Scaling dynamics of the ultracold Bose gas

The large-scale expansion dynamics of quantum gases is a central tool for ultracold gas experiments and poses a significant challenge for theory. In this work we provide an exact reformulation of the Gross-Pitaevskii equation for the ultracold Bose gas in a coordinate frame that adaptively scales with the system size during evolution, enabling simulations of long evolution times during expansion or similar large-scale manipulation. Our approach makes no hydrodynamic approximations, is not restricted to a scaling ansatz, harmonic potentials, or energy eigenstates, and can be generalized readily to noncontact interactions via the appropriate stress tensor of the quantum fluid. As applications, we simulate the expansion of the ideal gas, a cigar-shaped condensate in the Thomas-Fermi regime, and a linear superposition of counterpropagating Gaussian wave packets. We recover known scaling for the ideal gas and Thomas-Fermi regimes, and identify a linear regime of aspect-ratio preserving free expansion; analysis of the scaling dynamics equations shows that an exact, aspect-ratio invariant, free expansion does not exist for nonlinear evolution. Our treatment enables exploration of nonlinear effects in matter-wave dynamics over large scale-changing evolution.

Automated summary

In this study, we propose an exact reformulation of the Gross-Pitaevskii equation for ultracold Bose gas, which allows simulations of long evolution times during expansion or similar large-scale manipulation by using a coordinate frame that adaptively scales with the system size. Our approach does not make hydrodynamic approximations and is not limited to specific assumptions or potentials. We apply our method to simulate various scenarios including the expansion of an ideal gas, a cigar-shaped condensate in the Thomas-Fermi regime, and a linear superposition of counterpropagating Gaussian wave packets. We identify different scaling regimes and show that there is no exact, aspect-ratio invariant, free expansion for nonlinear evolution.