Experiments to Control Atom Number and Phase-Space Density in ...

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shutter instead. The shutter speed is less than 1 µs, enabling this camera to image atoms at very short times after the MOT beams are turned off. The second problem results from the particular imaging setup in this experiment. Figure 7.38 shows that the imaging beam axis coincides with one of the three MOT beam axes. Even though more than 95% of the MOT beam power are reflected at the reflective polarizer, approximately 5% of the power is transmitted and photons impinge on the CCD chip of the Stingray camera, saturating the pixels. Before an image is taken, all prior charge built-up has to be removed. Unfortunately not all charges can be removed during the brief shutter time. An image of the remaining charge built-up is shown in figure 8.7. Figure 8.7: Charge built-up on the Stingray CCD camera. This effect on the raw absorption image leads to a lot of noise in the absorption and optical density pictures, if the same effect is not visible in the probe image. It is therefore necessary to turn on the MOT beams again before taking the probe image. About 10 ms of MOT beam exposure prior to taking the probe image are sufficient to introduce the same effect on the probe image, so that this imaging artifact can successfully be removed from the absorption images. 8.3 Alignment of the CO2 Laser Dipole Trap Optical evaporation of atoms trapped in a red-detuned focused laser beam has been a very powerful technique to create degenerate gases. Using optical evaporation, degenerate gases of alkali atoms have been created in lithium [39, 111], sodium [112], 155
ubidium [113–115] and cesium[116]. Creating the tweezer trap using CO2 lasers (λ ≈ 10 µm) has been particularly interesting because of the low noise of these lasers as well as the very low scattering rate due to the large detuning. However, aligning the CO2 laser beam to the trapped cloud of atoms remains a challenging task. A standard technique to align an optical dipole trap beam is to image this beam at the location of the atoms directly onto a camera. This method is however not employ- able when working with a wavelength larger than the mid-infrared, due to the limited wavelength sensitivity of a CCD or CMOS camera and thus does not work for aligning a CO2 laser beam. The second technique is to look for a change in the scattering rate of the atoms in the presence of the CO2 laser beam. Any laser beam causes a change in the polarizabilites of the atoms by the AC-Stark shift, thus effectively changing the transition frequency. Therefore the scattering rate of atoms in the MOT changes, as the detuning of the MOT laser beams is varied. This technique has been used successfully in [117]. However, for lithium atoms the change in the scattering rate in the presence of the CO2 laser is vanishingly small, because the ground ( 2 S1/2) and excited ( 2 P3/2) state polarizabilities are nearly equal. For a 50 W laser beam focused to a waist of 50 µm the change in transition frequency at the peak intensity of the CO2 laser is less than10 MHz. The overall change in scattering rate is therefore negligible and makes alignment using this technique unviable for lithium atoms. In [118] an additional laser beam at a wavelength of 610 nm was therefore added. The scattering rate for this transition is highly sensitive to the alignment of the CO2 laser and quick alignment using this technique could be achieved. However, this method requires an additional laser system to drive the sensitive transition and is therefore in general difficult and expensive to implement. Instead the CO2 laser beam is aligned by using balanced lock-in detection from a split-image of the fluorescence signal. This method requires only a few optics, two photodiodes and a lock-in amplifier. Figure 8.8 shows a schematic of the experimental setup. About 3×10 8 6 Li atoms are loaded in a MOT during the course of 2 s. The fluorescence light of the MOT is collected at an angle from underneath the vacuum chamber using a mirror (M1) and a 25 mm diameter lens with focal length f = 75.6 mm (L1). This setup is determined by 156
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Copyright by Kirsten Viering 2012
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Experiments to Control Atom Number
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Acknowledgments First of all, I wou
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Experiments to Control Atom Number
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Table of Contents Acknowledgments v
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Chapter 7. Lithium Apparatus 92 7.1
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List of Tables 2.1 Physical propert
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4.1 Vacuum chamber . . . . . . . .
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7.39 Future vertical axis optics .
