Observation of Long-Lived Vortex Aggregates in Rapidly Rotating Bose-Einstein Condensates

We study the formation of large vortex aggregates in a rapidly rotating dilute-gas Bose-Einstein condensate. When we remove atoms from the rotating condensate with a tightly focused, resonant laser, the density can be locally suppressed, while fast circulation of a ring-shaped superflow around the area of suppressed density is maintained. Thus a giant vortex core comprising 7 to 60 phase singularities is formed. The giant core is only metastable, and it will refill with distinguishable single vortices after many rotation cycles. The surprisingly long lifetime of the core can be attributed to the influence of strong Coriolis forces in the condensate. In addition we have been able to follow the precession of off-center giant vortices for more than 20 cycles.

Figure 1

Different stages of the giant vortex formation process. (a) Starting point: BEC after evaporative spin-up. (b)–(h) Laser shone onto BEC for (b) 14 s, (c) 15 s, (d) 20 s, (e) 22 s, (f) 23 s, (g) 40 s, and (h) 70 s. Pictures are taken after 5.7-fold expansion of the BEC. Laser power is 8 fW. (i) Zoomed-in core region of (f).

Figure 2

Lattice reforming after giant vortex formation. (a) BEC after evaporative spin-up. (b) Effect of a 60 fW, 2.5 s laser pulse. (c),(d) Same as (b), but additional 10 s (c) and 20 s (d) in-trap evolution time after the end of the laser pulse. Images taken after a sixfold expansion of the BEC.

Figure 3

Core developing after a 5 ms short, 2.5 pW laser pulse. In-trap evolution time after the end of the pulse is (a) 0.5 ms, (b) 10.5 ms, (c) 20.5 ms, and (d) 30.5 ms. Images taken after sixfold expansion of BEC. (e) Zoomed-in core region of (c).

Figure 4

Oscillation of core area after an 8 pW, 5 ms laser pulse. Starting conditions are $2.5\times {10}^6$ atoms with rotation rate $0.9{\omega }_{\rho }$. Time given is the in-trap evolution time after the end of the pulse. Core area is normalized to mean of all data points. Inset: Initial condition $3.5\times {10}^6$ atoms with rotation rate $0.78{\omega }_{\rho }$; in-trap images taken after a 14 pW, 5 ms laser pulse followed by evolution time of (a) 20 ms, (b) 40 ms, (c) 60 ms, and (d) 80 ms.

Figure 5

Giant core precession. Cores created by a 10 ms off-centered laser pulse with a power of (a) 4.2 pW, (b) 33 pW, and (c) 470 pW. Time given in figure is in-trap evolution time after the end of the laser pulse. For reference, a naive expectation for the lifetime of a giant vortex given by the radius of the core divided by the speed of sound in the surrounding cloud would be 13 and 30 ms for (a) and (b), respectively.

Figure 6

Ring cut out of a BEC by a 125 ms long, off-centered laser pulse. Expansion image taken (a) directly after the end of the pulse (b) after an additional in-trap evolution time of 200 ms after the end of the pulse and (c) evolution time 2 s.