A cold cloud of atoms is like a swarm of fireflies drawn to one another. The attraction between atoms tries to pack them into a single dense ball. But quantum jitter, the unavoidable motion of particles, keeps the ball from collapsing. When the attraction is strong, the cloud breaks into separate clumps. That's how solitons are born: stable waves, like a lone crest that runs across water without spreading out.
In the lab, such clumps are made by cooling atoms to temperatures a few millionths of a degree above absolute zero. Alkali metal atoms, lithium or potassium, are held in traps made of laser light. The trap is shaped like a narrow tube, so atoms move only in one direction. Then the attraction acts along the tube, and the clumps come out elongated. They collide, pass through each other, and separate again, like two ghosts that can't merge.
Recently, scientists have spotted a new type of soliton — two-dimensional. They appear when the trap is not a tube but a flat plate. Such a soliton resembles a vortex that swirls and holds itself together. Vortex solitons were predicted long ago, but only now have they been seen in experiments. This matters because such vortices could be building blocks for future quantum devices.
The clumps of atoms don't live long, just fractions of a second. But in that time, you can measure how they emerge from quantum noise. It turns out that the birth of clumps is not a random process but a quantum one: it starts with tiny fluctuations that get amplified by attraction. This is like a snow avalanche that begins with a single snowflake falling from a branch.
The soliton is named after engineer John Scott Russell, who in 1834 spotted a solitary wave in a canal and chased it on horseback for several kilometers.
In science fiction, solitons often become weapons or means of communication, as in Stephen Baxter's novel "Soliton," where a wave carries information across spacetime.