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Ultracold matter

Laser-cooled YbOH molecules could aid the hunt for new physics

04 May 2020
An optical table and vacuum system bathed in yellow laser light
Cool result: the laser cooling setup. (Courtesy: Benjamin Augenbraun)

Physicists at Harvard University and Arizona State University in the US have succeeded in laser-cooling YbOH molecules – a crucial first step towards using these molecules to make precision measurements of the electron’s electric dipole moment (eEDM). Their work was augmented by a related effort, carried out by researchers at the California Institute of Technology (Caltech) and Temple University, to enhance the brightness of a beam of cold YbOH. The results appear in separate papers in New Journal of Physics, which (like Physics World) is published by IOP Publishing. 

Here, two members of the PolyEDM collaboration, Harvard PhD student Ben Augenbraun and Caltech PhD student Arian Jadbabaie, describe the project’s goals and achievements.

What was the motivation for the research?

Despite its many successes, the Standard Model of particle physics is an incomplete theory. Among other deficiencies, 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 have come from measurements of electrons in molecules that are a few degrees above absolute zero. Extending the current state-of-the-art requires probing large numbers (millions) of molecules over very long times (several seconds) – a task that calls for trapped, ultracold molecules at microkelvin temperatures. Our research tackles the challenges associated with both producing these molecules in large quantities and using a laser to reduce their temperature by many orders of magnitude.

What did you do?

We demonstrated laser cooling of YbOH, a heavy, polyatomic molecule with high sensitivity to BSM physics. We also showed that laser light can be used to enhance the chemical reactions that produce YbOH. 

Using a combination of precisely tuned lasers and magnetic fields, we 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, we needed to apply many lasers in order to prevent loss of molecules to vibrational levels that did not “see” the other laser wavelengths. In this way, we forced the molecules to absorb and re-emit hundreds of photons, exerting large forces on them using the momentum of the photon to cool the molecules. 

Before laser cooling, we produce cold YbOH in a beam using chemical reactions between atomic Yb and OH-containing molecules (for example water or methanol). We were able to make these chemical reactions 10 times more efficient by using laser light (at a different wavelength than is used for the cooling) to excite the Yb atoms to a long-lived state. Our corresponding theoretical computations show that this excited Yb state has enough energy to overcome reaction barriers. 

What was the most interesting or important finding?

YbOH is one of the most massive small molecules and has one of the most complex structures among laser-cooled molecules. Showing for the first time that simple techniques could be extended even to YbOH is an important step forward. The laser cooling is extremely rapid, taking a fraction of a millisecond and requiring the absorption and re-emission of only hundreds of laser photons. The efficiency of this cooling is a very 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, we perform the excitation and reactions in a cold environment full of helium gas. We found collisions with the helium effectively thermalize the additional molecules formed by the exothermic reaction. This is particularly useful for laser cooling, which requires a starting point of cold, slow molecules.

Why is this research significant?

Laser cooling of YbOH demonstrates that increasingly more complex polyatomic molecules can be brought under control, including at the single quantum state level. At ultracold temperatures, their additional complexity then becomes a feature, allowing for novel applications in precision measurement and beyond. 

Furthermore, the laser-based chemical enhancement we demonstrate can 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 can be a significant advantage for many experiments requiring cold molecules. 

Together, the combination of chemical enhancement and laser cooling will make possible measurements on trapped molecules that would search for BSM physics at scales far beyond those achievable by current particle collider technology. We expect this to extend the current reach of such experiments by several orders of magnitude. 

What will you do next?

The chemical enhancement could be further increased by optimizing the reactants involved and completely filling the reaction volume with the “catalysing” laser light.  

For laser cooling, the next step is to extend the cooling to 3D, which means cooling and confining the molecules to a small spatial volume – holding them in a trap of laser light. This requires laser slowing and the application of more lasers to extend the cooling force to that of nearly 10,000 photon “kicks”, up from the several hundred demonstrated presently. Once trapped, the molecules can be studied over many seconds, allowing their internal properties to be controlled and tracked with exquisite precision.  

The full results of both studies are reported in New Journal of Physics.

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