On the Way to Ultracold Hydrogen

Scientists are a step closer to isolating ultracold hydrogen, the simplest atom in the universe, which offers a potential method for further exploring the world of quantum mechanics Writing in Physical Review Research last month, a team of physicists and chemists at Columbia, UCLA, and Johns Hopkins University investigated calcium monohydride (CaH)’s propensity to spontaneously disintegrate—known as predissociation—and developed a laser-based methodology to confine the hydrogen-containing molecule at ultracold temperatures.

“Trapping hydrogen will be vital for us to understand nature and discover potentially unknown physics,” said first author Qi Sun, a physics PhD student at Columbia working with physicist Tanya Zelevinsky. Hydrogen, the first element on the periodic table, is the simplest atom:  It has just one proton and one electron. Theoretical physicists can therefore model its potential behaviors quite comprehensively, which experimentalists, in principle, should be able to validate in the lab. If theoretical and experimental values don’t match up, that would mean there’s an unknown discrepancy to explore between the best mathematical models of the universe and the actual physics that governs it.

Despite its natural abundance, hydrogen is challenging to isolate on its own. So researchers like Sun and others in the ZLab, as Zelevinsky’s group is known, are first working to trap a molecule that contains hydrogen using direct laser trapping and cooling techniques. These techniques are like throwing ping pong balls against an incoming basketball, said Sun: Blast enough ping pong balls, which correspond to photons of light from a laser, against the basketball, which corresponds to a molecule, and, eventually, the basketball will stop moving. “Once the molecule is trapped with optical force, it may be possible to split it apart to create an ultracold cloud of hydrogen” said Sun.

CaH is a relatively light molecule compared to other hydrogen-containing molecules the ZLab considered, so it should be relatively easy to cool down; however, it has a tendency to predissociate and disappear. In the new work, the team combined experimental techniques with quantum chemistry calculations to establish the probability of disintegration for CaH’s two excited states, which reach as low as 0.1%. With this information, they created a laser-cooling scheme for CaH and proposed a pathway for the controlled splitting of the molecule into its two atomic pieces. They hope to have trapped molecules next year.

Read More: Qi Sun et al. Probing the limits of optical cycling in a predissociative diatomic molecule. Physical Review Research 2023. DOI: 10.1103/PhysRevResearch.5.043070