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arXiv:1701.04948 · Physical Review A

Efficient repumping of a Ca magneto-optical trap

Michael Mills, Prateek Puri, Yanmei Yu, Andrei Derevianko, Christian Schneider, Eric R. Hudson

Published Sep 5, 2017• 25 citations
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Abstract

We investigate the limiting factors in the standard implementation of the Ca magneto-optical trap. We find that intercombination transitions from the $4s5p{\phantom{\rule{0.16em}{0ex}}}^{1}{P}_{1}$ state used to repump the electronic population from the $3d4s{\phantom{\rule{0.16em}{0ex}}}^{1}{D}_{2}$ state severely reduce the trap lifetime. We explore seven alternative repumping schemes theoretically and investigate five of them experimentally. We find that all five of these schemes yield a significant increase in the trap lifetime and consequently improve the number of atoms and peak atom density by as much as $\ensuremath{\sim}20$ times and $\ensuremath{\sim}6$ times, respectively. One of these transitions, at 453 nm, is shown to approach the fundamental limit for a Ca magneto-optical trap with repumping only from the dark $3d4s{\phantom{\rule{0.16em}{0ex}}}^{1}{D}_{2}$ state, yielding a trap lifetime of $\ensuremath{\sim}5$ s.

DOI
10.1103/physreva.96.033402

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Laser excitation of the $4s4p {}^{3}{P}_{2} -- 4s3d {}^{1}{D}_{2}$ transition in atomic calcium has been observed and the wavelength determined to 1530.5298(6) nm. The metastable $4s4p {}^{3}{P}_{2}$ 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 $4{s}^{2} {}^{1}{S}_{0} -- 4s4p {}^{1}{P}_{1}$ 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 ${}^{3}{P}_{2}$ atoms back up to the $4s3d {}^{1}{D}_{2}$ 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 $4s3d {}^{1}{D}_{2} -- 4s5p {}^{1}{P}_{1}$ transition. In the present experiment, we detected about $4.5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ trapped ${}^{3}{P}_{2}$ atoms, a relatively low atom density, and measured a lifetime of approximately 1 s, which is limited by background collisions.

Physical Review A•Jun 27, 2011•2 citations
We investigate the limiting factors in the standard implementation of the Ca magneto-optical trap. We find that intercombination transitions from the 4s5p1P14s5p{\phantom{\rule{0.16em}{0ex}}}^{1}{P}_{1}4s5p1P1​ state used to repump the electronic population from the 3d4s1D23d4s{\phantom{\rule{0.16em}{0ex}}}^{1}{D}_{2}3d4s1D2​ state severely reduce the trap lifetime. We explore seven alternative repumping schemes theoretically and investigate five of them experimentally. We find that all five of these schemes yield a significant increase in the trap lifetime and consequently improve the number of atoms and peak atom density by as much as \ensuremath∼20\ensuremath{\sim}20\ensuremath∼20 times and \ensuremath∼6\ensuremath{\sim}6\ensuremath∼6 times, respectively. One of these transitions, at 453 nm, is shown to approach the fundamental limit for a Ca magneto-optical trap with repumping only from the dark 3d4s1D23d4s{\phantom{\rule{0.16em}{0ex}}}^{1}{D}_{2}3d4s1D2​ state, yielding a trap lifetime of \ensuremath∼5\ensuremath{\sim}5\ensuremath∼5 s.
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 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.
The role of repumping in the magneto-optical trap (MOT) has often been seen as a necessary evil, requiring the introduction of either an electro-optical modulator or extra laser.1,2 However, repumping can be exploited to improve the performance of the MOT,3 or to study further the behavior of such a trap. We report results of a modified MOT using for repumping the D1 transitions 3S1/2 F = 1 to 3Pl/2 F = 1 and F = 2. Because these wavelengths are well displaced from the usual (D2) repump wavelengths, the direct repump influence on the trapping and cooling laser interaction should be significantly reduced. We also expect further possibilities for cold-atom spectroscopy within the MOT. The possibility of a dark trap with this repumping transition has been suggested.3
The theoretical description of the external degrees of freedom of atoms trapped inside a magneto-optical trap (MOT) often relies on the decoupling of the evolution of the internal and external degrees of freedom. That is possible thanks to much shorter timescales typically associated with the first ones. The electronic structure of alkaline-earth atoms, on the other hand, presents ultra-narrow transitions and metastable states that makes such an approximation invalid in the general case. In this article, we report on a model based on open Bloch equations for the evolution of the number of atoms in a magneto-optical trap. With this model we investigate the loading of the strontium blue magneto-optical trap under different repumping schemes, either directly from a Zeeman slower, or from an atomic reservoir made of atoms in a metastable state trapped in the magnetic quadrupolar field. The fluorescence observed on the strong 461~nm transition is recorded and quantitatively compared with the results from our simulations. The comparison between experimental results and calculations within our model allowed to identify the existence of the decay paths between the upper level of the repumping transition and the dark strontium metastable states, which could not be explained by electric dipole transition rates calculated in the literature. Moreover, our analysis pinpoints the role of the atomic movement in limiting the efficiency of the atomic repumping of the Sr metastable states.
The theoretical description of the external degrees of freedom of atoms trapped inside a magnetooptical trap (MOT) often relies on the decoupling of the evolution of the internal and external degrees of freedom.That is possible thanks to much shorter timescales typically associated with the first ones.The electronic structure of alkaline-earth atoms, on the other hand, presents ultra-narrow transitions and metastable states that makes such an approximation invalid in the general case.In this article, we report on a model based on open Bloch equations for the evolution of the number of atoms in a magneto-optical trap.With this model we investigate the loading of the strontium blue magnetooptical trap under different repumping schemes, either directly from a Zeeman slower, or from an atomic reservoir made of atoms in a metastable state trapped in the magnetic quadrupolar field.The fluorescence observed on the strong 461 nm transition is recorded and quantitatively compared with the results from our simulations.The comparison between experimental results and calculations within our model allowed to identify the existence of the decay paths between the upper level of the repumping transition and the dark strontium metastable states, which could not be explained by electric dipole transition rates calculated in the literature.Moreover, our analysis pinpoints the role of the atomic movement in limiting the efficiency of the atomic repumping of the Sr metastable states.
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.