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Laser-Cooled YbOH Molecules Could Support New Discoveries in Physics

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CAMBRIDGE, Mass., May 15, 2020 — 60th anniversary of the laser, Photonics Media.Researchers at Harvard University and Arizona State University have demonstrated laser cooling of polyatomic YbOH molecules, in a first step toward using these molecules to make precision measurements of the electron’s electric dipole moment (eEDM). The researchers also showed that laser light could be used to enhance the chemical reactions that produce YbOH. Their work was augmented by a related effort, carried out by researchers at Caltech and Temple University, to enhance the brightness of a beam of cold YbOH.

Using a combination of precisely tuned lasers and magnetic fields, the scientists demonstrated laser cooling of a beam of gaseous YbOH in one dimension to temperatures as low as several microkelvin. Because of the complex vibrational motions in YbOH, they needed to apply many lasers to prevent loss of molecules to vibrational levels that did not “see” the other laser wavelengths. They forced the molecules to absorb and reemit hundreds of photons, exerting large forces on them using the momentum of the photon to cool the molecules.

Laser-cooled polyatomic molecules for improved electron electric dipole moment searches. Harvard University and Arizona State University.
Cool result: The laser cooling setup. Courtesy of Benjamin Augenbraun.

Before laser cooling, the researchers produced cold YbOH in a beam using chemical reactions between atomic Yb and OH-containing molecules (for example, water or methanol). They were able to make these chemical reactions 10× more efficient by using laser light, at a different wavelength than what was used for the cooling, to excite the Yb atoms to a long-lived state.

The researchers’ theoretical computations showed that this excited Yb state has enough energy to overcome reaction barriers.

According to the researchers, the laser cooling is extremely rapid, taking a fraction of a millisecond and requiring the absorption and reemission of only hundreds of laser photons. They consider the efficiency of this cooling to be a promising sign for extending it in future experiments.

When using excited-state Yb, the chemical reactions producing YbOH are exothermic, with extra energy available that can heat the molecules. However, the researchers performed the excitation and reactions in a cold environment full of helium gas. They found that collisions with the helium effectively thermalized the additional molecules formed by the exothermic reaction. This could be useful for laser cooling, which requires a starting point of cold, slow molecules, the researchers said.

Laser-cooled polyatomic molecules for improved electron electric dipole moment searches, Harvard University and Arizona State University.
Reaction boost: The chemical enhancement setup, with the green enhancement light being sent into the molecular beam source. Courtesy of Arian Jadbabale.

The research team believes that laser cooling of YbOH indicates that increasingly more complex polyatomic molecules could be brought under control, including at the single quantum state level. At ultracold temperatures, the additional complexity of the molecules would become a feature allowing for new applications in precision measurement and beyond.

The team further believes that the laser-based chemical enhancement it demonstrated could be applied to other reactions, producing a variety of interesting molecules at low temperatures. Corresponding computations could help identify the optimal reactants for molecular production. Chemical enhancement could be a significant advantage for many experiments requiring cold molecules, the researchers said.

Laser-cooled YbOH molecules could aid the hunt for new physics. Despite its many successes, the Standard Model of particle physics is an incomplete theory. It explains neither the nature of dark matter nor why our universe is full of matter but not antimatter. These mysteries could be solved by the existence of new particles and interactions beyond the Standard Model (BSM). Despite many ongoing searches, no sign of BSM physics has been observed in the laboratory.

If studied at high enough precision, “common” particles, such as electrons, may exhibit minute but measurable effects from “new” interactions and particles (those that exist in nature but have not yet been seen in the lab).

Some of the best constraints on BSM physics, the researchers said, have come from measurements of electrons in molecules that are a few degrees above absolute zero. To extend the current state of the research, the researchers believe that it will be necessary to probe millions of molecules over periods of several seconds — a task that will require trapped, ultracold molecules at microkelvin temperatures. Their research into YbOH, a heavy, polyatomic molecule with high sensitivity to BSM physics, takes on the challenges associated with both producing molecules in large quantities and using a laser to reduce their temperature by many orders of magnitude.

The research was published in the New Journal of Physics (www.doi.org/10.1088/1367-2630/ab687b).  

Photonics.com
May 2020
GLOSSARY
laser cooling
A process and method by which manipulation and orientation of a given number of directed laser beams decreases the motion of a group of atoms or molecules such that their internal thermodynamic temperatures reach near absolute zero. The 1997 Nobel Prize in Physics was awarded to Steven Chu, Claude Cohen-Tannoudji and William D. Phillips for the development of methods to cool and trap atoms with laser light.
Research & TechnologyeducationAmericasHarvard UniversityArizona State UniversityCaltechTemple Universitylaserslight sourcesopticsnew physicsbeyond the Standard Modellaser coolingYbOH moleculesBSM physicslaser-cooled moleculesTunable Lasers

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