Toss a handful of peas into a pot of semolina porridge — the peas quickly sink, pushing the grains aside. But if you make the porridge very thick and sticky, the peas get stuck. That's roughly how light lithium atoms behaved in a cloud of heavy potassium atoms at temperatures just a few millionths of a degree above absolute zero.
Usually, a particle in a gas moves like a drunk pedestrian: a step left, a step right, and on average it drifts farther from the starting point. This is called diffusion. But in this experiment, the light atoms moved differently: as the temperature dropped, their drift from the starting point slowed down more than expected. Some atoms stopped moving altogether, even though they collided with heavy neighbors hundreds of times.
The explanation comes from quantum mechanics: the waves describing the atoms overlap and cancel motion in some places while reinforcing it in others. The heavy potassium atoms act as nearly stationary scatterers, creating a maze without walls for the light atoms. This behavior doesn't fit the usual model of a fluid made of independent particles, but it is well described by the picture of a random walk through a frozen disorder.
This experiment is a clean playground for studying how quantum interference stops motion. Similar effects are important for understanding conductivity in disordered materials and, possibly, for future quantum devices where controlling particle flows is essential.
The lithium and potassium atoms in this experiment are a billion times colder than room temperature — at such cold, their wave-like properties become dominant, and they behave not like tiny balls but like fuzzy clouds.
The freezing of particles in random disorder echoes the plot of 'The Zone of Dead Silence' from science fiction, where characters get stuck in an anomalous region, losing the ability to move.