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Laser Cooling and Bose Einstein Condensation
Published in Pradip Narayan Ghosh, Laser Physics and Spectroscopy, 2018
Schawlow first showed in the seventies that the optical force of laser radiation can be used to decelerate an atom. This can be accomplished by the absorption of a photon by an atom through a process of stimulated absorption if the laser radiation has a frequency resonant with the energy level difference of the atom. As the photon is absorbed, the momentum of the photon disappears since the photon does not exist. This can be considered as the result of a collision of a single atom with a single photon. In this case the law of conservation of linear momentum should hold. If an atom absorbs a photon coming from the opposite direction, according to the law of conservation of momentum, the momentum of the atom will be reduced since the atom receives a negative momentum from the incident photon as the atom and the photon are moving in opposite directions (Fig. 9.5). Thus the atom velocity may be reduced resulting in a deceleration of the atom. If the atom absorbs a photon moving in the same direction as the atom, it may be accelerated on the basis of the same argument. The laser beam moving from two opposite directions along a particular path can be absorbed by two different atoms that are moving in opposition to each other. We show below that in the process of Doppler cooling, we can create a situation in which the atom absorbs the photon coming from the opposite direction. Thus each atom may be decelerated in the process. Considering three mutually perpendicular directions atomic velocity components in all three directions may be reduced.
The effect of atomic response time in the theory of Doppler cooling of trapped ions
Published in Journal of Modern Optics, 2018
H. Janacek, A. M. Steane, D. M. Lucas, D. N. Stacey
Doppler cooling exploits the fact that an atom or ion counter-propagating with a laser beam and absorbing photons from it is subject to a retarding force [1–11]. The simplest practical system is that of an ion, trapped in a harmonic potential, illuminated by a single laser beam detuned to the red from a resonance transition [12–15]. The ion is more likely to absorb photons when travelling towards the source than when moving in the opposite sense, so that averaged over a cycle of the motion the momentum of the ion is reduced. An equilibrium temperature is reached when the cooling is balanced by the heating effects associated with spontaneous emission and the stochastic nature of the absorption process.