI second. This thesis reports on the construction and evaluation of a robust Sr optical lattice clock at NPL. We describe the apparatus needed to capture, cool, and load samples of neutral strontium atoms into a magic-wavelength, far off-resonant lattice trap at 813 nm. We provide details of our optical local oscillator - the "clock laser" - and how it is used to realise an Sr-referenced optical frequency standard. We rigorously characterise the various contributing factors which limit the performance of the clock, focusing on the standard measures of (1) frequency instability, and (2) systematic frequency uncertainty. Finally, we introduce new innovations for improving the accuracy of the 88Sr lattice clock, including methods of "modified hyper-Ramsey" and multi-photon spectroscopy of the clock transition.
A steady-state magneto-optical trap (MOT) of fermionic strontium atoms operating on the 7.5-kHz7.5\text{-kHz}7.5-kHz-wide $^{1}S_{0}\text{\ensuremath{-}}^{3}P_{1}$ transition is demonstrated. This MOT contains $8.4\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.16em}{0ex}}\mathrm{atoms}$, a loading rate of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.16em}{0ex}}\mathrm{atoms}/\mathrm{s}$, and an average temperature of $12\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$. These parameters make it well suited to serve as a source of atoms for continuous-wave superradiant lasers operating on strontium's mHz-wide clock transition. Such lasers have only been demonstrated using pulsed Sr sources, limiting their range of applications. Our MOT makes an important step toward continuous operation of these devices, paving the way for continuous active optical clocks.
Atomic Caesium clocks have made an astonishing development in the past few decades, with them gaining an order of magnitude in precision every decade. A new generation of atomic optical clocks is now emerging, which is moving to the forefront of timekeeping. By going from microwave to optical frequencies these clocks are now reaching fractional uncertainty levels in the 10\\(^-\\)\\(^1\\)\\(^8\\), opening a whole new range of exciting applications. This thesis presents the design and implementation of an experiment for laser cooling of strontium atoms, focusing on the mobility of the setup, opening a range of new applications as a mobile clock. The experimental apparatus applies a range of new techniques for strontium, such as the use of a two dimensional magneto-optical trap for pre cooling atoms, allowing for a compact and transportable system. A mobile laser system for laser cooling strontium atoms was realized. This includes a frequency doubling system for the main cooling transition. The constructed monolithic frequency doubling cavity has generated 300 mW of blue light and proven to be a stable setup suitable for a mobile system. The 3D-MOT was able to capture 4 * 10\\(^5\\) atoms with a long lifetime of 5.5 s, with further improvements for a significant increase in atom number currently being implemented.
Accurate time keeping is critical to the function and advancement of many human endeavours. Atomic clocks based on a microwave transition in atomic Cs have served as primary frequency standards to realise the SI second since their formal adoption in 1967. State-of-the-art, laser cooled, Cs fountains now routinely operate with fractional frequency uncertainties below the 10⁻¹⁵ level in laboratories throughout the world. Over the last decade, efforts to build frequency standards based on optical transitions have come to fruition and potential inaccuracies below the 10⁻¹⁷ level have been demonstrated. Such standards point to the future of time keeping and much development is being pursued to realise the potential of these 'next generation' optical clocks. One such approach, on which this work focusses, relies on a 'magic' wavelength optical lattice trap to provide (almost) Doppler- and recoil-free confinement of atoms during clock interrogation. At this 'magic' wavelength, light induced perturbation of the clock states is equal and so cancels precisely. This thesis describes the design and implementation of an optical lattice frequency standard apparatus based on 88Sr held in a 1D optical lattice. A review of the development of atomic frequency metrology is given from a historical viewpoint and the technology and limitations of current methods discussed. To tackle the blackbody radiation shift, which currently presents the greatest limitation to the Sr lattice clock, imposing at the 10⁻¹⁶ level, a scheme to measure the blackbody radiation shift coefficient at the 10⁻¹⁷ level is developed. Techniques to laser cool and trap atomic strontium are described and the implementation detailed. All laser systems required for clock operation are described including the design and implementation of extended-cavity diode lasers for precision spectroscopy. Details relevant to the eventual operation of the apparatus are discussed.
Clocks based on optical transitions in ions and atoms are quickly moving to the forefront of the frequency standards field mainly because of the high spec-tral resolution, and therefore the potential stability and accuracy, which can be achieved. In this thesis a new optical clock based on neutral atoms trapped in an optical lattice is presented, demonstrating spectroscopy of the clock transition free of any lineshape or accuracy degradation due to atomic motion. The system simu-lates a single trapped ion clock, but allows use of thousands of atoms for improved signal to noise ratio, and clock stability. High accuracy can also be achieved as the lattice can be designed to shift the energy of the two atomic clock states equally, such that the transition frequency is unchanged. Strontium is a natural candidate for such a clock as it offers an extremely narrow optical transition (∼1 mHz) based on atomic states which are very insensitive to external fields. The strontium level structure allows efficient laser cooling to 1 µK with diode laser sources, and a con-venient wavelength of 813 nm for the zero-differential Stark shift optical lattice. The strontium lattice clock system has allowed observation of high signal-to-noise
We report a two-color magneto-optical trap that continuously operates on the (5s2) 1S0−(5s5p) 1P1\mathrm{(5s^2)\:^1S_0-(5s5p)\:^1P_1}(5s2)1S0​−(5s5p)1P1​ and (5s5p) 3P2−(5s5d) 3D3\mathrm{(5s5p)\:^3P_2-(5s5d)\:^3D_3}(5s5p)3P2​−(5s5d)3D3​ transitions in 88Sr\mathrm{^{88}Sr}88Sr. Owing to the sub-Doppler cooling from the metastable (5s5p) 3P2\mathrm{(5s5p)\:^3P_2}(5s5p)3P2​ state, a bimodal momentum distribution is observed, with the cold part of the ensemble reaching a temperature of 105 μ\mathrmμμK. A detailed simulation is established to explain the data. The absolute frequency of the (5s5p) 3P2−(5s5d) 3D3\mathrm{(5s5p)\:^3P_2-(5s5d)\:^3D_3}(5s5p)3P2​−(5s5d)3D3​ transition is measured to be 603 976 473 ±{\pm}± 2 MHz using a frequency comb, improving the literature value. The accompanying magnetic trap is characterized, and is shown to contribute to the sequential production of ensembles with temperatures down to 45 μ\mathrmμμK.