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arXiv:2001.05210 · Physical Review A

Midinfrared magneto-optical trap of metastable strontium for an optical lattice clock

Richard Hobson, William Bowden, Alvise Vianello, Ian R. Hill, Patrick Robert Gill

Published Jan 14, 2020• 23 citations
arXivHTMLPDF

Abstract

We report on the realization of a magneto-optical trap (MOT) for metastable strontium operating on the 2.92-$\ensuremath{\mu}\mathrm{m}$ transition between the energy levels $5s5p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{2}$ and $5s4d{\phantom{\rule{0.16em}{0ex}}}^{3}{D}_{3}$. The strontium atoms are initially captured in a MOT operating on the 461-nm transition between the energy levels $5{s}^{2}{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}$ and $5s5p{\phantom{\rule{0.16em}{0ex}}}^{1}{P}_{1}$, prior to being transferred into the metastable MOT and cooled to a final temperature of 6 $\ensuremath{\mu}\mathrm{K}$. Challenges arising from aligning the midinfrared and 461-nm light are mitigated by employing the same pyramid reflector to realize both MOTs. Finally, the 2.92-$\ensuremath{\mu}\mathrm{m}$ transition is used to realize a full cooling sequence for an optical lattice clock, in which cold samples of $^{87}\mathrm{Sr}$ are loaded into a magic-wavelength optical lattice and initialized in a spin-polarized state to allow high-precision spectroscopy of the $5{s}^{2}{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}$ to $5s5p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{0}$ clock transition.

DOI
10.1103/physreva.101.013420

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We report on the realization of a magneto-optical trap (MOT) for metastable strontium operating on the 2.92-\ensuremathμm\ensuremath{\mu}\mathrm{m}\ensuremathμm transition between the energy levels 5s5p3P25s5p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{2}5s5p3P2​ and 5s4d3D35s4d{\phantom{\rule{0.16em}{0ex}}}^{3}{D}_{3}5s4d3D3​. The strontium atoms are initially captured in a MOT operating on the 461-nm transition between the energy levels 5s21S05{s}^{2}{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}5s21S0​ and 5s5p1P15s5p{\phantom{\rule{0.16em}{0ex}}}^{1}{P}_{1}5s5p1P1​, prior to being transferred into the metastable MOT and cooled to a final temperature of 6 \ensuremathμK\ensuremath{\mu}\mathrm{K}\ensuremathμK. Challenges arising from aligning the midinfrared and 461-nm light are mitigated by employing the same pyramid reflector to realize both MOTs. Finally, the 2.92-\ensuremathμm\ensuremath{\mu}\mathrm{m}\ensuremathμm transition is used to realize a full cooling sequence for an optical lattice clock, in which cold samples of 87Sr^{87}\mathrm{Sr}87Sr are loaded into a magic-wavelength optical lattice and initialized in a spin-polarized state to allow high-precision spectroscopy of the 5s21S05{s}^{2}{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}5s21S0​ to 5s5p3P05s5p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{0}5s5p3P0​ clock transition.
We report the magnetic trapping of metastable 3P2{}^{3}{P}_{2}3P2​ atomic strontium. Atoms are cooled in a magneto-optical trap (MOT) operating on the dipole-allowed ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ transition at 461 nm. Decay via ${}^{1}{\stackrel{\ensuremath{\rightarrow}}{{P}_{1}}}^{1}{\stackrel{\ensuremath{\rightarrow}}{{D}_{2}}}^{3}{P}_{2}$ continuously loads a magnetic trap formed by the quadrupole magnetic field of the MOT. Over 108{10}^{8}108 atoms at a density of $8\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3}$ and temperature of 1 mK are trapped. The atom temperature is significantly lower than what would be expected from the kinetic and potential energies of atoms as they are transferred from the MOT. This suggests the occurrence of thermalization and evaporative cooling in the magnetic trap.
Abstract We realize a two-stage, hexagonal pyramid magneto-optical trap (MOT) with strontium, and demonstrate loading of cold atoms into cavity-enhanced 1D and 2D optical lattice traps, all within a single compact assembly of in-vacuum optics. We show that the device is suitable for high-performance quantum technologies, focusing especially on its intended application as a strontium optical lattice clock. We prepare 2 × 10 4 spin-polarized atoms of 87 Sr in the optical lattice within 500 ms; we observe a vacuum-limited lifetime of atoms in the lattice of 27 s; and we measure a background DC electric field of 12 V m −1 from stray charges, corresponding to a fractional frequency shift of (−1.2 ± 0.8) × 10 −18 to the strontium clock transition. When used in combination with careful management of the blackbody radiation environment, the device shows potential as a platform for realizing a compact, robust, transportable optical lattice clock with systematic uncertainty at the 10 −18 level.
We report a two-color magneto-optical trap that continuously operates on the (5s2) 1S0−(5s5p) 1P1\mathrm{(5s^2)\:^1S_0-(5s5p)\:^1P_1}(5s2)1S0​−(5s5p)1P1​ and (5s5p) 3P2−(5s5d) 3D3\mathrm{(5s5p)\:^3P_2-(5s5d)\:^3D_3}(5s5p)3P2​−(5s5d)3D3​ transitions in 88Sr\mathrm{^{88}Sr}88Sr. Owing to the sub-Doppler cooling from the metastable (5s5p) 3P2\mathrm{(5s5p)\:^3P_2}(5s5p)3P2​ state, a bimodal momentum distribution is observed, with the cold part of the ensemble reaching a temperature of 105 μ\mathrmμμK. A detailed simulation is established to explain the data. The absolute frequency of the (5s5p) 3P2−(5s5d) 3D3\mathrm{(5s5p)\:^3P_2-(5s5d)\:^3D_3}(5s5p)3P2​−(5s5d)3D3​ 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 μ\mathrmμμK.
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.
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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.