When your heart beats, blood courses through your veins in waves of pressure. These pressure waves manifest as your pulse, a regular rhythm unperturbed by the complex internal structure of the body. Scientists call such robust waves solitons, and in many ways they behave more like discrete particles than waves. Soliton theory may aid in the understanding of tsunamis, which—unlike other water waves—can sustain themselves over vast oceanic distances.Solitons can arise in the quantum world as well. At most temperatures, gas atoms bounce around like billiard balls, colliding with each other and rocketing off into random directions. Near absolute zero, however, certain kinds of atoms suddenly start behaving according to the very different rules of quantum mechanics, and begin a kind of coordinated dance. Under pristine conditions, solitons can emerge inside these ultracold quantum fluids, surviving for several seconds.Curious about how solitons behave in less than pristine conditions, scientists at NIST’s Physical Measurement Laboratory, in collaboration with researchers at the Joint Quantum Institute (JQI), have added some stress to a soliton’s life. They began by cooling down a cloud of rubidium atoms. Right before the gas became a homogenous quantum fluid, a radio-frequency magnetic field coaxed a handful of these atoms into retaining their classical, billiard ball-like state. Those atoms are, in effect, impurities in the atomic mix. The scientists then used laser light to push apart atoms in one region of the fluid, creating a solitary wave of low density—a “dark” soliton.