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Dynamic control of a narrow-line red magneto-optical trap for strontium atoms under microgravity

Jie Ren, Chihua Zhou, Benquan Lu, Wei Tan, Jian Xia, Guodong Zhao, Hongxin Zou, Hong Chang

Published Aug 1, 2026• 1 citation

Abstract

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.

DOI: 10.1063/5.0346328

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•Apr 6, 2007•0 citations

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.

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

^{88}

Sr to microkelvin temperature with an integrated-photonics system

We report on experiments generating a magneto-optical trap (MOT) of 88-strontium (

^{88}

Sr) 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~μ

K 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.

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.

Optical atomic clocks with precisions and accuracies in the 10

^-

^1

^8

level [1] are now the most advanced man-made timekeeping devices. They outperform the microwave cesium atomic clocks that realize the SI definition of the second. Scaling down the size of optical atomic clocks may open the door to a range of industrial and space applications. In this thesis, the design and preliminary results of a compact strontium cooling system are presented. In the first cooling stage, the high power 461 nm laser with 300 mW output features a modular design, while smaller laser sources for demonstrating a strontium magneto-optical trap have also been investigated. An innovative design that couples a spectroscopy cell directly into the scientific chamber reduces the overall size and power consumption of the system. Additionally, using strontium oxide as a source of strontium atoms suitable for optical clocks has achieved initial success. For the first time, a single-beam MOT configuration is applied to strontium. In this novel apparatus, the blue MOT and red MOT broadband cooling phases are able to trap 5x10

^6

and 1000 atoms, respectively. This work shows promising progress towards developing a functional miniaturized strontium optical lattice clock.

The intercombination line of Strontium at 689nm is successfully used in laser cooling to reach the photon recoil limit with Doppler cooling in a magneto-optical traps (MOT). In this paper we present a systematic study of the loading efficiency of such a MOT. Comparing the experimental results to a simple model allows us to discuss the actual limitation of our apparatus. We also study in detail the final MOT regime emphasizing the role of gravity on the position, size and temperature along the vertical and horizontal directions. At large laser detuning, one finds an unusual situation where cooling and trapping occur in the presence of a high bias magnetic field.