A stirred cup of coffee, like any normal fluid, can swirl around with an arbitrary speed and angular momentum. That’s not the case for superfluids. They can rotate only by winding themselves in vortices whose circulation is quantized. Mathematically, the vortices are simply zero-density points or lines around which the wavefunction’s phase wraps by integer multiples of 2π (see the article by Joe Vinen and Russell Donnelly, Physics Today, April 2007, page 43). That constraint has turned superfluids such as liquid helium and Bose–Einstein condensates (BECs) into model systems for studying turbulence, largely because the quantized vortices that embody that messy state in a superfluid are themselves discrete, well defined, and detectable.1 

Because their vortices can be directly caught on camera, BECs are particularly appealing to study. The dilute atoms interact so weakly that the extent of the especially low-density region around a vortex can be as...

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