Extended Data Fig. 3: Scanning the interaction switch-off time.

T_π is the duration of the spin flip from a strongly interacting |1⟩–|3⟩ to an (almost) non-interacting |1⟩–|4⟩ mixture at the beginning of the TOF sequence (inset). When we increase T_π, the magnitude of the pair correlations reduces greatly above a threshold of T_π = 100 μs (red circles). The reason is that scattering events during the TOF expansion redistribute the momenta between the participating atoms and destroy the correlations that were present between the in situ momenta. We model the effect by assuming that each scattering event between two atoms annihilates all the in situ correlations for those particles. The only parameters that enter the model (solid line) are the scattering rate λ_sc of the |1⟩–|3⟩ mixture at the magnetic offset field (B₀ = 750) and the in situ density. The dashed line shows the same model prediction but at one of the highest scattering rates used in our experiments (B₀ = 695, E_B/ħω_r = 1.97). It follows that at the spin flip time of T_π = 300 ns that we use for our experiments, no scattering is expected to occur during the TOF at any interaction strength setting. The mean kinetic energies E_kin (blue squares) show a similar dependence on T_π. This indicates again that only for T_π ≪ 1/λ_sc (with λ_sc ≲ 50 kHz) the true in situ momentum distribution is obtained after the TOF sequence. The error bars are obtained from the counts in each bin of the momentum distribution and the correlation function respectively and by assuming Poissonian statistics.