A robust and modular cesium magneto-optical trap using divergent laser beams

The gist: The authors demonstrate a robust and modular cesium magneto-optical trap optimized for inertial quantum sensing and precision metrology applications using diverging laser beams.

System Design and Robustness

The MOT achieves stable operation through two primary design features: (1) using diverging cooling laser beams to reduce unwanted reflections, and (2) mounting optical elements in a fiber-coupled, compact, and modular cage rigidly attached to the vacuum chamber The system successfully traps 4×108 cesium (Cs) atoms with less than 20% fluctuation in atom number over a dataset taken for 50 hours This configuration preserves independent control of all laser beam properties—size, collimation, pointing, position, polarization, and intensity—while reducing space taken by beam delivery systems and improving mechanical stability

Cooling and Trapping Mechanisms

The system utilizes polarization-gradient cooling (PGC) to produce atom samples with temperatures below 10 μK, similar to those achieved in conventional MOTs that use collimated laser beams This result is achieved by using diverging laser beams, which have the advantage of reducing instabilities in atom sample numbers and spatial distribution typically caused by reflections from vacuum chamber viewports Furthermore, the system observes trapping of 2×10 7 Cs atoms in a non-conventional MOT geometry where the cooling laser beams are diagonal to the principal axis of the quadrupole magnetic field

Experimental Setup and Components

The conventional MOT configuration uses three pairs of red-detuned, collimated, counterpropagating laser beams along each Cartesian axis to Doppler cool the atoms through velocity-dependent radiation pressure forces The MOT light contains two different tones: a “trap” component red-detuned by ΔTrap= 2pi×12 MHz = 2.3 Γ from the 62S1/2, F = 4 → 62P3/2, F′ = 5 cycling transition, and a “repump” component resonant with the 62S1/2, F = 3 → 62P3/2 transition at 852 nm The laser beam delivery system is designed to be compact, modular, robust against drift, and can be disassembled and reassembled with minimal need for optical realignment

Optimization Results

The optimization of the loading process showed that the atom number increases with larger magnetic field gradients up to an optimum value of 6 G/cm, after which higher values cause instability and decreased atom number The optimized frequency detuning found for the trap laser is ΔTrap= 2pi×12 MHz or 2.3 Γ The PGC process was optimized by scanning beam intensity, frequency detuning, and PGC duration to achieve low temperatures The resulting temperatures obtained are comparable to those achieved by conventional MOTs that use collimated laser beams, confirming the effectiveness of the PGC process in cooling MOTs that use diverging laser beams

Stability and Alternative Geometries

Long-term operation over more than 50 hours showed that the atom number is stable to within 20%, with fluctuations correlated with variations in the lab temperature of up to 5°F A diagonal-beam MOT configuration was also investigated, trapping up to 2.6×10 7 Cs atoms and achieving temperatures of 10 μK However, this diagonal-beam configuration is much more sensitive than the conventional MOT configuration to changes in the polarization and pointing of the laser beams, causing a reduction in atom number over timescales of tens of minutes This resulting Cs MOT is suitable for atom-interferometric applications in precision quantum metrology and quantum inertial sensing

Conclusion

The authors have described an implementation of a Cs MOT that achieves long-term stability in MOT atom number and temperature at the 20% level by using (1) diverging laser beams to suppress reflections from the vacuum chamber and (2) a compact, robust, and modular cage-system to deliver laser beams They have optimized the MOT atom number and temperature by tuning beam intensity, frequency detuning, magnetic field gradient, and loading time They achieve samples with several ×10 8 atoms at sub-Doppler temperatures of less than 10 μK They also demonstrated a diagonal-beam MOT configuration, which can create atom samples with more than 10 7 atoms and temperatures near 10 μK However, this configuration is more susceptible to long-term drift in experimental parameters The resulting Cs MOT is suitable for atom-interferometric applications in precision quantum metrology and quantum inertial sensing.

