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arXiv:2411.16402 · Physical Review A

Two-photon cooling of calcium atoms

Wojciech Adamczyk, Silvan Koch, Claudia Politi, Henry Fernandes Passagem, Christoph Fischer, P.I. Filippov, Florence Berterottière, Daniel Kienzler, Jonathan Home

Published Apr 1, 2025• 2 citations
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Abstract

We demonstrate cooling of calcium atoms using a two-photon transition from the ${}^{1}{S}_{0}$ ground state to the upper $4s5s\phantom{\rule{3.33333pt}{0ex}}{}^{1}{S}_{0}$ state via the ${}^{1}{P}_{1}$ intermediate state. We achieve temperatures as low as $260\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{K}$ in a magnetooptical trap (MOT), well below the Doppler limit (${T}_{\text{D}}=0.8\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{K}$) of the ${}^{1}{P}_{1}$ state. We characterize the temperature, lifetime, and confinement of the MOT over a range of experimental parameters, observing no reduction in lifetime due to coupling to the higher state. We perform theoretical simulations of the cooling scheme and observe good agreement with the experimental results. The two-photon cooling scheme presented in this work provides an alternative to the standard Doppler cooling applied to alkaline-earth atoms, based on a sequence of two magnetooptical traps. The advantages of our scheme are the possibility of varying the effective linewidth of the ${}^{1}{P}_{1}$ state, a higher transfer efficiency (close to $100%$), and a more straightforward experimental implementation.

