Twisted layers of atoms in a material form a pattern of tiny regions, domains, each of which can be electrified in one of two directions, like a compass needle pointing north or south. Previously, such materials were studied only as a whole, measuring thousands of domains at once, and their collective behavior was like a choir where individual voices are not heard.
Now physicists have built a bridge a few atoms wide that touches only one domain in twisted molybdenum disulfide. This bridge works like a narrow door for electric current. When the voltage is changed, the current through the bridge does not change smoothly but jumps between two levels, like a switch that clicks. Moreover, the domain remembers which way it was electrified even when the voltage is removed: the compass needle stays in place.
Comparison with large samples revealed something interesting: when there are many domains, their needles can point in opposite directions and cancel each other out. So the material as a whole stops being electrified, even though each domain individually is electrified. This is like people in a crowd pulling in different directions: each is strong, but together they stand still.
Such a bridge can be used as an ultra-sensitive sensor: it notices when even a single needle flips — a single atomic displacement. This opens the way to electronics where states are written and read at the level of a single domain, not a whole crystal.
A domain in a twisted material does not flip its polarization instantly but over a certain time, like an hourglass that needs to be turned over for the sand to start flowing the other way.
In Greg Egan's novel "Permutation City," people live in a simulation where matter consists of discrete cells with memory — the domains from this work resemble such cells in a real crystal.