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a. Preparation of the magnetic trap 'reservoir'

Dans le document The DART-Europe E-theses Portal (Page 80-83)

Part I – Abstract

I.4. EXPERIMENTAL WORK

I.4.1. a. Preparation of the magnetic trap 'reservoir'

In this paragraph the sequence for the preparation of a 'reservoir' of Cs atoms with the use of magnetic trapping is presented. The atoms are loaded from the Cs vapor in the MOT 1, and guided by the 'pushing-guiding' laser to MOT 2. There, they are pass subsequently form a C-MOT phase and a molasses phase in order for their density to be increased and their temperature to be lowered, they are polarized in the |F=3,mF=-3> state and loaded in the magnetic trap. These atoms are detected with fluorescence imaging and their temperature is measured with the Time-Of-Flight (TOF) method.

The experimental sequence used for the preparation of the magnetic trap is schematically shown in Fig. I.4.1. Initially, the trapping and re pumping lasers for MOT 1 and 2 are ON, as well as for the pushing laser. The detuning of the MOT 2 trapping laser is ~3Γ (Γ=5.2MHz) slightly to the red with respect to MOT 1,(the MOT 1 is realized a bit more compressed, since it has to be as large as possible). The MOT2 loading process is saturated after five seconds, where the subsequent phase of C-MOT starts. Then, the MOT 1 trapping and repumper as well as the pushing lasers are turned off. These lasers are controlled by acousto-optic modulators which give the ability to modulate their intensity and frequency, which have respond times smaller than microsecond.

Nevertheless, the minimum intensity that they transmit cannot be zero, i.e. their 'extinction ratio' is limited. In order to correct this, and to avoid the presence of photon leak in the atomic sample, whenever a laser is turned off, additional to lowering its intensity with the AOM, the laser path is blocked with a small cooper screens. These light metallic leaves are mounted in small motors, and the overall system (called as 'click clack' due to the characteristic sound) has response times of some microseconds (except for the pusher click clack which has response time of ~12 ms, due to the large dimensions of the pusher beam). These time delays were frequently measured and updated in the LabVIEW piloting program where they were taken into account automatically during the experimental sequence.

In the subsequent C-MOT phase the MOT 2 trapping laser frequency is brought to ~5Γ, and the repumper intensity is slightly lowered. In the C-MOT phase the task is to increase the atomic sample's density. Usually, in such a phase the magnetic field gradient is increased with respect to the one used for the MOT. Nevertheless, in this experiment, the magnetic trap loading, as well as the loading of the dipole trap where found to be better when the magnetic field gradient is not increased at this point.

The repumper intensity is further decreased and the MOT 2 trapping frequency is brought to

~8Γ, in the subsequent molasses phase. Then the repumper is blocked and the atoms are left radiated by the trapping laser alone for ~1 ms, in order for them to be optically pumped to the F = 3 state. The polarization laser is applied for a period of time that can be as short as 500 µs. In the same time, the magnetic field gradient is turned off, and a small bias magnetic field is applied. The role of this magnetic field is to define a direction with which the polarized atoms are 'aligned' with, and to avoid depolarization due to stray magnetic fields. This bias field is available by the same coils that generate the field necessary to bring the system in the vicinity of a Feshbach resonance during the evaporation process. Its use in the polarization sequence is discussed in the next paragraph along were the polarization is discussed.

The magnetic field gradient during the MOT and C-MOT phase has been held at the value of

~15 G/cm while it was turned off during the molasses phase. The magnetic field gradient used for the magnetic trap is available by the same coils which provide magnetic field gradients up to some hundreds G/cm. As previously explained, control over the magnetic field gradient is achieved by controlling the current provided in the coils, with the use of a MOSFET switch. which has response times of some microseconds.

After being trapped for a variable time interval, the atoms are detected via fluoresce imaging and their temperature is measured with the TOF method. For the fluorescence imaging to be achieved the atoms are shined with the trapping and the repumper laser light for ~2 ms. The reason that the fluorescence method is preferred is mainly its simplicity. The alternative absorption imaging is more complicated and its realization is more time consuming. Additionally, the VISION software used with this method is configured to analyze automatically images of the dipole trap, and Fig. I.4.1: Experimental sequence used for the preparation of the magnetic trap reservoir, see text for details.

configuring it for different types of traps demands great effort. Finally, absorption imaging is not necessary for this step, since fluorescence imaging exhibits no problems for these densities. An example of the detection of the magnetic trap with the use of fluorescence imaging is shown in the following picture.

The atoms are imaged after having performed ballistic expansion under the influence of Fig. I.4.2: Images of the magnetically trapped atoms after having performed free

ballistic expansion for 5 (a), 10 (b) and 15 ms (c).

Fig. I.4.3: Fitting the expansion of the atomic cloud dimensions. σz and σR stand for the size in the point where the density of the atomic cloud reaches the 1/e of its peak value. The fitting is done with Igor, according to equation 3.10. The temperature is found to be 47.3 ± 1.8 µK for the axial and 40.75 ± 3.7 µK for the radial direction.

gravity for 5, 10 and 15 ms. In Fig. I.4.3 we see plots of the dimensions of the expanding cloud in the radial and axial direction versus the TOF. These data points are fitted according to Eq. 3.10 and the radial and axial temperature is determined with the process described in paragraph I3.3.a. Here the atomic cloud has ~108 atoms at a temperature of ~45 µK.

Dans le document The DART-Europe E-theses Portal (Page 80-83)