Vortices in a two-dimensional fermionic superfluid
Quantized vortices are a hallmark of superfluidity. They have proven to be an indispensable probe for systems ranging from superfluid helium to type-II superconductors. In ultracold two-dimensional (2D) Fermi gases, where phase fluctuations are enhanced and the Berezinskii-Kosterlitz-Thouless (BKT) mechanism governs the superfluid transition, their observation has remained elusive. Here, we report on the controlled creation of quantized vortices in a homogeneous 2D gas in the BEC--BCS crossover. By continuously tuning the velocity of two moving obstacles, we observe quantized steps in the absolute vortex charge. Matter-wave interferometry reveals the characteristic phase winding, quantized in units of $2π$. Measurements on 2D and 3D gases show a striking difference: only in 2D does the vortex number grow with temperature, reflecting thermal activation of BKT vortex pairs. Our approach provides evidence of fermionic superfluidity in 2D and offers a means to study BKT physics, vortex-controlled transport and resistance, as well as to detect unconventional superfluidity in spin-imbalanced 2D gases.