Stable molecule trapped with deep ultraviolet light for the first time

Researchers from the Department of Molecular Physics at the Fritz Haber Institute have demonstrated the first magneto-optical trap of a stable “closed-shell” molecule: aluminum monofluoride (AlF). They were able to cool AlF with lasers and selectively trap it in three different rotational quantum levels—breaking new ground in ultracold physics.

Their experiments open the door to advanced precision spectroscopy and quantum simulation with AlF. The work has been accepted for publication in Physical Review Letters and is currently available on the arXiv preprint server.

Cooling matter to temperatures near absolute zero (0 K, -273.15°C) acts like a microscope for quantum mechanical behavior, bringing physics that is normally blurred out into sharp focus. Classic historical examples include the 1911 discovery of superconductivity in mercury metal cooled near 4 K, and anomalous thermal behavior in molecular hydrogen due to its “ortho” and “para” spin states. These phenomena confounded classical physics theories of the time, driving both the evolution of quantum mechanics, as well as efforts to reach ever lower temperatures.

Following the invention of the laser, physicists realized that cooling cycles could be implemented via the interaction of matter with light. The effect of an individual particle of light (a “photon”) is tiny, but when accumulated many thousands of times in a cycle, laser cooling becomes extremely powerful: ultimate temperatures achieved can be around one thousandth to one millionth of a degree above zero Kelvin (10^-3—10^-6 K). This is typically called the ultracold regime.

Magneto-optical trapping

For almost 40 years now, it has been possible to prepare ultracold neutral atoms in “magneto-optical traps.” Here, multiple trapping laser beams combine with a correctly chosen magnetic field to confine particles and cool them to around one thousandth of a degree above absolute zero temperature. This key technique has, for instance, led to today’s optical atomic clocks, prototype atom-based quantum computers and simulators, and the observation of new phases of matter.

A little over 10 years ago, researchers succeeded in laser cooling and trapping a diatomic molecule—the simplest chemical compound possible, but already with a much more complicated energy structure than an atom. While there are strong motivations to bring molecules into the ultracold regime, this complexity presents a considerable challenge. Before now, only a handful of reactive molecules with unpaired electrons (often called “spin-doublet” species) have been loaded into magneto-optical traps.

The challenge of trapping chemically stable molecules

In their present study, the research team from the Molecular Physics Department present experiments that could revolutionize physics with ultracold molecules: They demonstrate the first magneto-optical trap of a “spin-singlet” molecule, aluminum monofluoride (AlF). AlF has an extremely strong chemical bond, which, in combination with other properties, makes it chemically inert when compared to all other laser-cooled molecules. Thanks to its properties, it is easier to produce with high efficiency in the lab, and unlikely to be lost in ultracold experiments via chemical reactions.

But why is this groundbreaking step only being taken now? Molecules requiring a lot of energy to be ripped apart also tend to have very large energy gaps between their electronic states. As a consequence, the laser wavelengths required for cooling are pushed further and further into the ultraviolet, significantly increasing the experimental challenges.

Cooling AlF required four laser systems, each with a wavelength near 227.5 nm. This is far in the “deep ultraviolet” part of the spectrum, and the shortest wavelength used to trap any atom or molecule so far. Trapping AlF has required new innovations in laser technology and optics, for which strong industry-academic collaboration has been essential.

The electron configuration matters

It is not only its chemical stability that makes AlF so promising for quantum science. Another unique aspect of the experiment is that AlF can be laser-cooled and trapped in multiple rotational quantum levels. The FHI team were able to switch between three different rotational levels in the trap, by simply finely tuning the laser wavelengths used, and expect that higher rotational levels could be trapped with different molecular sources to those currently in use.

This feature distinguishes AlF from the other laser-cooled molecules produced to date: for these molecules, only one rotational level has been cooled and trapped, and extension to different rotational levels is much more challenging.

“The dream for us would be to trap AlF from a compact, inexpensive vapor source, similar to what is used for the alkali atoms,” says Sid Wright, who joined the AlF project in 2020 and currently leads the FHI team. “In initial experiments, we have seen that AlF can survive collisions with room temperature vacuum walls—even thermalizing—which is highly promising.”

A long journey in the lab yields promising results

To reach this milestone took almost eight years of hard work in the laboratory: first with detailed study of the spectroscopic properties of AlF, followed by development and testing of deep-UV technology for the trap, and finally the laser-slowing and magneto-optical trapping itself. “This has been a huge team effort, and our results are in several respects down to the fantastic research environment, technical support and resources in the Molecular Physics Department,” says Eduardo Padilla, the lead graduate student on the project.

The recent results expand the possibilities of ultracold molecular physics, and laser-cooled AlF will likely enable new precision measurements and quantum control of molecules. A particularly interesting aspect of AlF is the presence of a long-lived “metastable” electronic state, for which the electron spins combine to make a so-called “spin-triplet.” The metastable state can be reached from the ground state by another ultraviolet transition, and this opens the door to even colder temperatures.