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The development of techniques to la
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Ptorr 10 5 10 6 10 7 10 8 10 9 260
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Property Symbol Value Reference Ato
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2.3.2 Hyperfine Structure So far th
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The Wigner-Eckhart theorem can be u
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The total magnetic moment of an ato
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interaction. The shift in the energ
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(a) (b) scattering length a 0 Energ
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a laser field with frequency ω. Th
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Here ω0 is the resonance frequency
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Optical molasses do not provide a s
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J=1 J=0 -1 Δ σ + σ - σ - 0 +1 -
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phase-space density is defined as
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which maintains a constant η = U/k
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2.12.1 Saturated Absorption Spectro
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Figure 2.22: Saturation absorption
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Signal 0 Signal (a) (b) Signal Freq
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(a) (b) (c) (d) Figure 2.24: Absorp
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y more complex energy level structu
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ight side of the box. An irreversib
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time (a) (b) (c) Figure 3.3: Single
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them to move from B to A, but close
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Figure 4.1: Vacuum chamber. Photogr
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( ( Figure 4.3: Helicoflex seal. (a
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88 mm 4.2.3 Auxiliary Coils 62 mm Q
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5 2 P 3/2 5 2 S 1/2 384. 230 484 46
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distribution of the MOT master lase
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shifts. First a frequency shift of
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transition. However, the laser freq
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4.3.1.4 Upper MOT Horizontal Slave
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provides the beam for the upper MOT
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demon beam λ λ horizontal MOT bea
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scholar, Florian Schreck. It is wri
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Atoms are then optically pumped to
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ferred into the |F = 1,mF = 0〉 or
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5.2 Branching Ratios and Population
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effect of the reflected beam at zer
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y the repulsive trough beams, gaini
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Figure 5.9: Atom number as a functi
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Figure 5.10: Radius of the magnetic
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on the collision rate of atoms insi
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ferred via the single-photon coolin
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By comparison an atomic Fock state
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of atoms in the two hyperfine groun
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difference in the lifetimes is dete
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Chapter 7 Lithium Apparatus Creatin
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angle valve rotary feedthrough ion
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an ion gauge controller (Granville
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viewports (MDC, 450002) are attache
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not trapped by the MOT impinge on t
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Should contamination of the vacuum
Page 121 and 122: Figure 7.12: The assembled Zeeman s
Page 123 and 124: 7.2.2 MOT Coils A second set of coi
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Page 127 and 128: are required to create the high mag
Page 129 and 130: The coils are then checked for shor
Page 131 and 132: + B A Figure 7.23: Feshbach coil H-
Page 133 and 134: ensure that no particulates build u
Page 135 and 136: 118 Figure 7.26: Optical layout of
Page 137 and 138: The error signal shows three lines,
Page 139 and 140: into the CO2 laser. The optical lay
Page 141 and 142: Thorlabs PDA10A Coupler ZDC-10-1 to
Page 143 and 144: Imaging ber Imaging ber from Imagin
Page 145 and 146: Table 7.4 summarizes the beam power
Page 147 and 148: CO2 laser 40 MHz AOM f=2 in f=2 in
Page 149 and 150: 132 Figure 7.38: Optical layout aro
Page 151 and 152: Feshbach coil objective lenses atom
Page 153 and 154: MOT beams are no longer traveling a
Page 155 and 156: The second card (NI6733) offers eig
Page 157 and 158: Averaging over the magnetic subleve
Page 159 and 160: 2 2 P 3/2 D 2=670.977nm 2 2 S 1/2 m
Page 161 and 162: The right hand side of equation 7.1
Page 163 and 164: = E0 1 −n2 √2 exp σ0 ± iE0
Page 165 and 166: esidual potentials besides gravitat
Page 167 and 168: Figure 8.1: MOT loading. Typical lo
Page 169 and 170: A linear ramp typically changes the
Page 171: A typical MOT is density-limited at
Page 175 and 176: that most of the noise is due to th
Page 177 and 178: (a) (c) y . x z X=0.0mm Z=2.5mm 1)
Page 179 and 180: Figure 8.11: Different shapes of MO
Page 181 and 182: ZnSe lens f=5 in knife edge ZnSe wi
Page 183 and 184: shown in figure 8.13. By matching t
Page 185 and 186: Figure 8.15: Atom number and temper
Page 187 and 188: 275 Hz. Using these values the beam
Page 189 and 190: system. Maybe the easiest way to ac
Page 191 and 192: Appendix A Magic Wavelength of Hydr
Page 193 and 194: transitions for the excited state p
Page 195 and 196: Bibliography [1] J.J. Thomson. Cath
Page 197 and 198: [22] C.J. Foot. Atomic Physics. Oxf
Page 199 and 200: [46] E.D. Black. An introduction to
Page 201 and 202: [67] J.L. Hanssen. Controlling Atom
Page 203 and 204: [88] D.S. Naik, C.G. Peterson, A.G.
Page 205 and 206: [110] J.W. Goodman. Introduction to
Page 207: [132] C. Degenhardt, H. Stoehr, U.
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