In a two-dimensional (2D) classical fluid, a large-scale flow structure
emerges out of turbulence, which is known as the inverse energy cascade where
energy flows from small to large length scales. An interesting question is
whether this phenomenon can occur in a superfluid, which is inviscid and
irrotational by nature. Atomic Bose-Einstein condensates (BECs) of highly
oblate geometry provide an experimental venue for studying 2D superfluid
turbulence, but their full investigation has been hindered due to a lack of the
circulation sign information of individual quantum vortices in a turbulent
sample. Here, we demonstrate a vortex sign detection method by using Bragg
scattering, and we investigate decaying turbulence in a highly oblate BEC at
low temperatures, with our lowest being ∼0.5Tc​, where Tc​ is the
superfluid critical temperature. We observe that weak spatial pairing between
vortices and antivortices develops in the turbulent BEC, which corresponds to
the vortex-dipole gas regime predicted for high dissipation. Our results
provide a direct quantitative marker for the survey of various 2D turbulence
regimes in the BEC system.Comment: 8 pages, 8 figure