Acknowledgments

The authors thank Poul Jessen, Jason Jones and Jae Hoon Lee for helpful discussions; Charles Condos, Stanley Pau and Dalziel Wilson for sharing equipment; Richard Schickling, Paul Schickling, Jerry Edwards (Universal Cryogenics), and Brayden James for technical assistance; Stone Guymon and Scarlett Bolton for contributions to experiment assembly; Justin Murante for software support This material is based upon work supported by the University of Arizona. P. Luna is supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-2137419. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Conflict of Interest The authors have no conflicts to disclose. Author Contributions Paola Luna Data Curation (lead), Formal Analysis (lead), Investigation (lead), Methodology (supporting), Project Administration (supporting), Visualization (lead), Writing/Original Draft Preparation (lead), Writing – review and editing (equal) Kenneth Nakasone Investigation (lead), Methodology (supporting), Project Administration (supporting), Resources equal, Validation equal, Writing – review and editing equal Andrei Zhukov Investigation (lead), Resources equal, Validation equal, Writing/Original Draft Preparation supporting, Writing – review and editing equal Philip Wondrak Data Curation (lead), Investigation supporting, Writing – review and editing equal Brian P. Anderson Conceptualization (lead), Writing – review and editing equal Benjamin F. Kanzer Investigation supporting, Writing – review and editing equal Cristian D. Panda Conceptualization (lead), Investigation lead, Methodology lead, Project Administration lead, Supervision lead, Writing/Original Draft Preparation supporting, Writing – review and editing equal References 1. W. Ren et al., Natl. Sci. Rev., 7(12), 1828–1836 (2020) 2. R. Elvin et al., Opt. Express, 27, 38359–38366 (2019) 3. M. Kasevich and S. Chu, Appl. Phys. B, 54, 321–332 (1992) 4. J. M. McGuirk et al., Phys. Rev. A, 65, 033608 (2002) 5. G. Rosi et al., Nature, 510, 518–521 (2014) 6. X. Wu et al., Sci. Adv., 5, eaax0800 (2019) 7. J. R. Williams et al., Nat. Commun., 15, 6414 (2024) 8. Y. Zhou et al., Phys. Rev. A, 110, 033313 (2024) 9. M. Filzinger et al., Phys. Rev. Lett., 130, 253001 (2023) 10. M. Abe et al., Quantum Sci. Technol., 6, 044003 (2021) 11. P.-Y. Zhao et al., Chin. Phys. Lett., 25, 3631–3633 (2008) 12. P.-Y. Zhao et al., Chin.

1. Bold beam geometry optimization for robust trapping: The system can be optimized by selecting laser beam divergence to reduce unwanted reflections from the chamber viewports, which directly addresses spatial gradients that affect MOT stability. This ensures a more reliable and stable atom sample number, achieving less than 20% fluctuation in atom number over a dataset taken for 50 hours.

2. Polarization-gradient cooling (PGC) implementation: The improved system can implement PGC using the specific laser parameters found to be optimal, such as ΔTrap= 2pi×12 MHz or 2.3 Γ and IPGC = 0.25 mW/cm2, resulting in temperatures comparable to conventional systems, achieving sub-Doppler temperatures of less than 10 μK.

3. Modular, fiber-coupled optical delivery: The AI system can utilize the compact and modular cage system where each laser beam is built from optical elements that can be mounted directly on the vacuum chamber, preserving independent control over properties like size, collimation, pointing, position, polarization, and intensity. This allows for rapid assembly and disassembly while maintaining high performance.

4. Diagonal-beam MOT capability: The system can be configured as a diagonal-beam MOT where the propagation axes of four trapping laser beams are at approximately 45° relative to the principal axis of the quadrupole field. This configuration can trap up to 2.6×107 Cs atoms and achieve temperatures of 10 μK, enabling experiments requiring non-conventional magnetic field alignments for inertial sensing.

5. Long-term operational stability monitoring: The system can be designed with feedback loops that monitor parameters like laser pointing and polarization to mitigate drift, addressing the observation that drift in these experimental parameters causes a reduction in atom number and changes in the spatial distribution of the atom sample over timescales of tens of minutes.

Magneto-optical traps (MOTs) are a workhorse technology for neutral-atom quantum sensing, simulation, and computing. Here, we demonstrate a MOT optimized for inertial quantum sensing and precision metrology applications. Our MOT traps 4 times10 8 cesium (Cs) atoms in a robust, modular system that achieves stable operation through two design features: (1) we use divergent cooling laser beams to reduce unwanted reflections and (2) optical elements are mounted in a fiber-coupled, compact, and modular cage rigidly attached to the vacuum chamber. Following polarization-gradient cooling (PGC), the system produces atom samples with temperatures below 10 μ K, similar to those achieved in conventional MOTs that use collimated laser beams. In addition, we observe trapping of 2 times10 7 Cs atoms in a non-conventional MOT geometry, where the cooling laser beams are diagonal to the principal axis of the quadrupole magnetic field.