c. Loading a crossed dipole trap from a magnetic trap reservoir

Dans le document The DART-Europe E-theses Portal (Page 86-93)

Part I – Abstract

I.4. EXPERIMENTAL WORK

I.4.1. c. Loading a crossed dipole trap from a magnetic trap reservoir

The experimental sequence used for the study of the loading of a dipole trap from a magnetic trap reservoir is shown in Fig. I.4.8. It is similar with the sequence shown in Fig. I.4.1, with the difference of adding the dipole trap, applying the polarization twice and performing the data acquisition with the use of absorption imaging. Again the atoms are loaded in MOT 2 for ~5 s

Fig. I.4.7: (a) Fluorescence image of the magnetic trap, (b) after polarization to the |F=3,mF=-3> and (c) after polarization in the |F=3,mF=3> state.

and are subjected to compression in the C-MOT phase, cooled in the molasses phase and polarized.

Then they are loaded in the magnetic trap and kept for a variable time labeled ttrap until the dipole trap is turned on. The duration of the superposition of the dipole trap with the magnetic trap is labeled tload and it is one of the main parameters under study. At the end of the loading process, the magnetic field gradient of the magnetic trap is removed and the atoms are polarized once more in the |F=3,mF=3> state. This process is necessary in order to minimize the inelastic two body losses and thus increase the lifetime of the dipole trap. Nevertheless, when the loading of the dipole trap is studied and the number of atoms in the dipole trap is detected shortly after the end of the loading process, this second polarization step is not necessary and is omitted. When the lifetime of the atoms in the dipole trap is studied, the dipole trap stays on for time thold after the end of the loading process and the atoms are detected with the use of absorption imaging.

In this study fluorescence imaging has been replaced by absorption imaging, a fact that has increased the duration of our experimental sequences by 2 s. The reason for this increase is the fact that the data acquisition in absorption imaging makes use of three images, one of the atoms in the dipole trap and the absorption laser together, one with the dipole trap without atoms and one of the background, as explained in I.3.5.b. In the sequence shown in Fig.4.8 we see the absorption camera been triggered four times; this is due to the fact that an initial shot is necessary for the camera's chip to 'flush' the previous images. The use of absorption imaging increases the duration of our sequences and adds one more diode laser in the set-up, nevertheless it is necessary. Apart the technique's increased resolution and, in general, its suitability for the study of dipole traps, our main reason for the choice of absorption imaging is that the use of VISION with combination with the LabVIEW piloting program allowed for the automation of the study. The sequence shown in Fig.4.5 is executed repeatedly and the parameter under study is modified. Each time the data are recorded by VISION along with the value of the parameter under study.

In Fig. I.4.9 we see such a study of the parameter ttrap which is the time that the atoms spend in the magnetic trap before the dipole trap is turned on. The time ttrap is varied from the initial value of 5 to 300 ms. The dipole trap is loaded for 15 ms and then it is imaged by means of absorption and VISION calculates the atom number and the temperature according to the methods discussed in I.3.3.b. The magnetic field gradient used for the magnetic trap is ~150 G/cm, while the dipole trap's waist is 89 µm (corresponding to the measurement shown in Fig.3.10). The short overlap between the magnetic trap and the dipole trap (5 ms) as well as the small power of the dipole trap (~30W) Fig. I.4.8: Experimental sequence used for the study of the loading of a dipole trap from a magnetic trap reservoir, see text for details.

explain the small atom number. As we see, the number of transferred atoms in the dipole trap and their temperature is unvaried with respect to this parameter.

This measurement showed that ttrap is not a critical parameter, thus it can be eliminated and indeed, in the following of this study the starting point of the dipole trap and the magnetic trap coincide. The following parameter to be studied is the time in which the dipole and the magnetic trap coexist, tload. This parameter is perhaps the most important of this study, since, the technique aims to improve loading by increasing the loading time with the use of the reservoir. However, as Fig. I.4.10 shows, even though loading is initially improved as tload is increased, this increase reaches its maximum in less than 10 ms, and gives its place to very violent losses.

Fig. I.4.9: Evolution of the atoms number (a) and temperature (b) of the dipole trap with respect to ttrap. The laser power is ~30W, and the waist ~98 µm.

The incompatibility of this result with the predictions of [Comp06] is striking, since it predicted that the loading process would last ~10 times more. At the time were the experiment was conducted our efforts were oriented towards the examination of the experimental conditions, in order to identify and to eliminate possible imperfections that could be responsible for the technique's poor performance. Among the various elements that were examined during the period that the experiment was conducted, there was the overlap of the dipole and the magnetic trap. The overlap of the two traps has to be precise and the dipole trap has to be realized as close as possible to the center of the magnetic trap. This is necessary on the one hand in order for the loading to be optimized. When a dipole trap is loaded by a MOT or a molasses, the center of the quadrupole magnetic field gradient of the MOT is not the best position for its realization as demonstrated for the case of 87Rb in [Szcze09] and as we have verified for the case of Cs in the lab as well. For the case of Cs atoms loaded from a magnetic to a dipole trap, overlap in the center of the magnetic trap is necessary. If the dipole trap is realized in the presence of high magnetic field gradient the two body losses between atoms in different magnetic sublevels can be very violent. The way this parameter was controlled was to realize magnetic trap of very small dimensions (~200 µm) with the use of high magnetic field gradients (~250 G/cm). We took advantage of the fact that the two different imaging systems, the fluorescence and the absorption imaging, were realized in different (almost) orthogonal directions, in order to verify the good overlap of the dipole and the 'small' magnetic trap as seen in Fig. I.4.11.

Fig. I.4.10: Evolution of the atoms number (a) and temperature (b) of the dipole trap with respect to tload. Here the laser power is risen to ~60W while the waist is kept at ~98 µm.

The study was repeated for various conditions. In Fig. I.4.12 we see the evolution of the atom number in the dipole time versus the loading time for (a) waist ~89 µm but power ~2 times larges (65 W) and (b) for a waist ~130 µm. The final number of atoms is never sufficiently increased. Again the same trend is observed: the number of atoms initially increases with the loading time, until finding its maximum in relatively small times and then it decreases rapidly. For the case of the 89 µm trap this maximum is found at ~50 ms while for the case of the 130 µm trap the maximum is found in ~80 ms. A final parameter to be studied is the value of the magnetic field gradient used to realize the magnetic trap. In Fig. I.4.13 we see the result of such a study for the 89 µm and 60 W dipole trap1.

1 The number of atoms detected in the dipole trap depends a lot on the time tload . As tload increases, the number of atoms in the dipole trap decreases, initially rapidly (spilling) and more smoothly later, depended on the nature of collisions in the trap. In the studies presented here the traps were detected ~10ms after the end of the loading proccess, except for the measurements shown in Figs.4.11.a and 4.12. where this time was set at 5ms. This is the Fig. I.4.11: Magnetic trap of small dimensions (~200µm) imaged with fluorescence imaging in (a) and absorption imaging in (c). Dipole trap fluorescence (b) and absorption (d) images. The two imaging techniques were realized in vertical directions a fact that allows to test the realization of the dipole trap in the center of the magnetic trap.

Fig. I.4.12: Evolution of the number of atoms in the dipole trap with respect to tload. In part (a) the trap's dimensions are still~89µm while the power is now ~60W. In part (b) the trap's dimensions is 130µm.

Fig. I.4.13: Modification of the final atom number in the dipole trap with respect to the magnetic field gradient, for a trap of ~89 µm and ~60 W.

Dans le document The DART-Europe E-theses Portal (Page 86-93)