Bose-Einstein Condensation into Nonequilibrium States Studied by Condensate Focusing

We report the formation of Bose-Einstein condensates into nonequilibrium states. Our condensates are much longer than equilibrium condensates with the same number of atoms, show strong phase fluctuations, and have a dynamical evolution similar to that of quadrupole shape oscillations of regular condensates. The condensates emerge in elongated traps as the result of local thermalization when the nucleation time is short compared to the axial oscillation time. We introduce condensate focusing as a new method to extract the phase-coherence length of Bose-Einstein condensates.

Figure 1

Condensate half length $L_z(t,\tau )$ versus evolution time $t$ for three different expansion times $\tau$. The dark grey line is a guide to the eye. The light grey line represents a fit to a damped quadrupole shape oscillation.

Figure 2

Condensate half length $L_z(t,\tau )$ versus expansion time $\tau$ after 11 ms of evolution (ten point averages; see 25). The grey (dashed) line is a fit to the scaling equations including (excluding) phase fluctuation broadening. Inset: Axial density profile showing phase fluctuation stripes ($\tau =6.8\mathrm{m}\mathrm{s}$).