Ruchka et al. (Quantum Sci. Technol. 7, 045011, 2022) introduced optical tweezers for ultracold atoms formed by micrometer-scale lenses printed directly onto the tips of optical fibers, and demonstrated vacuum compatibility, robustness against rubidium vapor, and a magneto-optical trap located about 125 micrometers from the fiber tip - but not the trapping of single atoms. The open problem is the experimental completion of this concept: load a single atom into the dipole trap of the printed fiber-lens tweezer, detect it deterministically through the fluorescence collected by the same printed lens and fiber, and manipulate it in the trap. This requires overcoming the background-light scatter of macroscopic cooling beams that currently makes such single-atom detection challenging; the experimental realization of the designed very-high-NA printed lenses (in principle up to NA 0.99) with chromatic focal matching, and the long-term environmental qualification of the polymer micro-optics, remain open supporting directions of the problem. As of August 2026 no published work has demonstrated any of these steps for atoms.

Background

Optical tweezers - tightly focused laser beams whose dipole force confines a polarizable particle near the focus - have become a standard tool for trapping and manipulating single neutral atoms, with applications in quantum simulation, quantum computing, optical clocking, and single-photon generation. In a far-detuned dipole trap, atoms are confined in the intensity maximum of a red-detuned beam with minimal photon scattering, so the trap is conservative and long-lived. Conventionally, such tweezers are generated and observed with bulk high-numerical-aperture (NA) microscope objectives and many free-space optical elements, which makes the setups large, expensive, and sensitive to laboratory conditions. The numerical aperture of the focusing optic sets how tightly the light can be focused: higher NA gives a smaller trap waist, a deeper trap for given power, and a larger fraction of the atom's fluorescence that can be collected back through the same optic.

Two-photon polymerization is a 3D printing technique in which a femtosecond laser is focused into a photoresist so that polymerization occurs only in the tiny focal volume (a 'voxel'); scanning the focus writes arbitrary free-form micro-optics, including lenses printed directly onto the cleaved tip of an optical fiber (Gissibl et al., Nature Communications 7, 11763, 2016). Such printed fiber lenses had been used for endoscopic imaging, particle trapping in liquids, and photon collection from quantum dots. A magneto-optical trap (MOT) is the standard laser-cooling configuration - six beams plus a magnetic-field gradient - that collects and cools a dilute atomic gas close to the Doppler limit; it is typically the first stage from which single atoms are transferred into dipole traps.

Ruchka et al. (Quantum Sci. Technol. 7, 045011, 2022) proposed and began to validate a tweezer concept in which a lens printed on a fiber tip both focuses a trapping beam (for rubidium, typically 1064 nm) into a microscopic dipole trap and collects the trapped atom's fluorescence (780 nm) back into the fiber. They characterized two printed designs experimentally (an aspheric lens of NA = 0.45 and a total-internal-reflection lens), verified vacuum compatibility, showed no performance degradation after long-term exposure to high-pressure rubidium vapor, measured excellent agreement (<2%) between designed and measured focal properties, and observed no Raman-shifted light that would spoil single-photon-level detection. Ray-tracing simulations indicated that the printed-lens concept allows, in principle, NAs up to about 0.99, corresponding to fluorescence-detection efficiencies exceeding 40%, with the trap and fluorescence focal spots matched to within 1-2 micrometers. In an exploratory vacuum experiment they formed a macroscopic six-beam MOT of about 10^6 atoms centered roughly 125 micrometers from the fiber tip. What they did not demonstrate is the actual single-atom sequence: no atom has been loaded into, detected in, or manipulated in a printed-lens trap.

The authors identified the immediate obstacle to single-atom detection, together with further open directions: (a) background light scattered from the large-diameter beams of the macroscopic MOT renders the detection of the faint fluorescence of a single atom in the fiber trap challenging, a limitation they expect can be fully overcome using integrated, miniaturized cooling and trapping setups such as grating, pyramidal, or chip-based MOTs; (b) the very-high-NA printed designs (up to NA = 0.99) exist so far only on paper, with surface quality, chromatic control, and working-distance trade-offs untested; and (c) the anticipated deployment environments - alkali-vapor exposure over long timescales, cryostats, and field or space operation - impose chemical, optical, and thermal stability requirements on the polymer structures that remain largely unqualified. In parallel, an adjacent flat-optics route has advanced rapidly: single atoms have been trapped with a dielectric metasurface lens (Hsu et al., PRX Quantum 3, 030316, 2022), in holographic metasurface tweezer arrays with more than 1000 trapped strontium atoms (Holman et al., Nature 649, 859, 2026), and in an in-vacuum metasurface microtrap array (Li et al., Optics Express 33, 51085, 2025). These metasurface demonstrations, however, use macroscopic chip-scale optics illuminated through free space; they do not implement the fiber-integrated printed refractive lens with fluorescence collected through the same fiber, and thus leave the printed fiber-lens concept itself uncompleted.

