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Download Atomic and Molecular Physics for Physicists Ben-Gurion University of the Negev
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Ben-Gurion University of the Negev Atomic Atomic and and Molecular Molecular Physics Physics for for Physicists Physicists Ron Folman Chapter 11: New experimental procedures with atom-light interaction (absorption and fluorescence imaging, magneto optical trap). Main Reference: Any modern text book or Review paper on atom physics (also thesis from our laboratory) Exercises: Dudi Moravchik. www.bgu.ac.il/atomchip www.bgu.ac.il/nanofabrication www.bgu.ac.il/nanocenter Before we start looking at experiments, we need to know how to see atoms? Namely how to be able to measure the result of the experiments. IMAGING ATOMS Fluorescence Imaging Atom Chip Detector Cold atomic cloud 87Rb atoms Homework: πhν3 1. Why is saturation light intensity defined as: Is ≡ 2 3τc 2. the ccd has a chip the size of 1cm^2 at a distance of 50cm from 10^8 Rb Atoms. The atoms receive saturation light intensity. How many photons does the ccd receive? Absorption Imaging Take an object e.g. a key, in a dark room i.e. with a very feeble source of light And you will find out that the best way to know its shape is not to look at the Reflected light but rather at the shadow….. Resonant light f1 f1 Results from our lab Homework: the laser beam has a size of 1cm^2 with 10mW of power illuminating 10^8 Rb atoms. How many photons does the ccd miss? Detector MOT: Magneto-Optical-Trap Lets look in one dimension: Mirror (atomchip) MOT problem: the chip blocks two of the 6 beams. Solution: trick the atoms to think there are 6 beams. Namely we use only 4 beams. Homework: 1. Why do the helicities change upon reflection (this enables the atomchip setup)? 2. Until now we saw that all sigma (circular polatization) signs and/or helicities are the same for counter propagating beams in the MOT. Why is the above picture (also explaining a Mirror MOT) different? Hint: the energy level diagram will also look different, but its just different notation explaining the same physics. Explaining the MOT with a different notation The quantum axis is now not an arbitrary axis in space like before but parallel to the local magnetic field or the photon propagation direction. Let look at the angular momentum conservation in a simplified J=1/2 system. Magnetic field in the MOT B J in the lowest energy state of the EXCITED atom (this will be the one interacting with the light as the light is red-detuned) B J J J J (or flipped depending on the sign of the Lande factor) (or flipped depending on the sign of the Lande factor) The atom ground state will be opposite as the J transfer from the photon is J=1. Where should the photons’ J point to? Photon P Photon P To conserve J the answer is: Photon P Photon P Namely, relative to the magnetic field axis or the photon momentum axis, the counter propagating photons in a MOT are both right handed or both left handed circ. polarized Homework 1. Why cant we build an all optical trap from the 6 beams of a MOT (i.e. without the magnetic field)? 2. Why can U shaped wire simulate a 3D quadrupole field? Beyond the doppler limit γ d TD = 2k B Sisyphus cooling Final result: the recoil limit Tr = Homework: calculate both limits. ( hν ) m 2