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OnderzoekVan Swinderen InstituteFundamental Interactions and Symmetries (TRIµP)

Experimental Results

We succeeded in trapping all the stable isotope of calcium (40Ca, 42Ca, 43Ca, 44Ca, 46Ca, and 48Ca) in our magneto-optical trap. The trap is loaded from a deflected, Zeeman slowed atomic beam. The deflection stage is an important step in decreasing the 40Ca background relative to the other isotopes. An important step in this experiment is the realization of single-atom counting. A lens system is inserted in the trapping chamber in order to collect as much fluorescence as possible. An essential step in reducing the background light level was painting the inside of the vacuum chamber black. Great care must be taken to suppress undesirable background light.

The trap has a temperature of a few milli-Kelvin, as is typical for alkali-earth MOTs. This corresponds to an average speed for the atoms of about half a meter per second. A picture (figure 2) is shown of the trapped calcium atoms. More close-up you can see picture 3. The picture is taken with a CCD camera. Figure 4 is just a fancy plot, showing the fluorescence intensity. At the moment the fluorescence of the MOT is recorded by a CCD camera, interfaced through a labview program. This enables us to subtract background light, average multiple images and change camera parameters in realtime.

Figure 2: Picture of trapped calcium atoms
Figure 2: Picture of trapped calcium atoms
Figure 3:CCD-camera picture of trapped calcium atoms
Figure 3:CCD-camera picture of trapped calcium atoms
Figure 4: surface plot of fluorescence intensity 
Figure 4: surface plot of fluorescence intensity 

 

We are also doing doppler free saturation absorption spectroscopy. This has been done in a small vacuum chamber that we have built that we use to stabilize the laser. A similar oven as the one connected to the MOT-chamber produces a beam of calcium atoms. Two counter-propagating laser beams of the same frequency are used to excite atoms in the beam. The laser beams are oriented perpendicular to the atom beam. We operate the oven at temperatures of up to 650 ˚C. We can clearly see the lamb-dip in the absorption signal (figure 5) and we observe doppler-free signals of the isotopes 40Ca, 42Ca, 43Ca, 44Ca and 48Ca. (figure 6).

Figure 5: Lamb dip
Figure 5: Lamb dip
Figure 6: Doppler free saturation absorption spectroscopy
Figure 5: Doppler free saturation absorption spectroscopy

As the laser frequency is scanned and we look at the MOT fluorescence with the CCD camera, we can see the different isotopes being trapped. This is shown in figures 8 and 9, below. We find that only the 9/2 component of 43Ca is trapped. The intensities of the trapped isotopes corresponds within a few percent to those listed in literature. Only from the 43Ca we trap less due to the hyperfine structure and the reduced Doppler fore.

Figure 8: Fluorescence intensity of the trap as a function of laser frequency
Figure 8: Fluorescence intensity of the trap as a function of laser frequency
Figure 9: Frequency scan around the isotope43Ca
Figure 9: Frequency scan around the isotope43Ca
The figure below (11 dec 2002) shows all the isotopes we have trapped so far. Fluorescence signals from three different measurements has been combined in this graph, in order to be able to see in the same picture 40Ca (97%) as well as 46Ca (0.004%).
Figure 10: All the trapped isotopes so far
Figure 10: All the trapped isotopes so far
The final limit in the sensitivity is the ability to detect single atoms in the trap. Using a repump laser at 672 nm the trapping time of the atoms can be extended from 20 ms to 160 ms. As a result, the steady state number of trapped atoms also increases by a factor of 8. Using low oven temperatures and switching off the Zeeman slower we have been able to detect single 40Ca atoms in the trap. This is demonstrated in the figure below, where three different graphs are shown for different oven temperatures. It can be seen that the average trapping time of the atoms is much longer then 20 ms, and from unloading measurements of the trap we find an average trapping time of 160 to 180 ms.
Figure 11: The fluorescence signal of a few trapped calcium atoms. Shown are three different graphs for three different oven temperatures.
The fluorescence signal of a few trapped calcium atoms. Shown are three different graphs for three different oven temperatures.
Last modified:20 June 2014 10.19 a.m.