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arXiv:1806.00838

A Robust Narrow-Line Magneto-Optical Trap using Adiabatic Transfer

Juan A. Muniz, Matthew A. Norcia, Julia Cline, James K. Thompson

Published Jun 3, 2018• 10 citations
arXivHTMLPDF

Abstract

We characterize the properties of a new form of magneto-optical trap (MOT) that relies on non-equilibrium population dynamics associated with narrow-linewidth optical transitions. We demonstrate this mechanism using the 7.5 kHz linewidth transition in both bosonic and fermionic strontium isotopes. In contrast to standard narrow-line MOTs based on imbalances in equilibrium scattering rates, our system benefits from a less complex laser system, a larger capture fraction from higher temperature samples, robustness against experimental perturbations and the ability to operate in the presence of large inhomogeneous broadening of the atomic transition frequency.

DOI
10.48550/arxiv.1806.00838

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We characterize the properties of a new form of magneto-optical trap (MOT) that relies on non-equilibrium population dynamics associated with narrow-linewidth optical transitions. We demonstrate this mechanism using the 7.5 kHz linewidth transition in both bosonic and fermionic strontium isotopes. In contrast to standard narrow-line MOTs based on imbalances in equilibrium scattering rates, our system benefits from a less complex laser system, a larger capture fraction from higher temperature samples, robustness against experimental perturbations and the ability to operate in the presence of large inhomogeneous broadening of the atomic transition frequency.
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.
We report on the realization of a magneto-optical trap (MOT) for europium atoms using a narrow-line cooling transition with a natural linewidth of 97 kHz. Europium is a strongly dipolar species with interesting properties, such as the presence of hyperfine structure and the absence of electronic orbital angular momentum. Our starting point of the MOT is continuous capturing and cooling of optically pumped metastable europium atoms. We have employed simultaneous MOT for the metastable and ground-state atoms. The trapped metastable atoms are successively pumped back to the ground state and then continuously loaded to the narrow-line MOT, where up to $4.7\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms are captured. A spin-polarized sample at a temperature of $6\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and with a peak number density of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ is obtained through the compression process, resulting in a phase space density of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$.
We realize narrow-linewidth magneto-optical traps of 46^{46}46Ti, 48^{48}48Ti and 50^{50}50Ti atoms based on a 1040 nm-wavelength transition, cooling atoms to a minimum temperature in one dimension of Tz=990(20)T_z=990(20)Tz​=990(20) nK and a three-dimensional temperature of T3D=1.28(7)T_\mathrm{3D}=1.28(7)T3D​=1.28(7) μμμK. Atoms are pre-cooled in a broad-line magneto-optical trap and then transferred with about 25% efficiency to the narrow-line trap. We operate the narrow-line trap in two stages over 85 ms. First, a single cooling beam, blue-detuned from the narrow-linewidth atomic resonance, optically pumps and traps the atoms on a two-dimensional surface where the Zeeman shift from the applied spherical quadrupole magnetic field brings the light nearly to resonance. Second, four additional beams, counter-propagating in the transverse directions, cool and compress the atoms in all dimensions. The high magnetic moment of the laser cooling state makes the dynamics of the narrow-line titanium trap similar to those of other magnetic atoms. We measure the lifetime of the excited state of the transition to be τ=8.2(9)τ=8.2(9)τ=8.2(9) μμμs, indicating a transition linewidth of γ/2π=20(2)γ/2π=20(2)γ/2π=20(2) kHz, and also measure isotope shifts on the narrow-line transition. We use Stern-Gerlach separation on the ultracold Ti gas to measure the mJm_JmJ​-distribution in the narrow-line magneto-optical trap, finding over 98% of the atoms in the stretched spin state.
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.
Laser cooling on weak transitions is a useful technique for reaching ultracold temperatures in atoms with multiple valence electrons. However, for strongly magnetic atoms a conventional narrow-line magneto-optical trap (MOT) is destabilized by competition between optical and magnetic forces. We overcome this difficulty in Er by developing an unusual narrow-line MOT that balances optical and magnetic forces using laser light tuned to the blue side of a narrow (8 kHz) transition. The trap population is spin polarized with temperatures reaching below 2 muK. Our results constitute an alternative method for laser cooling on weak transitions, applicable to rare-earth-metal and metastable alkaline earth elements.
We report the experimental realization of strontium magneto-optical trap (MOT) operating on the intercombination transition 1 S 0 − 3 P 1 at 689 nm, namely red MOT. A 689 nm laser used for cooling and trapping is injection locked to a master laser, whose linewidth is narrowed to 150 Hz by locking to a high finesse optical reference cavity. 88 Sr atoms pre-cooled and trapped by the broad 1 S 0 − 1 P 1 transition at 461 nm are transferred to the red MOT with the help of a time sequence controller. The transfer ratio is about 20% and the red MOT's temperature is estimated to be less than 20 μK by the time-of-flight (TOF) image analysis.