In an ultracold cloud of atoms cooled to near absolute zero, regions with different spin orientations emerge, like patches with different magnetization. The boundaries between them are called domain walls. Usually they are stationary, but in this experiment the walls started moving on their own.
The impetus came from spin currents — flows of spin that arise from exchange collisions between atoms. When an atom with spin up collides with an atom with spin down, they can swap orientations, and the spin effectively flows from one to the other. As a result, the wall begins to crawl, like a wave in a stadium where each spectator stands up and sits down, passing the motion to the neighbor.
By varying the angle between the spin directions in neighboring domains, the physicists controlled the strength and direction of the currents. In one regime, exchange collisions are so strong that the wall freezes in place — held by a coherent 'lock.' In another regime, diffusion dominates, and the wall races along like a sled down a hill. The transition between these regimes resembles shifting gears in a car.
Numerical solutions of the quantum Boltzmann equation confirmed that the key parameter is the transverse phase gradient. It determines where and how fast the wall will creep. This means that the phase can be used as a steering wheel for spin flows.
Such control opens the door to atomtronics — an analog of electronics where neutral atoms replace electrons. Domain walls could become switches or transmission lines in future quantum circuits.
Domain walls in an ultracold gas can live for more than a second — by microscopic standards, that's an eternity.
In Greg Egan's novel 'Permutation City,' quantum states are used as carriers of information, and here domain walls could become memory cells.