FIGURE 3–2 The original demonstration of Bose-Einstein condensation (BEG) in a dilute gas of rubidium atoms. These false-color images show the velocity distribution of a cloud of cold atoms near and below the BEG critical temperature. Left: Above the transition temperature, the atomic velocities show that this is an ordinary cold gas at an extraordinary temperature of 400 billionths of a degree above absolute zero (400 nK) . Center: Further cooling to 200 nK leads to the onset of BEG, shown as a clumping of atoms near zero velocity. Right: Still more cooling to 50 nK shows nearly all of the remaining gas in the BEG phase. SOURCE: E.Cornell, C.Wieman, JILA.

slightest resistance, to—as if by magic—overcome friction. Superfluidity is very much analogous to superconductivity, the ability of a metal to conduct electricity without any loss. Indeed, in a very real sense superconductivity is superfluidity, with the electrical current being carried along through the superconducting metal as a superfluid of electrons. A superfluid is to a regular fluid what a superconductor is to a regular electrical conductor. Until the mid-1990s the only laboratory superfluids known were liquids of both helium isotopes, 4He and the rarer 3He.

In principle, a superfluid gas flowing around a closed loop can keep flowing (or “persist”) forever. At a conceptual level, persistent superflow around a loop can be understood as analogous to a twisted loop of ribbon. Imagine taking a length of ribbon, putting a twist in it, and then bringing the two ends of the ribbon together and permanently gluing them. The twist in the loop of ribbon represents the flow



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