If you ask an undergraduate student, “What do neutron stars, metals, nuclei, and atoms have in common?” you might hear the answer, “They are made of neutrons, protons, quarks, and electrons.” While it is common for students to have heard about the existence of those microscopic particles and their individual properties like charge, spin, and color, it is far less common to hear from them that neutrons, protons, quarks, electrons, and atoms may exhibit a collective and macroscopic property called superfluidity, in which a large number of particles can flow coherently without any friction or dissipation of heat.
Neutron stars, metals, nuclei, and ultracold atoms have revealed their amazing superfluid state in ingenious experiments (see figure 1 for estimated critical temperatures of fermion superfluids). The first discovery of superfluid behavior was made nearly 100 years ago, when Heike Kamerlingh Onnes in 1911 cooled a metallic sample of mercury to...