DOI
10.1103/physreva.111.043101

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We demonstrate cooling of calcium atoms using a two-photon transition from the 1S0{}^{1}{S}_{0}1S0​ ground state to the upper 4s5s1S04s5s\phantom{\rule{3.33333pt}{0ex}}{}^{1}{S}_{0}4s5s1S0​ state via the 1P1{}^{1}{P}_{1}1P1​ intermediate state. We achieve temperatures as low as 260\textmuK260\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{K}260\textmuK in a magnetooptical trap (MOT), well below the Doppler limit (TD=0.8mK{T}_{\text{D}}=0.8\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{K}TD​=0.8mK) of the 1P1{}^{1}{P}_{1}1P1​ state. We characterize the temperature, lifetime, and confinement of the MOT over a range of experimental parameters, observing no reduction in lifetime due to coupling to the higher state. We perform theoretical simulations of the cooling scheme and observe good agreement with the experimental results. The two-photon cooling scheme presented in this work provides an alternative to the standard Doppler cooling applied to alkaline-earth atoms, based on a sequence of two magnetooptical traps. The advantages of our scheme are the possibility of varying the effective linewidth of the 1P1{}^{1}{P}_{1}1P1​ state, a higher transfer efficiency (close to 100100%100), and a more straightforward experimental implementation.
The possibility of laser cooling of alkaline-earth-metal atoms and ytterbium atoms using a two-photon transition is analyzed. We consider a ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{S}_{0}$ transition with excitation in near resonance with the 1P1{}^{1}{P}_{1}1P1​ level. This greatly increases the two-photon transition rate, allowing an effective transfer of momentum. The experimental implementation of this technique is discussed and we show that for calcium, for example, two-photon cooling can be used to achieve a Doppler limit of $123\ensuremath{\mu}\mathrm{K}.$ The efficiency of this cooling scheme and the main loss mechanisms are analyzed.
A new possibility of laser cooling of Calcium atoms using a two-photon transition is analyzed We investigate the possibility of Doppler cooling of Calcium using its two-photon (4s 2 ) 1 S0 - (4s5s) 1 S0 transition, with excitation in near resonance with the (4s4p) 1 P1 level. The two-photon absorption of l aser beams at 423 and 1 034 nm greatly increases the two-photon rate, allowing an effective transfer of momentum. The ex perimental i mplementation o f t his technique is discussed a nd we show that two-photon cooling can be used to achieve a temperature limit of about 1/7 of the onephoton Doppler limit reached in a conventional MOT.
A two-photon mechanism for cooling atoms below the Doppler temperature is analyzed. We consider the magnesium ladder system $(3{s}^{2})^{1}S_{0}\ensuremath{\rightarrow}(3s3p)^{1}P_{1}$ at 285.2nm285.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}285.2nm followed by the $(3s3p)^{1}P_{1}\ensuremath{\rightarrow}(3s3d)^{1}D_{2}$ transition at 880.7nm880.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}880.7nm. For the ladder system quantum coherence effects may become important. Combined with the basic two-level Doppler cooling process this allows for reduction of the atomic sample temperature by more than a factor of 10 over a broad frequency range. First experimental evidence for the two-photon cooling process is presented and compared to model calculations. Agreement between theory and experiment is excellent. In addition, by properly choosing the Rabi frequencies of the two optical transitions a velocity independent atomic dark state is observed.
Laser cooling and trapping of the heavy alkaline-earth-metal element barium has been achieved based on the strong $6{s}^{2}\text{ }{^{1}S}_{0}\text{\ensuremath{-}}6s6p\text{ }{^{1}P}_{1}$ transition. The excited state decays to a large fraction into metastable DDD states. Two schemes were implemented where three additional laser-driven transitions provide closed cooling cycles. This results in a fraction of 50(20)% of the trapped atoms in one of the metastable states. A total efficiency of $0.4(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ for slowing a thermal atomic beam and capturing atoms into a magneto-optical trap was obtained. Trap lifetimes of more than 1.5 s were observed. They are shortened at high laser intensities by three photon photoionization losses. The developed scheme can be transferred to other systems with large leakage from the cooling transition, such as Ra which is interesting for sensitive fundamental symmetry research.
Laser excitation of the 4s4p3P2−−4s3d1D24s4p {}^{3}{P}_{2} -- 4s3d {}^{1}{D}_{2}4s4p3P2​−−4s3d1D2​ transition in atomic calcium has been observed and the wavelength determined to 1530.5298(6) nm. The metastable 4s4p3P24s4p {}^{3}{P}_{2}4s4p3P2​ atoms were magnetically trapped in the quadrupole magnetic field of a magneto-optical trap. This state represents the only ``loss'' channel for the calcium atoms when laser cooled on the 4s21S0−−4s4p1P14{s}^{2} {}^{1}{S}_{0} -- 4s4p {}^{1}{P}_{1}4s21S0​−−4s4p1P1​ transition. A rate equation model shows that an order of magnitude more atoms are trapped in this state compared with those taking part in the main cooling cycle. Excitation of the 3P2{}^{3}{P}_{2}3P2​ atoms back up to the 4s3d1D24s3d {}^{1}{D}_{2}4s3d1D2​ state provides a means of accessing these atoms. Efficient repumping is achieved if the 1530-nm laser is used in conjunction with a 672-nm laser driving the 4s3d1D2−−4s5p1P14s3d {}^{1}{D}_{2} -- 4s5p {}^{1}{P}_{1}4s3d1D2​−−4s5p1P1​ transition. In the present experiment, we detected about $4.5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ trapped 3P2{}^{3}{P}_{2}3P2​ atoms, a relatively low atom density, and measured a lifetime of approximately 1 s, which is limited by background collisions.
We report on a system for atomic beam deceleration and magneto-optical trapping of calcium atoms that uses the 1S0–1P1 transition, in which a single laser is used to trap and slow the atoms. The slower laser beam is focused near the magneto-optical trap’s center, which has a waist size much smaller than the atomic cloud such that its influence on the trapped atoms is greatly reduced. We also investigate the theoretical possibility of cooling by use of a two-photon (4s2)1S0–(4s5s)1S0 transition. Excitation near resonance with the 1P1 level results in an equilibrium temperature seven times smaller than the Doppler limit of the 1S0–1P1 transition.