Catching a single molecule in a beam of light and nearly stopping its motion is like trying to hold a dandelion fluff in a gust of wind. A particle that's too hot slips out of the trap, so physicists first tame its thermal jitters with laser cooling.
The authors applied a technique called Sisyphus cooling to strontium hydroxide (SrOH) molecules. The name harks back to the myth of Sisyphus: the particle, like the boulder, endlessly rolls uphill, losing energy, but unlike the myth, it slows down more with each cycle. Here, they used a two-dimensional version: cooling occurred along two directions simultaneously, not just one. The result: the number of molecules entering the magneto-optical trap, a light cage where particles are held by magnetic fields and laser beams, increased by a factor of 12.
Life in the trap isn't eternal: molecules collide with each other and escape. The measured loss rate (the speed at which particles disappear due to pairwise collisions) was approximately 4×10⁻¹⁰ cm³/s. That's like a room full of people where someone exits through the door every second — except here, the "door" is opened by mutual shoves.
The method is universal: it works for all known molecules that are directly laser-cooled — both symmetric and asymmetric tops. The increase in trapped particles will directly boost the search for ultralight dark matter and enable the use of polyatomic molecules as probes to detect new particles with masses far beyond the reach of the Large Hadron Collider.
The temperature of the molecules after cooling is millionths of a kelvin — colder than interstellar space, where it's still "warm" due to the cosmic microwave background.
In Peter Watts's novel Blindsight, alien life forms use quantum effects to survive — experiments like this with ultracold molecules might one day help us understand whether such biochemistry is possible in reality.