2007 Cs Experiment


Nov/7 - Runaway Evaporation to Cs BEC

Accelerating evaporative cooling of atoms into Bose-Einstein condensation in optical traps

C.-L. Hung, X. Zhang, N. Gemelke and C. Chin

Physical Review A 78, 011604(R) (2008)

The time-of-flight (TOF) absorption images above are taken at various stages in our evaporative cooling path. The TOF expansion time is 70 ms. During the first 64 ms, atoms are levitated with an external field gradient of 31.5 G/cm, followed by a 6 ms free fall. In the first two pictures, atoms are still thermal with a Gaussian momentum distribution. In the later pictures, bimodality is seen, showing the phase transition into a Bose-Einstein condensate.

 

We employ a novel evaporative cooling scheme realized by tilting the dipole trap. The idea is shown in (a). Trap depth U decreases with time when a magnetic field gradient is applied to the atoms. In (b), we show the magnetic field gradient B'(t) levitates the atoms against gravity and evaporate hot atoms upward.

Performance of our new trap-tilting based forced evaporation:

 

(a) phase space density,

(b) collision rate, and

(c) particle number are shown during the evaporation process.

We adopt two evaporation paths: an efficient, 4 s path (black dots) and a fast, 1.8 s path (red circles).

The dashed line in (a) shows simple exponential increase. Our 4 s path outperforms the dashed line, which, together with the increasing collision rate, demonstrate the first runaway evaporative cooling of cold atoms in an optical trap.

 

  We compare the performance of our new evaporation cooling scheme with theoretical models. For all models, we assume an initial collision rate of 133/s and truncation parameter of 6.2~6.8 and negligible collision loss. This set of parameters best simulates our experiment condition. The performance of our experiment (black dot) is very close to the tilted trap model we formulate assuming 3 dimensional evaporation. Note that all 1D models cannot explain the fast evaporation speed we observe.

 

 

Dipole trapping

  Our dipole trap is formed by intersecting two laser beams on the horizontal (x-y) plane; both beams are extracted from a single-mode, single frequency Yb fiber laser operating at the wavelength of 1064 nm, frequency offset by 80 MHz, focused to a beam diameter of 540 micron (620 micron) and intensity of 1.9 W (1.6 W) in the y-(x-) direction.

Optical cooling: molasses and degenerate Raman sideband cooling

Here are three snapshots of ultracold atoms, the yellowish blobs in the images, when they are released in free space. In free space, two things happen: atoms drop downward due to gravity and also expand due to their finite temperature. Here atoms are about 5 micro-Kelvin, after optical molasses cooling.
Using degenerate Raman-sideband cooling, we achieved a much lower temperature. In this figure, we show that the atoms hardly expand within the free fall time of 25 ms. The temperature of these atoms is 470nK. The fine fringes are due to imperfection of our imaging system, which we have fixed now.