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arXiv:2302.02992 · Physical Review Applied

Atom-Number Enhancement by Shielding Atoms From Losses in Strontium Magneto-Optical Traps

Jonatan Höschele, Sandra Buob, Antonio Rubio-Abadal, Vasiliy Makhalov, Leticia Tarruell

Published Jun 5, 2023• 15 citations
arXivHTMLPDF

Abstract

We present a scheme to enhance the atom number in magneto-optical traps of strontium atoms operating on the 461-nm transition. This scheme consists of resonantly driving the ${}^{1}{\mathrm{S}}_{0}\ensuremath{\rightarrow}{}^{3}{\mathrm{P}}_{1}$ intercombination line at 689 nm, which continuously populates a short-lived reservoir state and, as expected from a theoretical model, partially shields the atomic cloud from losses arising in the 461-nm cooling cycle. We show a factor of 2 enhancement in the atom number for the bosonic isotopes ${}^{88}\mathrm{Sr}$ and ${}^{84}\mathrm{Sr}$, and the fermionic isotope ${}^{87}\mathrm{Sr}$, in good agreement with our model. Our scheme can be applied in the majority of strontium experiments without increasing the experimental complexity of the apparatus, since the employed 689-nm transition is commonly used for further cooling. Our method should thus be beneficial to a broad range of quantum science and technology applications exploiting cold strontium atoms, and could be extended to other atomic species.

DOI
10.1103/physrevapplied.19.064011

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We present a scheme to enhance the atom number in magneto-optical traps of strontium atoms operating on the 461-nm transition. This scheme consists of resonantly driving the 1S0\ensuremath→3P1{}^{1}{\mathrm{S}}_{0}\ensuremath{\rightarrow}{}^{3}{\mathrm{P}}_{1}1S0​\ensuremath→3P1​ intercombination line at 689 nm, which continuously populates a short-lived reservoir state and, as expected from a theoretical model, partially shields the atomic cloud from losses arising in the 461-nm cooling cycle. We show a factor of 2 enhancement in the atom number for the bosonic isotopes 88Sr{}^{88}\mathrm{Sr}88Sr and 84Sr{}^{84}\mathrm{Sr}84Sr, and the fermionic isotope 87Sr{}^{87}\mathrm{Sr}87Sr, in good agreement with our model. Our scheme can be applied in the majority of strontium experiments without increasing the experimental complexity of the apparatus, since the employed 689-nm transition is commonly used for further cooling. Our method should thus be beneficial to a broad range of quantum science and technology applications exploiting cold strontium atoms, and could be extended to other atomic species.
We report on a technique to improve the continuous loading of atomic strontium into a magnetic trap from a magneto-optical trap. This is achieved by adding a depumping laser tuned to the $^{3}P_{1}\ensuremath{\rightarrow}^{3}S_{1}$ (688-nm) transition. The depumping laser increases atom number in the magnetic trap and subsequent cooling stages by up to 65% for the bosonic isotopes and up to 30% for the fermionic isotope of strontium. We optimize this trap loading strategy with respect to the 688-nm laser detuning, intensity, and beam size. To understand the results, we develop a one-dimensional rate equation model of the system, which is in good agreement with the data. We discuss the use of other transitions in strontium for accelerated trap loading and the application of the technique to other alkaline-earth-like atoms.
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.
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.
A dynamic magneto-optical trap, which relies on the rapid randomization of population in Zeeman substates, has been demonstrated for fermionic strontium atoms on the $^{1}S_{0}\ensuremath{-}^{3}P_{1}$ intercombination transition. The obtained sample, $1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms at a temperature of $2\text{ }\ensuremath{\mu}\mathrm{K}$ in the trap, was further Doppler cooled and polarized in a far-off resonant optical lattice to achieve 2 times the Fermi temperature.
ABSTRACT In a ceramic vapor cell we have created a robust Sr magneto-optical trap that stores about 108 atoms with lifetimes>200 ms. We eliminate the 5p 1P1—4d 1D2—*5p 3P2 leak and achieve a 10-fold improvement in trap lifetime byrepumping the 5p 3Po,2 dark states with 679 nm and 707 nm light. The observed lifetime is now limited by coldcollision losses, and we have preliminary measurements of the 2-body loss rate. Direct readout of the trap velocity distribution is possible using the narrow (7 kHz) 52 'S0----5p 3P1 intercombination line at 689 nm. We can also cool with this narrow transition and have achieved a 40-fold 1-D velocity compression for about 5% of the trapped atoms by applying this second-stage cooling. Keywords: Laser cooling, laser trapping, alkaline earths, cold collisions, frequency metrology 1. INTRODUCTION Laser cooling and trapping techniques have advanced to become an important tool of the atomic physics community, and in recent years there has been considerable interest in expanding the range of trapped elements. The alkalineearths, in particular, possess unique characteristics which make them interesting to trap. Studies of long-rangeatom-atom interactions are simplified because they have no hyperfine structure, and their narrow intercombinationtransitions make them excellent candidates for optical frequency standards. Furthermore, Doppler cooling with thesenarrow transitions can reduce temperatures to near the recoil limit.The most abundant alkaline earth isotopes have I =
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.