Simultaneous sub-Doppler laser cooling of fermionic 6Li and 40K 

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Principle of a 2D-MOT

A 2D-MOT transversally connes atoms, captured from a background vapor, and outcouples an atomic beam in the longitudinal direction through a dierential pumping tube. The tube also acts as a velocity lter which yields a transversally and longitudinally cold atomic beam arriving in the 3D-MOT chamber. A longitudinal molasses cooling increases the ux of a 2D-MOT[157], because atoms spend more time in the transverse cooling region. In our setup we use a 45-angled mirror inside the vacuum chamber allowing for two independent, counter propagating, longitudinal molasses beams, labeled long+ and long- in Fig. 2.6. In our new setup, we implement an independent pushing beam that out-couples the atomic beam through a small hole in the center of the 45-angled mirror.

Characterization of the 2D-MOT upgrade

The performance of the 2D-MOT is characterised by the loading rate of the 40K-MOT. Its dependence on several 2D-MOT parameters 6 has been already studied in the thesis of Armin Ridinger [148]. Here we focus on the upgrades of the 2D-MOT setup, namely, a new strategy to control the pressure in the 2D-MOT glass cell (see Cold point on page 18), and the integration of an independent pushing beam (see Independent pushing beam on page 19).

6Li-40K dual-species MOT

In this section we present the implementation and performance of our 6Li-40K dualspecies MOT. Combining three orthogonal pairs of counter-propagating red-detuned laser beams with a magnetic quadrupole eld cools and magneto-optically connes atoms around the eld zero [156]. Hence the name magneto-optical trap. We use bichromatic MOT-beams for both 6Li and 40K with a cooling and repumping frequency. The latter is needed for alkali atoms in general and ensures that the atoms stay in the almost cycling cooling transition. For 6Li the repumping frequency even contributes to the cooling.
Because of the very small hyperne structure of the excited-state, the 6Li atoms very likely leave the cooling transition. For a dual-species MOT it is important to minimize the light-induced interspecies collisions to avoid losses. We are able to limit these losses to less than 10%, by using low magnetic eld gradients and low intensity repumping light for both atomic species [125].

Optical molasses – D1 sub-Doppler cooling

To increase the phase-space density of the 6Li-40K mixture we apply an optical molasses phase in combination with a preceding compressed MOT phase (CMOT). The CMOT phase increases the densities of the atomic samples, before the D1 molasses phase cools them to deep sub-Doppler temperatures. Here we describe the implementation of the 6Li-40K dual-species molasses. Its characterization is presented in Chapter 4.

Compressed MOT

To increase the density of the MOT we apply a compressed MOT phase. In 5 ms the magnetic gradient is linearly ramped from 8G/cm to 45G/cm. At the same time the frequencies of the cooling and repumping light are brought closer to resonance while the beam intensities are reduced. For 40K the density increases from 7 × 1010 at./cm3 to 3.7×1011 at./cm3 while the temperature increases from 240 μK to 2mK. The 6Li density increases from 2.6 × 1010 at./cm3 to 1.8 × 1011 at./cm3 and the temperature decreases from 1mK to 800 μK.

D1 laser system

Figure 2.10 shows the level schemes of the D1 lines for 6Li and 40K as well as the molasses beams. The D1 sub-Doppler cooling scheme requires two more separate laser systems, because the AOMs of the D2 systems can only be tuned by some tens of MHz (see Fig. 2.11), while the frequency separations of the D1 and D2 lines for 6Li and 40K, are 10 Ghz and 11 Thz, respectively (see Fig. 2.2).

Transfer to the magnetic quadrupole trap

After the optical cooling phase, the atoms are transferred to a magnetic quadrupole trap. For an ecient transfer, the atoms need to be optically pumped to magnetically trappable states (see Section 5.1.2). Therefore, in presence of a small bias eld, a short spin polarization light pulse is applied before the magnetic trap is switched on. The magnetic trapping eld is created by the same coil pair as for the MOT. It takes 3ms to ramp up the magnetic gradient from zero to 45G/cm. From there, the gradient is linearly ramped to its nal value of 145G/cm in 500ms.

