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arXiv:2311.02941

Optical pumping of ${\mathrm{5s4d} ^1\mathrm{D}_2}$ strontium atoms for laser cooling and imaging

Jens Samland, Shayne Bennetts, Chun-Chia Chen, Rodrigo González Escudero, Florian Schreck, Benjamin Pasquiou

Published Nov 6, 2023• 0 citations
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Abstract

We present a faster repumping scheme for strontium magneto-optical traps operating on the broad ${\mathrm{5s^2} ^1\mathrm{S}_0} - {\mathrm{5s5p} ^1\mathrm{P}_1}$ laser cooling transition. Contrary to existing repumping schemes, we directly address lost atoms that spontaneously decayed to the ${\mathrm{5s4d} ^1\mathrm{D}_2}$ state, sending them back into the laser cooling cycle by optical pumping on the ${\mathrm{5s4d} ^1\mathrm{D}_2} - {\mathrm{5s8p} ^1\mathrm{P}_1}$ transition. We thus avoid the $\sim 100 \, \mathrm{μs}$-slow decay path from ${\mathrm{5s4d} ^1\mathrm{D}_2}$ to the ${\mathrm{5s5p} ^3\mathrm{P}_{1,2}}$ states that is part of other repumping schemes. Using one low-cost external-cavity diode laser emitting at $448 \, \mathrm{nm}$, we show our scheme increases the flux out of a 2D magneto-optical trap by $60 \, \%$ compared to without repumping. Furthermore, we perform spectroscopy on the ${\mathrm{5s4d} ^1\mathrm{D}_2} - {\mathrm{5s8p} ^1\mathrm{P}_1}$ transition and measure its frequency $ν_{\mathrm{^{88}Sr}} = (668917515.3 \pm 4.0 \pm 25) \, \mathrm{MHz}$. We also measure the frequency shifts between the four stable isotopes of strontium and infer the specific mass and field shift factors, $δν_\text{SMS} ^{88,86} = -267(45) \, \mathrm{MHz}$ and $δν_\text{FS} ^{88,86} = 2(42) \, \mathrm{MHz}$. Finally, we measure the hyperfine splitting of the ${\mathrm{5s8p} ^1\mathrm{P}_1}$ state in fermionic strontium, and deduce the magnetic dipole and electric quadrupole coupling coefficients $A = -4(5) \, \mathrm{MHz}$ and $B = 5(35) \, \mathrm{MHz}$. 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.

