Deep inside a laboratory at Columbia University, 230 calcium monohydride molecules have been caught in a hovering embrace of laser light — and they've been cooled to a temperature so extreme it defies everyday intuition. The molecules now exist at less than one millikelvin, hovering just above the boundary of absolute zero.
This achievement, published in Physical Review Letters by researchers from Columbia University and Indiana University Bloomington, marks the first time a metal hydride molecule has been trapped using a magneto-optical trap (MOT). For years, physicists have successfully trapped individual atoms this way, but molecules present a tougher challenge. Their extra vibrational and rotational movements make them far more complex to cool and contain.
The team overcame this difficulty by developing a new cryogenic buffer-gas beam source and redesigning the laser cooling scheme specifically for calcium monohydride (CaH) — a molecule made of one calcium atom bonded to one hydrogen atom. Using carefully arranged laser beams and magnetic fields, they slowed a fast-moving beam of CaH molecules down to near-rest and confined roughly 230 of them inside the trap.
Jinyu Dai, the paper's first author, explained the broader purpose of this delicate work. "As nature's simplest atom, hydrogen provides an ideal platform that could enable some of the most precise tests of fundamental physics," he told Phys.org. "Our work lays the groundwork for producing ultracold hydrogen through the dissociation of ultracold metal hydride molecules."
The implications stretch well beyond this single experiment. By demonstrating that metal hydrides — despite their additional complexity and predissociative loss channels — can indeed be laser cooled and trapped in the ultracold regime, the researchers have opened a new platform for studying ultracold quantum chemistry. One promising avenue involves photodissociation of the CaH molecule to produce ultracold atomic hydrogen, which could eventually serve as an ideal system for high-precision tests of the Standard Model and measurements of fundamental constants.
The team is already pushing further. "Further cooling and trapping of the molecule are underway to achieve higher phase-space densities," Dai said. The next exciting step will involve using dissociation spectroscopy to study ultracold chemistry and produce ultracold hydrogen — bringing researchers closer than ever to probing the foundations of physics with unprecedented precision.