Problem Statement

Demonstrate experimentally the complete single-atom sequence in a 3D-printed fiber-lens optical tweezer, as posed by Ruchka et al.: (1) load a single laser-cooled atom into the microscopic dipole trap formed by a lens printed on an optical-fiber tip; (2) detect that atom deterministically through the fluorescence collected by the same printed lens and fiber, at the single-photon level; and (3) manipulate and control the atom in the printed trap, meaning that the atom's position or internal state is actively changed in a non-trivial way (for example by transporting the atom within the trap, or by preparing it in and reading it out from a chosen internal state). Achieving this requires overcoming the enabling issue the authors identified as the current obstacle: (a) the background light scattered by macroscopic magneto-optical-trap (MOT) cooling beams, which currently renders single-atom detection challenging, must be suppressed sufficiently for the detection to be deterministic. Two further supporting directions remain open: (b) experimental realization of the designed very-high-numerical-aperture (NA; in principle up to NA ≈0.99\approx 0.99) printed lens geometries with the required surface quality, chromatic matching of the trapping and fluorescence focal spots, and working distance; and (c) qualification of the long-term chemical and optical stability of the polymer micro-optics under the relevant operating environments, including extended alkali-vapor exposure, cryogenic operation, and field-deployment thermal and mechanical conditions. The scientific target is a single atom observed to load into, survive in, be detected in, and be actively manipulated - its position or internal state non-trivially changed - within the printed-lens trap, with the single-atom fluorescence detection succeeding against the background light of the cooling beams by whatever means that background is suppressed. The printed lens is any already-realized design; realizing the highest-NA design class remains an open direction (b).

References

  1. Pavel Ruchka, Sina Hammer, Marian Rockenhäuser, Ralf Albrecht, Johannes Drozella, Simon Thiele, Harald Giessen, Tim Langen, 'Microscopic 3D printed optical tweezers for atomic quantum technology', Quantum Science and Technology 7, 045011 (2022). DOI: 10.1088/2058-9565/ac796c; arXiv:2206.11090, https://arxiv.org/abs/2206.11090
  2. Ting-Wei Hsu, Wenqi Zhu, Tobias Thiele, Mark O. Brown, Scott B. Papp, Amit Agrawal, Cindy A. Regal, 'Single atom trapping in a metasurface lens optical tweezer', PRX Quantum 3, 030316 (2022). DOI: 10.1103/PRXQuantum.3.030316; arXiv:2110.11559, https://arxiv.org/abs/2110.11559
  3. Seyed Khalil Alavi, Jose Manuel Monterrosas Romero, Pavel Ruchka, Sara Jakovljević, Harald Giessen, Sungkun Hong, 'Compact vacuum levitation and control platform with a single 3D-printed fiber lens', Photonics Research 14(1), 201-212 (2026). DOI: 10.1364/PRJ.569207; arXiv:2504.15734, https://arxiv.org/abs/2504.15734
  4. Aaron Holman, Yuan Xu, Ximo Sun, Jiahao Wu, Mingxuan Wang, Zezheng Zhu, Bojeong Seo, Nanfang Yu, Sebastian Will, 'Trapping of single atoms in metasurface optical tweezer arrays', Nature 649, 859-865 (2026). DOI: 10.1038/s41586-025-09961-5; arXiv:2411.05321, https://arxiv.org/abs/2411.05321
  5. Donghao Li, Qiming Liao, Beining Xu, Thomas Zentgraf, Emmanuel Narvaez Castaneda, Yaoting Zhou, Keyu Qin, Zhongxiao Xu, Heng Shen, Lingling Huang, 'In vacuum metasurface for optical microtrap array', Optics Express 33, 51085 (2025). DOI: 10.1364/OE.580201; arXiv:2407.05918, https://arxiv.org/abs/2407.05918
  6. F. Wang, N. Cooper, D. Johnson, B. Hopton, T. M. Fromhold, R. Hague, A. Murray, R. McMullen, L. Turyanska, L. Hackermüller, 'Additive Manufacturing for Advanced Quantum Technologies', arXiv:2503.11570 (2025), https://arxiv.org/abs/2503.11570
  7. T. Gissibl, S. Thiele, A. Herkommer, H. Giessen, 'Sub-micrometre accurate free-form optics by three-dimensional printing on single-mode fibres', Nature Communications 7, 11763 (2016). DOI: 10.1038/ncomms11763, https://doi.org/10.1038/ncomms11763
  8. Chengyu Fang, Minjeong Kim, Mark Saffman, Jennifer T. Choy, Mikhail Kats, 'Metasurfaces for neutral-atom trapping', arXiv:2605.30498 (2026), https://arxiv.org/abs/2605.30498
  9. Yu Zhang, Jianan Liu, Fenghui Hu, Zhen Wang, Zhihai Liu, Yifan Qin, Yaxun Zhang, Jianzhong Zhang, Xinghua Yang, Libo Yuan, 'Optical fiber tweezers: From fabrication to applications', Optics and Laser Technology 175, 110681 (2024). DOI: 10.1016/j.optlastec.2024.110681, https://doi.org/10.1016/j.optlastec.2024.110681