Torque and Angular-Momentum Transfer in Merging Rotating Bose-Einstein Condensates

When rotating classical fluid drops merge together, angular momentum can be advected from one to another due to the viscous shear flow at the drop interface. It remains elusive what the corresponding mechanism is in inviscid quantum fluids such as Bose-Einstein condensates (BECs). Here we report our theoretical study of an initially static BEC merging with a rotating BEC in three-dimensional space along the rotational axis. We show that a solitonlike sheet, resembling a corkscrew, spontaneously emerges at the interface. Rapid angular-momentum transfer at a constant rate universally proportional to the initial angular-momentum density is observed. Strikingly, this transfer does not necessarily involve fluid advection or drifting of the quantized vortices. We reveal that the corkscrew structure can exert a torque that directly creates angular momentum in the static BEC and annihilates angular momentum in the rotating BEC. Uncovering this intriguing angular-momentum transport mechanism may benefit our understanding of various coherent matter-wave systems, spanning from atomtronics on chips to dark matter BECs at cosmic scales.

Figure 1

(a) Schematic of the potential $\tilde{U}(\tilde{\mathbf{r}})$ used in Eq. (2). (b) Initial density and phase profile of the BECs with a single vortex line at the center of the lower condensate. The plot shows the density isosurface at 50% of the bulk density.

Figure 2

Merging dynamics of the two condensates when they are (a) static and (b) corotate at $\tilde{t}=0$. (c) Condensate density evolution when only the lower condensate contains a vortex line at $\tilde{t}=0$. The color plots at $\tilde{t}=3$ and 6 show the instantaneous phase profiles. The solid yellow lines represent the locations of the vorticity singularities. (d) Evolution of the angular-momentum density ${\tilde{L}}_z$ corresponding to (c). The plots show ${\tilde{L}}_z$ isosurface at 10% of the initial bulk value.

Figure 3

(a) Evolution of the total angular momentum ${\tilde{L}}_T$ in the upper and lower condensate regions. (b) Angular-momentum contribution from fluid advection in the upper condensate before vortices drift to this region.

Figure 4

(a) Profiles of the integrated torque exerted by the soliton sheet in the upper condensate region. (b) Profiles of the soliton sheet and the vortex line created by showing the density isosurface at 50% of the bulk density.

Figure 5

(a) Evolution of the condensate density when the lower condensate contains three vortex lines. (b) The angular-momentum transfer rate $d{\tilde{L}}_T/d\tilde{t}$ versus the initial angular-momentum density ${\tilde{L}}_z(0)$ for cases with various initial vortex configurations. The barely visible error bars represent the uncertainties in the linear fit as shown in Fig. 3. The dashed line is a linear fit to the data.