In 2006, Zoran Hadzibabic, Peter Krüger, Marc Cheneau, Baptiste Battelier and Jean Dalibard split a cooled rubidium-87 cloud into two independent planar gases using an optical lattice. After release, the expanding sheets overlapped; absorption imaging recorded their matter-wave interference. Smooth fringe waviness revealed thermal phase fluctuations, while sharp dislocations indicated isolated vortices. The initial evaporative-cooling endpoint controlled the preparation; central fringe contrast served as a degeneracy proxy because direct thermometry in the lattice was difficult.[ref]
Berezinskii-Kosterlitz-Thouless Crossover in a Trapped Atomic Gas by Zoran Hadzibabic and others. Pages 1–2, preparation and readout; Fig. 1; temperature control and Fig. 2
Loss of extended coherence coincided with increased vortex signatures. The optical trap and finite cloud produced a crossover; tightly bound pairs were unresolved, and short-wavelength modes could contaminate high-temperature counts. These were observations of the vortex-unbinding mechanism, not a direct measurement of phase stiffness or a sharp homogeneous universal stiffness jump. The authors left the sample's superfluidity open.[ref]
Berezinskii-Kosterlitz-Thouless Crossover in a Trapped Atomic Gas by Zoran Hadzibabic and others. Pages 3–4, Figs. 3–4; geometry, invisible pairs, detection limits and final paragraph on unmeasured superfluidity
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