Optical Cooling Limits

Meanwhile, experiments have shown that, in fact, temperatures lower than this calculated Doppler limit can be reached [1168-1170]. How is this possible  

The electric field E(z,t) of the standing light wave causes a shift and broadening of the atomic Zeeman levels (ac Stark shift) that depends on the saturation parameter, which in turn depends on the transition probability, the polarization of E, and the frequency detuning co] — coq. It differs for the different Zeeman transitions. Since the a transition starting from g-1/2 is three times as intense as that from g+1/2 (Fig. 9.28b), the light shift A of g-1/2 is three times the shift A+ of g+1/2. If the 

The z-dependent energy shift of the ground-state sublevel therefore follows the curve of Fig. 9.28c. For those z values where a linearly polarized light field is present, the transition probabilities and light shifts are equal for the two sublevels. 

Because the population density (indicated by the magnitude of the dots in Fig. 9.28c) is larger in the minimum than in the maxima of the potentials pot(z), on the average the atom climbs uphill more than downhill. It transfers part of its kinetic energy to photon energy and is therefore cooled. 

Depending on the polarization of the two counterpropagating laser beams the lin J. lin configuration can be used with two orthogonal linear polarizations or the a -a configuration with a circularly polarized a+ wave, superimposed by the reflected cr wave. With Sisyphus cooling temperatures as low as 5-10 pK can be achieved. 

For an atom at rest with the level scheme of Fig. 14.19b, the energies and the populations of the two ground-state sublevels g- /i, g+1/2 depend on the location z. For example, for z = A/8 the atom rests in a a light field and is therefore optically pumped into the g-1/2 level, giving the stationary population probabilities (g i/2) = 1 and g+ /i) = 0, while for z = (3/8)A the atom is pumped by light into the g+i/2-level. 

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SUB-DOPPUIR COOLING In 1988 the NIST-Gaithersburg group made careful measurements of the temperature of atoms laser cooled in optical molasses, cind found temperatures significantly below the Doppler cooling limit 113). The initial measurements on laser cooled sodium atoms gave temperatures of about 40 pK, about six times lower tham the predicted lower limit of 240 pK. 

The simultaneous determination of Co and Ni is also made at pH 8 in the presence of pyrophosphate. The EDTA is added to the mixture of coloured complexes of these metals to bind the Cu and Zn admixtures into the inactive complexes. The optical density of the solution is measured at 530, 555 and 580 nm. The solution is heated to the boiling point to destmct the complex formed by Ni with PAR, and then is cooled. Again the measurements of optical density ai e performed at the same wavelengths. The Ni concentration is calculated from the variation in the optical density, and the Co concentration is calculated from the final values of optical density. The detection limits for these metals are 4 and 2 p.g/dm, respectively. 

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