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When two superconductors are coupled with one another through an insulating junction, a so-called Josephson supercurrent may flow without creating any potential difference, sustained merely by a phase difference between the superconducting wave functions. We have observed the charge-neutral analogue of this phenomenon using an ultracold gas of strongly interacting fermionic atoms, revealing the quintessential sinusoidal relationship between the supercurrent and the superfluid phase. Our experiments demonstrate the profound connection between the supercurrent magnitude and the nature of superfluid states, which has allowed us to quantify the condensate density across the BCS-BEC crossover of ultracold Fermi gases, playing the role of the superfluid order parameter. W. J. Kwon et al. |