Realization of a Super-Tonks-Girardeau gas with strong attractive interactions

We realize a 1D quantum gas with strong attractive interactions. For this, we load a Bose-Einstein condensate of Cs atoms into an array of ``tube-like" 1D traps generated by a 2D optical lattice and exploit the tunability of the interaction strength near a Feshbach resonance. When the 3D scattering length is increased towards and beyond the length scale set by the tight transversal confinement, we observe a confinement-induced resonance (CIR) that allows us to tune the effective 1D-interaction parameter $g_{\mathrm{1D}}$ to large positive and negative values. By tuning to large positive values of $g_{\mathrm{1D}}$, we first observe the transition from a 1D mean field system to a Tonks-Girardeau (TG) gas as evidenced by a change of the breathing mode frequency $\omega_B$ along the weakly confining direction from $\sqrt{3}$ to $2$ in units of the (bare) dipole oscillation frequency $\omega_D$. When the CIR is crossed to enter the strong attractive interaction regime, $\omega_B$ increases beyond 2 (in units of $\omega_D$), giving evidence of the regime of hard-rod interactions known as the super-Tonks-Girardeau (sTG) regime$^{1}$. We find, in contrast to the TG regime, that the release energy strongly depends on $g_{\mathrm{1D}}$, and that the sTG gas is surprisingly stable despite the fact that the interaction is strongly attractive. \newline $^{1}$ G. Astrakharchik et al., Phys. Rev. Lett. {\bf 95}, 190407 (2005).