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arXiv:2507.02693 · Physical Review A

Sub-Doppler cooling of bosonic strontium in a two-color magneto-optical trap

Milán János Negyedi, Shubha Deutschle, Florian Jessen, József Fortágh, L. Sárkány

Published May 12, 2026• 1 citation
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Abstract

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.

DOI
10.1103/d345-62yg

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We report experimental evidence of Sisyphus cooling in atoms with a 1S0{}^{1}{S}_{0}1S0​ ground state. Since J=0,J=0,J=0, any cooling mechanism which requires multiple sublevels can only occur in the isotopes which have nuclear spin $I\ensuremath{\ne}0.$ Ytterbium has seven stable isotopes and offers a unique system in which we can study cooling on $F=\stackrel{\ensuremath{\rightarrow}}{0}1$ (168,170,172,174,176Yb),{(}^{168,170,172,174,176}\mathrm{Yb}),(168,170,172,174,176Yb), $F=1/\stackrel{\ensuremath{\rightarrow}}{2}3/2$ (171Yb),{(}^{171}\mathrm{Yb}),(171Yb), and $F=5/\stackrel{\ensuremath{\rightarrow}}{2}7/2$ (173Yb),{(}^{173}\mathrm{Yb}),(173Yb), depending on the selection of isotope. We have trapped each of the seven stable isotopes of ytterbium in a magneto-optical trap (MOT) using the strong ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ transition, and transferred them into a second MOT which uses the much narrower ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ intercombination transition. We have measured the temperature of isotopes 171Yb,{}^{171}\mathrm{Yb},171Yb, 173Yb,{}^{173}\mathrm{Yb},173Yb, and 174Yb{}^{174}\mathrm{Yb}174Yb in the ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ MOT, as a function of the intensity and detuning of the trapping laser. The temperature of 174Yb{}^{174}\mathrm{Yb}174Yb was found to increase more rapidly with intensity than predicted by Doppler cooling theory, in agreement with earlier work on alkaline-earth atoms. In the odd isotopes the temperature was found to decrease with increasing angular momentum, as observed in earlier experiments and three-dimensional simulations.
We present a detailed investigation of strontium magneto-optical trap (MOT) dynamics. Relevant physical quantities in the trap, such as temperature, atom number and density, and loss channels and lifetime, are explored with respect to various trap parameters. By studying the oscillatory response of a two-level 1S0–1P1 88Sr MOT, we firmly establish the laser cooling dynamics predicted by Doppler theory. Measurements of the MOT temperature, however, deviate severely from Doppler theory predictions, implying significant additional heating mechanisms. To explore the feasibility of attaining quantum degenerate alkaline-earth samples via evaporative cooling, we also present the first experimental demonstration of magnetically trapped metastable 88Sr. Furthermore, motivated by the goal of establishing the fermionic isotope 87Sr as one of the highest-quality, neutral-atom-based optical frequency standards, we present a preliminary study of sub-Doppler cooling in a 87Sr MOT. A dual-isotope (87Sr and 88Sr) MOT is also demonstrated.
Optical pumping of 5s4d1D2{\mathrm{5s4d} ^1\mathrm{D}_2}5s4d1D2​ strontium atoms for laser cooling and imaging
We present a faster repumping scheme for strontium magneto-optical traps operating on the broad 5s21S0−5s5p1P1{\mathrm{5s^2} ^1\mathrm{S}_0} - {\mathrm{5s5p} ^1\mathrm{P}_1}5s21S0​−5s5p1P1​ laser cooling transition. Contrary to existing repumping schemes, we directly address lost atoms that spontaneously decayed to the 5s4d1D2{\mathrm{5s4d} ^1\mathrm{D}_2}5s4d1D2​ state, sending them back into the laser cooling cycle by optical pumping on the 5s4d1D2−5s8p1P1{\mathrm{5s4d} ^1\mathrm{D}_2} - {\mathrm{5s8p} ^1\mathrm{P}_1}5s4d1D2​−5s8p1P1​ transition. We thus avoid the ∼100 μs\sim 100 \, \mathrm{μs}∼100μs-slow decay path from 5s4d1D2{\mathrm{5s4d} ^1\mathrm{D}_2}5s4d1D2​ to the 5s5p3P1,2{\mathrm{5s5p} ^3\mathrm{P}_{1,2}}5s5p3P1,2​ states that is part of other repumping schemes. Using one low-cost external-cavity diode laser emitting at 448 nm448 \, \mathrm{nm}448nm, we show our scheme increases the flux out of a 2D magneto-optical trap by 60 %60 \, \%60% compared to without repumping. Furthermore, we perform spectroscopy on the 5s4d1D2−5s8p1P1{\mathrm{5s4d} ^1\mathrm{D}_2} - {\mathrm{5s8p} ^1\mathrm{P}_1}5s4d1D2​−5s8p1P1​ transition and measure its frequency ν88Sr=(668917515.3±4.0±25) MHzν_{\mathrm{^{88}Sr}} = (668917515.3 \pm 4.0 \pm 25) \, \mathrm{MHz}ν88Sr​=(668917515.3±4.0±25)MHz. We also measure the frequency shifts between the four stable isotopes of strontium and infer the specific mass and field shift factors, δνSMS88,86=−267(45) MHzδν_\text{SMS} ^{88,86} = -267(45) \, \mathrm{MHz}δνSMS88,86​=−267(45)MHz and δνFS88,86=2(42) MHzδν_\text{FS} ^{88,86} = 2(42) \, \mathrm{MHz}δνFS88,86​=2(42)MHz. Finally, we measure the hyperfine splitting of the 5s8p1P1{\mathrm{5s8p} ^1\mathrm{P}_1}5s8p1P1​ state in fermionic strontium, and deduce the magnetic dipole and electric quadrupole coupling coefficients A=−4(5) MHzA = -4(5) \, \mathrm{MHz}A=−4(5)MHz and B=5(35) MHzB = 5(35) \, \mathrm{MHz}B=5(35)MHz. Our experimental demonstration shows that this simple and very fast scheme could improve the laser cooling and imaging performance of cold strontium atom devices, such as quantum computers based on strontium atoms in arrays of optical tweezers.