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Magnetic transport

The lifetime of the atoms, prepared in the dual species MOT, and the optical access can be increased by transferring the trapped cloud to the UHV science cell. We use a magnetic transport technique with a 90 corner [153], yielding additional optical access on the transport axis. The magnetic transport is realized with 14 overlapping coil pairs and a pushing coil. Time varying currents continuously move the magnetic trap center from the MOT chamber to the science cell over a total distance of 64 cm. A detailed discussion of the magnetic transport is presented in Section 5.3.

Optically plugged magnetic quadrupole trap

In the science cell, we perform RF evaporative cooling in an optically plugged magnetic quadrupole trap. The optical plug avoids atom loss caused by Majorana spin- ips by repelling the atoms from the trap center. The steep, linear connement of the quadrupole trap leads to high elastic collision rates, enabling an ecient evaporative cooling.

Table of contents :

1 Introduction 
1.1 Quantum gases
1.2 Ultracold atoms { a highly controllable model system
1.3 Quantum degenerate Fermi gases
1.4 Fermi-Fermi mixtures with two dierent atomic species
1.5 Sub-Doppler laser cooling
1.6 Thesis outline
2 Experimental setup 
2.1 Design
2.2 Vacuum system
2.3 D2 laser system
2.4 6Li Zeeman slower
2.5 40K 2D-MOT
2.5.1 Principle of a 2D-MOT
2.5.2 Experimental setup
2.5.3 Characterization of the 2D-MOT upgrade
2.6 6Li-40K dual-species MOT
2.6.1 Experimental setup
2.7 Optical molasses { D1 sub-Doppler cooling
2.7.1 Compressed MOT
2.7.2 D1 laser system
2.7.3 Implementation of the D1 molasses
2.8 Magnetic trapping
2.8.1 Transfer to the magnetic quadrupole trap
2.9 Magnetic transport
2.10 Optically plugged magnetic quadrupole trap
2.10.1 Coils
2.10.2 Optical plug
2.10.3 RF evaporative cooling
2.11 Optical dipole trap
2.11.1 Power stabilization
2.11.2 ODT2
2.12 Optical setup { Science Cell
2.13 Diagnostic tools
2.13.1 Fluorescence monitoring
2.13.2 Absorption imaging
2.13.3 Experiment control and data acquisition
2.14 Conclusion
3 High power 671 nm laser system 
3.1 Introduction
3.2 First generation
3.2.1 The Nd:YVO4 gain medium
3.2.2 Crystal structure
3.2.3 Absorption
3.2.4 Thermal eects in solid-state lasers
3.2.5 Performance
3.3 Second generation: Intracavity-frequency-doubling
3.3.1 The fundamental laser
3.3.2 Ecient intracavity second-harmonic generation
3.3.3 Tuning behavior and nonlinear-Kerr-lens mode locking
3.3.4 Conclusion
3.4 Third generation: Power scaling
3.4.1 Scheme
3.4.2 Infrared laser
3.4.3 Doubling cavity
3.4.4 Locking scheme
3.4.5 Experimental results
3.5 Conclusion
4 Simultaneous sub-Doppler laser cooling of fermionic 6Li and 40K 
4.1 Prelude: Laser cooling
4.1.1 Doppler cooling
4.1.2 Bright optical molasses
4.1.3 Sub-recoil laser cooling
4.1.4 Velocity selective coherent population trapping (VSCPT)
4.1.5 Gray optical molasses
4.2 D1 sub-Doppler laser cooling
4.2.1 40K D1 molasses
4.2.2 6Li D1 molasses
4.2.3 Raman-detuning dependance
4.2.4 Simultaneous D1 cooling of 6Li and 40K
4.3 Conclusion
5 Magnetic trapping, transport and evaporation 
5.1 Magnetic trapping
5.1.1 Principles of magnetic trapping
5.1.2 Transfer from the D1 molasses to the magnetic quadrupole trap
5.2 Thermalization and non-ergodicity
5.2.1 Thermalization experiment
5.2.2 Non-ergodicity
5.3 Magnetic transport
5.3.1 Algorithm { Keynote
5.3.2 Algorithm { Calculating the time-dependent transport currents
5.3.3 Experimental results
5.4 Evaporative cooling
5.4.1 Principle of evaporative cooling
5.4.2 Cooling approach
5.5 Conclusion
6 Conclusion 
Acknowledgements
Bibliography 

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