DOI
10.48550/arxiv.2311.02941

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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.
We present a faster repumping scheme for strontium magneto-optical traps operating on the broad 5 s 2 1 S 0 − 5 s 5 p 1 P 1 laser cooling transition. Contrary to existing repumping schemes, we directly address lost atoms that spontaneously decayed to the 5 s 4 d 1 D 2 state, sending them back into the laser cooling cycle by optical pumping on the 5 s 4 d 1 D 2 − 5 s 8 p 1 P 1 transition. We thus avoid the ∼ 100 µ s -slow decay path from 5 s 4 d 1 D 2 to the 5 s 5 p 3 P 1 , 2 states that is part of other repumping schemes. Using one low-cost external-cavity diode laser emitting at 448 nm , we show our scheme increases the flux out of a 2D magneto-optical trap by 60 % compared to without repumping. Furthermore, we perform spectroscopy on the 5 s 4 d 1 D 2 − 5 s 8 p 1 P 1 transition and measure its frequency ν Sr 88 = ( 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, δ ν SMS 88 , 86 =<
We explore the efficacy of optical pumping on the $5s4d\phantom{\rule{0.16em}{0ex}}^{1}D_{2}\text{\ensuremath{-}}5s8p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}$ (448nm)(448\phantom{\rule{0.16em}{0ex}}\mathrm{nm})(448nm) transition in a magneto-optical trap (MOT) of Sr atoms. The number of trapped atoms is enhanced by a factor of 12.0(6)12.0(6)12.0(6) relative to the case without repumping light, which is six times as large as that obtained using the pumping transition $5s4d\phantom{\rule{0.16em}{0ex}}^{1}D_{2}\text{\ensuremath{-}}5s6p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}$ (717nm)(717\phantom{\rule{0.16em}{0ex}}\mathrm{nm})(717nm). This enhancement is limited by decay pathways that bypass the 5s4d1D25s4d\phantom{\rule{0.16em}{0ex}}^{1}D_{2}5s4d1D2​ state, namely $5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}\ensuremath{\rightarrow}5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{1}\ensuremath{\rightarrow}5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{0}$ and $5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}\ensuremath{\rightarrow}5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{2}\ensuremath{\rightarrow}5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$, which account for 8% of the total loss of the trapped atoms. We determine the decay rates for the $5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}\ensuremath{\rightarrow}5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{1}$ and $5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}\ensuremath{\rightarrow}5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{2}$ transitions to be $66(6)\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ and $2.4(2)\ifmmode\times\else\texttimes\fi{}{10}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, respectively. Furthermore, we experimentally demonstrate, when the trap beam diameter is small, escape of atoms in the 5s4d1D25s4d\phantom{\rule{0.16em}{0ex}}^{1}D_{2}5s4d1D2​ state, which has a relatively long lifetime of $400\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{s}$, becomes a dominant loss mechanism, and that the 448nm448\phantom{\rule{0.16em}{0ex}}\mathrm{nm}448nm pumping light effectively suppresses this escape. Our findings will contribute to improved laser cooling and fluorescence imaging in cold strontium atom platforms, such as quantum computers based on optical tweezer arrays.
Spectroscopy of the 5s5p5s5p5s5p $ ^3 P_0 \rightarrow 5s5d$ $ ^3 D_1 $ transition of strontium using laser cooled atoms
This article presents spectroscopy results of the 5s5p  3P0→5s5d  3D15s5p{\;^3}P_0 \rightarrow 5s5d{\;^3}D_15s5p3P0​→5s5d3D1​ transition in all isotopes of laser cooled Sr atoms and the utility of this transition for repumping application. By employing the $5s5p{\;^{3} P_{0}} \rightarrow 5s5d{\;^3}D_1 $ (483 nm) transition in combination with the excitation of 5s5p  3P2→5s6s  3S15s5p{\;^3}P_2 \rightarrow 5s6s{\;^3}S_15s5p3P2​→5s6s3S1​ (707 nm) transition, we observe a significant increase (∼\sim∼ 13 fold) in the steady state number of atoms in the magneto-optic trap (MOT). This enhancement is attributed to the efficient repumping of Sr atoms that have decayed into the dark 5s5p  3P25s5p{\;^3}P_25s5p3P2​ state by returning them to the ground state 5s2  1S05s^2{\;^1}S_05s21S0​ without any loss into the other states. The absolute transition frequencies were measured with an absolute accuracy of 30 MHz. To support our measurements, we performed Fock-space relativistic coupled-cluster calculations of the relevant parameters in Sr. To further increase the accuracy of the calculated properties, corrections from the Breit, QED and perturbative triples were also included. The calculated branching ratio for the repumping state confirms the significantly increased population in the 3P1{^3}P_13P1​ state. Thereby, leading to an increase of population of atoms trapped due to the enhanced repumping. Our calculated hyperfine-splitting energies are in excellent agreement with the measured values. Moreover, our calculated isotope shifts in the transition frequencies are in good agreement with our measured values.
We describe repumping and spectroscopy of laser-cooled strontium (Sr) atoms using the (5s5p) 3 P 2 –(5s4d) 3 D 2 transition. Atom number in a magneto-optical trap is enhanced by driving this transition because Sr atoms that have decayed into the (5s5p) 3 P 2 dark state are repumped back into the (5s 2 ) 1 S 0 ground state. Spectroscopy of 84 Sr, 86 Sr, 87 Sr and 88 Sr improves the value of the (5s5p) 3 P 2 –(5s4d) 3 D 2 transition frequency and determines the isotope shifts for the transition accurately enough to guide laser-cooling experiments with less abundant isotopes.
We investigate a repumping scheme for magneto-optically trapped calcium atoms. It is based on excitation of the 4s3d1D24s3{d}^{1}{D}_{2}4s3d1D2​--4s5p1P14s5{p}^{1}{P}_{1}4s5p1P1​ transition at 672672672 nm with an extended cavity diode laser. The effect of the repumping is approximately a factor of three increase in trap lifetime and a doubling of the trapping efficiency from a Zeeman slowed thermal beam. Added to this, the 672-nm laser repumps atoms from an otherwise dark state to yield an overall increase in detected fluorescence signal from the magneto-optic trap (MOT) of more than an order of magnitude. Furthermore, we report isotope shift measurements of the 672672672-nm transition, for the first time, for four naturally occurring even isotopes. Using available charge radii data, the observed shifts, extending up to 4.34.34.3 GHz, display the expected linear dependence in a King plot analysis. The measured shifts are used to determine the isotope shifts of the remaining 41,43,46Ca^{41,43,46}\mathrm{Ca}41,43,46Ca isotopes. These might be of interest where less abundant isotopes are used enabling isotope selective repumping, resulting in enhanced trapping and detection efficiencies.
We report narrow-line laser cooling and trapping of strontium atoms down to the photon recoil temperature. 88Sr{}^{88}\mathrm{Sr}88Sr atoms precooled by the broad $^{1}S_{0}\ensuremath{-}^{1}P_{1}$ transition at 461 nm were further cooled in a magneto-optical trap using the spin-forbidden transition $^{1}S_{0}\ensuremath{-}^{3}P_{1}$ at 689 nm. We have thus obtained an atomic sample with a density over ${10}^{12}\mathrm{cm}{}^{\ensuremath{-}3}$ and a minimum temperature of 400 nK, corresponding to a maximum phase space density of ${10}^{\ensuremath{-}2}$ which is 3 orders of magnitude larger than the value that has been obtained by magneto-optical traps to date. This scheme provides us an opportunity and system to study quantum statistical properties of degenerate fermions as well as bosons.