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Applied Sciences

Analysis of Narrow-Line Laser Cooling and Trapping of Sr Atoms in Microgravity Environments

Jie Ren, Hui Liu, Xiaotong Lu, Hong Chang

Published Jul 17, 2020• 4 citations
Publisher

Abstract

Obtaining ultracold alkaline earth(-like) atoms in space encounters the problem of performing narrow-line laser cooling in microgravity environments ( μ -gEs). This paper reports an analysis of the magneto-optical trap (MOT) based on the narrow-line transition in 88 Sr, while paying special attention to the role of the gravity. This analysis suggests the MOTs based on narrow-line transitions cannot be cold and dense enough in a μ -gE. We thus propose a strategy: that one can use a dual-frequency MOT to realize a low-temperature, high density, and high transfer efficiency, narrow-line red MOT in μ -gEs.

DOI
10.3390/app10144928

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Obtaining ultracold alkaline earth(-like) atoms in space encounters the problem of performing narrow-line laser cooling in microgravity environments ( μ -gEs). This paper reports an analysis of the magneto-optical trap (MOT) based on the narrow-line transition in 88 Sr, while paying special attention to the role of the gravity. This analysis suggests the MOTs based on narrow-line transitions cannot be cold and dense enough in a μ -gE. We thus propose a strategy: that one can use a dual-frequency MOT to realize a low-temperature, high density, and high transfer efficiency, narrow-line red MOT in μ -gEs.
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.
The microgravity environment modifies the resonance condition that is formed with the participation of gravity in the 689 nm narrow-line magneto-optical trap (red MOT) of a ground-based strontium optical clock. Consequently, a cooling scheme optimized under standard 1g ground-commissioning conditions cannot be directly transferred to an in-orbit space optical clock. In this work, we establish a one-dimensional semiclassical Monte Carlo stochastic dynamics model along the ground-gravity direction to study the cooling dynamics, spatial distribution, and position compensation of a 88Sr narrow-line MOT under microgravity. The simulations show that directly applying ground-optimized parameters in microgravity leads to heating, cloud broadening, and displacement from the lattice position calibrated on the ground. Reducing the detuning can produce a smaller spatial distribution, but this behavior is mainly phase-space truncation and cannot maintain a sufficient effective atom number. We propose a dynamic-control scheme in which the laser detuning and intensity are continuously ramped, achieving an atomic temperature below 3 μK under microgravity while retaining more than 98% of the atoms in the MOT, outperforming the 1g ground-gravity reference case. On this basis, magnetic-gradient screening is used to obtain a cloud size matched to the optical-lattice mode. After an additional compensation magnetic field is introduced, the deviation between the narrow-line MOT center and the lattice position is ∼2.4 μm, far smaller than the transverse scale of the lattice beam. These results provide a physical picture and theoretical guidance for in-orbit commissioning of the narrow-line MOT in a strontium optical clock on the Chinese Space Station.
We demonstrate for the first time the two-color cooling and trapping of alkaline-earth atoms in a grating magneto-optical trap (gMOT). The trap is formed by a single incident laser beam together with four secondary beams that are generated via diffraction from a nanostructured wafer. A grating structure for a gMOT operating with strontium atoms is optimized and fabricated. We trap 10610^6106 88^{88}88Sr atoms on the 1^11S0_00​ →\rightarrow→ 1^11P1_11​ transition at 461  nm461\;\mathrm{nm}461nm and transfer 25  %25\;\%25% of these atoms to the second cooling stage on the narrower 1^11S0_00​ →\rightarrow→ 3^33P1_11​ intercombination transition at 689  nm689\;\mathrm{nm}689nm, preparing a sample of 2.5×1052.5\times 10^52.5×105 atoms at 5  μ5\;μ5μK. These results demonstrate for the first time the applicability of the gMOT technology in conjunction with two widely differing wavelengths and enable the continued miniaturization of alkaline-earth based quantum technologies like optical atomic clocks.
Laser cooling 88^{88}88Sr to microkelvin temperature with an integrated-photonics system
We report on experiments generating a magneto-optical trap (MOT) of 88-strontium (88^{88}88Sr) atoms at microkelvin temperature, using integrated-photonics devices. With metasurface optics integrated on a fused-silica substrate, we generate six-beam, circularly polarized, counter-propagating MOTs on the blue broad-line, 461 nm, and red narrow-line, 689 nm, Sr cooling transitions without bulk optics. By use of a diverging beam configuration, we create up to 10 mm diameter MOT beams at the trapping location. To frequency stabilize and linewidth narrow the cooling lasers, we use fiber-packaged, integrated nonlinear waveguides to spectrally broaden a frequency comb. The ultra-coherent supercontinuum of the waveguides covers 650 nm to 2500 nm, enabling phase locks of the cooling lasers to hertz level linewidth. Our work highlights the possibility to simplify the preparation of an ultracold 88Sr gas for an optical-lattice clock with photonic devices. By implementing a timing sequence for control of the MOT lasers and the quadrupole magnetic-field gradient, we collect atoms directly from a thermal beam into the blue MOT and continuously cool into a red MOT with dynamic detuning and intensity control. There, the red MOT temperature is as low as 2 μ2~μ2 μK and the overall transfer efficiency up to 16%. We characterize this sequence, including an intermediate red MOT with modulated detuning. Our experiments demonstrate an integrated photonics system capable of cooling alkaline-earth gases to microkelvin temperature with sufficient transfer efficiencies for adoption in scalable optical clocks and quantum sensors.
Sr-atoms are of special interest for an optical frequency standard since a large number of atoms can be cooled down efficiently to ultra-low temperatures in a magneto-optical trap (MOT). The ultra-narrow line on the forbidden 1 S 0 -> 3 P 0 transition of 87 Sr ( 3 P 0 natural lifetime is approx. 100 s), and the ability to interrogate Sr atoms in a perturbation free dipole trap promise un-precedented low inaccuracy level and relative frequency instability, well below 10 -15 tau -1/2 . We will show the realization of an efficient atomic source of cold Sr atoms and the construction of an ultra-stable laser for the interrogation of the atoms.
Laser cooling has become a powerful tool for producing quantum gases and has enabled the study of a wide range of phenomena only accessible at ultracold temperatures. This thesis will describe the implementation and characterization of a magneto-optical trap (MOT) for laser cooling strontium to microkelvin temperatures operating on the narrow 689nm transition (the “red” MOT). The red MOT is the second cooling stage required to create quantum degenerate gases of strontium, bridging the temperatures and densities produced by the first cooling stage (the 461 nm “blue” MOT) and the shallow conservative traps for evaporation to quantum degeneracy.