Simultaneous Trapping of Rb and Cs in a Vapor-Cell MOT

Amy C. Sullivan and George A. Ruff
Department of Physics, Bates College
Lewiston, ME 04240


We have developed a magneto-optical trapping apparatus which permits simultaneous cooling and trapping of cesium and rubidium in a compact octagonal vapor cell. The background vapor pressures of each atomic species are independently controllable, as are the intensities and frequencies of the respective trapping and repump lasers. Trap sizes and positions are determined by a pair of CCD cameras which view the trap from orthogonal directions, and loading times for each species are determined by PMT observations of trap fluorescence.

The Vapor Cell

The vapor cell is made from pyrex glass plates, fused together at the edges. The atomic vapors are generated by getters mounted in the sidearm of a conflat tee. The vapor pressure of each species is controlled by adjusting the current in the corresponding getter.

Saturated Absorption Spectra

Two weak probe beams are sent through a cesium cell into a pair of photodiodes. A strong, counter-propagating pump beam overlaps one of the weaker probe beams. This pump beam causes Lamb dips in the Doppler curve of the weaker beam. When the two photodiode signals are subtracted, the background Doppler curve disappears, revealing the hyperfine structure as seen above. The repump line shows the 6S1/2(F=3) 6P3/2(F'=2,3,4) transitions while the trapping line shows the 6S1/2(F=4) 6P3/2(F'=3,4,5) transitions of cesium. The trap line is locked slightly to the red of the F = 4, F' = 5 transition. The frequency of the repump laser is not as critical and can be locked either to the F = 3, F' = 3 or to the F=3, F' = 4 transition.(1) By using saturated absorption spectroscopy as described for Cs, the hyperfine structure of 87Rb is shown. The above repump line shows the 2S1/2(F=1) 2P3/2(F'=0,1,2) transitions and the trapping line shows the 2S1/2(F=2) 2P3/2(F'=1,2,3) transitions. Here the trap laser is locked slightly to the red of the F = 2, F' = 3 transition and the repump is again not as critical, but is locked to the F=1, F' = 2 transition.(2)

The Locking Electronics

The two inputs to this locking circuit come from the photodiodes in the saturated absorption setup. One signal is negative and the other is positive. They are added to show the hyperfine structure of the atom. The circuit is used in two ways to lock the laser frequency to the desired atomic transition. The output of the first integrator goes directly to the current input of the laser controller. This is used for rapid adjustments in the laser frequency. The output of the first integrator is then integrated again. The second integrator attempts to lock the laser current to its initial set point. The output of this second integrator goes to a PZT which controls the diffraction grating thereby altering the length of the external laser cavity. These two outputs keep the laser locked to the atomic transition needed to see a trap.(3)

Optical Table

Another View

Optical Schematic

Each saturated absorption cell is used to lock a trap laser and a repump laser. The lasers each contain an internal beamsplitter, producing two output beams. The output beams from all four lasers are combined before entering the trap cell. Thick glass plates are used to pick off probe beams for saturated absorption spectroscopy. The trap beams are then combined using nonpolarizing beamsplitters to preserve the horizontal polarization of the trapping beams. These elliptical beams are then sent through anamorphic prisms to stretch the beams horizontally so that the resulting beams are circular. These are broadened by 4X telescopes and sent through quarter wave plates to make the beams circularly polarized. The shape and polarization of the repump beams are less important, and these beams are sent into the trap cell on only one axis.

Fill time (logarithmic curve)

When one of the trapping laser beams going to the trap is blocked, the trap disappears. After unblocking this beam, the trap slowly reforms. This experiment is performed with the Cesium trap, the fluorescence from which is detected by a PMT. The above graph shows the log of this exponential growth, with the 1/e trap fill time as calculated from the slope of the line. The background pressure for this trap was relatively low, resulting in a slow fill time of 2.68s.


1. Christopher Roy Monroe, Ph.D thesis, University of Colorado (1992).

2. C. Wieman, G. Flowers, and S. Gilbert, AJP, 63, 317-330 (1995).

3. G. D. Rovera, G. Santarelli, and A. Clairon, Rev. Sci. Intrum., 65, 1502- 1505 (1994).