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The physics in two-dimensional (2D) systems is very different from what weobserve in three-dimensional (3D) systems. Thermal fluctuations in 2D systemsare enhanced, and they prevent the conventional Bose-Einstein condensation(BEC) at non-zero temperatures by destroying the long-range order. However, aphase transition to a superfluid phase is still expected to occur in a 2Dsystem along with an emergence of a quasi-long-range order, explained by theBerezinskii-Kosterlitz-Thouless (BKT) mechanism. Within the BKT mechanism, auniversal jump of the superfluid density in a 2D Bosonic system wastheoretically predicted by Nelson and Kosterlitz, and was first observed in 2D\textsuperscript{4}He films by Bishop and Reppy. Recent experiments in trappedultracold 2D Bose gas systems have shown signatures of the BKT transition, andits superfluidity. However, the universal jump in the superfluid density wasnot observed in these systems. Here we report the first observation of theuniversal jump in the superfluid density using an optically trapped ultracold2D Bose gas. The measured superfluid phase space density at the BKT transitionagrees well with the predicted value within our measurement uncertainty.Additionally, we measure the phase fluctuations in our density profiles to showthat the BKT transition occurs first, followed by the BEC transition.

Observation of the universal jump across the Berezinskii-Kosterlitz-Thouless transition in two-dimensional Bose gases