Fritz Haber Institute Traps Stable Molecule, Opening New Frontier in Ultracold Physics

Researchers at the Fritz Haber Institute's (FHI) Department of Molecular Physics have achieved a significant advance in ultracold physics by successfully engineering the first magneto-optical trap for a stable, closed-shell molecule: aluminum monofluoride (AlF). This breakthrough permits the precise cooling and selective confinement of AlF molecules across three distinct rotational quantum states. The comprehensive findings detailing this endeavor have been submitted for peer review in Physical Review Letters and are currently available on the arXiv preprint server, signaling a new era for molecular control.

The pursuit of this level of control is fundamentally rooted in probing quantum mechanics at its most basic level. Cooling matter to temperatures near absolute zero—the ultracold regime—has historically been the precursor to major discoveries, including the observation of superconductivity in materials such as mercury metal. For decades, the evolution of quantum understanding has been driven by this quest for lower temperatures. The introduction of the laser enabled cooling cycles, leveraging matter-light interactions, to reach temperatures mere millionths or thousandths of a Kelvin above absolute zero.

For nearly forty years, research in this field focused on preparing ultracold neutral atoms within magneto-optical traps, which laid the groundwork for technologies like highly accurate optical atomic clocks and early quantum computers. However, the inherent complexity of molecular energy structures posed a major obstacle to trapping molecules. Previously, only reactive molecules possessing unpaired electrons, known as spin-doublet species, could be successfully loaded into these sensitive traps. The FHI team's success with AlF, a chemically inert spin-singlet molecule due to its robust chemical bond, overcomes this long-standing limitation.

This demanding achievement required overcoming substantial technical challenges. Molecules like AlF, which require significant energy to dissociate, often necessitate laser wavelengths deep within the ultraviolet spectrum for effective cooling. The successful trapping of AlF mandated the operation of four separate laser systems tuned near 227.5 nm, marking the shortest wavelength ever used to confine any atom or molecule to date. This requirement spurred novel advancements in both laser technology and optical systems, reflecting strong collaborative ties between academic research and industry partners.

A unique capability demonstrated by the FHI team is the ability to laser-cool and trap AlF across multiple rotational quantum levels by skillfully tuning the laser wavelengths to switch between three different quantum states. This nuanced control over molecular orientation and energy is anticipated to create unprecedented opportunities in precision measurements and quantum control of molecular systems. Sid Wright, who leads the FHI research group, noted the long-held goal of trapping AlF using a readily available, compact vapor source. Initial observations indicate that AlF shows remarkable resilience, surviving collisions with the walls of the room-temperature vacuum chamber, which is a vital sign for practical applications.

This milestone represents the culmination of nearly eight years of dedicated work, including extensive spectroscopic analysis and the development of deep-ultraviolet technology. Eduardo Padilla, the lead graduate student, stressed that the achievement resulted from a massive, integrated team effort, supported by the comprehensive resources of the Molecular Physics Department. Furthermore, the existence of a long-lived metastable electronic state within laser-cooled AlF offers an exciting pathway toward achieving even colder temperatures, promising to expand the scope of precision spectroscopy and quantum simulations.

Mastering molecular control has broader implications, as seen in related research. Techniques used to cool molecules like AlF are directly pertinent to advancing quantum sensing, where the extreme sensitivity of ultracold molecules can be used to detect minute changes in fundamental physical constants. In a parallel effort, researchers have used similar laser-cooling techniques on molecules such as strontium monohydride (SrH) to test fundamental symmetries, illustrating the growing utility of these ultracold molecular platforms across various physics disciplines.