Researchers at the Fritz Haber Institute of the Max Planck Society, led by Dr. Sid Wright, have successfully demonstrated the first magneto-optical trap of a chemically stable, spin-singlet molecule: aluminum monofluoride (AlF). This breakthrough, marks a significant advance in ultracold molecular physics, enabling new avenues for precision spectroscopy and quantum simulation.
Padilla-Castillo, J. E., Wright, S., Truppe, S., & Fritz-Haber-Institut der Max-Planck-Gesellschaft. (2025). Magneto-optical trapping of aluminum monofluoride. arXiv. https://doi.org/10.48550/arxiv.2506.02266
The team produced a molecular beam of AlF and applied laser cooling techniques, using four laser systems near 227.5 nanometers in the deep ultraviolet. This represents the shortest wavelength employed so far to trap any atom or molecule. The researchers were able to selectively trap AlF in three different rotational quantum levels by fine-tuning the laser frequencies. The ability to access multiple rotational states distinguishes AlF from previously laser-cooled molecules, which have generally been limited to a single rotational level.
Fritz Haber Institute of the Max Planck Society, led by Dr. Sid Wright stated,
“The dream for us would be to trap AlF from a compact, inexpensive vapor source, similar to what is used for the alkali atoms. In initial experiments, we have seen that AlF can survive collisions with room temperature vacuum walls; even thermalizing; which is highly promising.”
AlF is chemically inert due to its strong bond and spin-singlet electronic configuration. This stability allows it to survive in ultracold experiments without undergoing unwanted chemical reactions. Its robust nature, combined with favorable optical properties, makes it easier to produce in high efficiency in the laboratory. These qualities open opportunities for experiments that require prolonged trapping and precise control of molecular states.
Cooling and trapping molecules is inherently more complex than atoms because of their multiple energy levels and vibrational modes. Previous efforts have focused on reactive, spin-doublet molecules, but stable molecules like AlF require laser wavelengths in the ultraviolet due to large energy gaps between electronic states. Overcoming these technical challenges required innovations in laser technology and optical design, supported by strong collaboration between academic research and industry partners.
The experiments conducted by the FHI team involved careful management of molecular beam production, magnetic field gradients, and vacuum conditions. The researchers observed that AlF can survive collisions with room-temperature surfaces and even thermalize, suggesting potential for more compact and stable sources similar to those used for alkali atoms.
This achievement follows nearly eight years of work, beginning with detailed spectroscopic studies of AlF, development of deep ultraviolet laser systems, and optimization of trapping conditions. Graduate student Eduardo Padilla led much of the laboratory effort, highlighting the importance of a collaborative research environment and technical support in realizing this milestone.
The successful trapping of AlF expands the possibilities of ultracold molecular physics. The molecule’s long-lived metastable states, accessible through ultraviolet transitions, could allow even lower temperatures and greater quantum control. The work also provides a foundation for future molecular quantum devices, including sensors, quantum simulators, and precision measurement platforms.
This research illustrates the intersection of physics and engineering in ultracold molecular studies. Producing stable molecules in magneto-optical traps requires precise engineering of lasers, vacuum systems, and molecular sources. The ability to control multiple rotational states introduces new opportunities for experiments that probe fundamental physics and develop quantum technologies.
The demonstration of deep-ultraviolet trapping of a stable molecule represents a significant step toward integrating ultracold molecular systems into practical applications. By addressing both the technical and scientific challenges, the FHI team has set the stage for further advances in molecular quantum control and ultracold chemistry.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
