Dynamics of Rotating Condensates Probed by Bragg Scattering

Raman Lab Research Projects

 
 

    Vortices are a hallmark of superfluids and have applications throughout the study of fluid mechanics and condensed matter physics. In recent years gaseous Bose-Einstein condensates (BECs) under rotation have become an important test bed for predictions of the behavior of quantized vortices. 


    In a previous work we have introduced two-photon Doppler sensitive Bragg scattering to study the rotation of sodium BECs, here we present a more detailed discussion of the method and some of its possible applications, particularly to learn about the dynamics of vortices in those systems.  We analyze the microscopic flow field and present laboratory measurements of the velocity profile. The remarkable feature of this method is that unlike time-of-flight imaging, Bragg scattering is sensitive to the direction of rotation and therefore to the phase of the condensate. We also show that it is possible to apply the technique as a non-destructive probe of the vortex flow field using a sequence of two Bragg pulses.

    Above:  Microscopic velocity field of a vortex state. Velocity field of a single vortex is shown on the left.  The arrows indicate the direction of the local velocity vector whose magnitude is proportional to the arrow length.  On the right we have the projection vx  of the velocity field, as measured by Bragg scattering (resonance condition).

    Microscopic velocity field of a vortex lattice. Shown are (a) column density profile in the trap (approximate solution, left) and TOF images (experiment, center). On the right, the velocity projection VNx is shown for a vortex lattice; (b) Spectral density of Bragg scattered atoms for -2.7 ± 0.3 mm/s (left), ± 0.3 mm/s (center) and 2.1 ± 0.3 mm/s (right), showing microscopic as well as macroscopic flow.

    Non-destructive measurement sequence for in-Situ probe of the rotation. Two consecutive pulses separated by a hold time  t can be used to non-destructively probe the rotating cloud.  Shown are 10 ms TOF images after t = 7 ms (a), 5 ms (b), and 2.5 ms (c).  The two diffracted groups appear as two tilted stripes on either side of the central, undiffracted cloud.  Motion within the trap causes the first group of atoms (pulse 1) to change its tilt angle due to kinematics considerations, as explained in the text.