Dynamical cooling of trapped gases: One-atom problem (original) (raw)
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Chemical Physics, 2001
Optimal control theory (OCT) is applied to laser cooling of molecules. The objective is to cool vibrations, using shaped pulses synchronized with the spontaneous emission. An instantaneous in time optimal approach is compared to solution based on OCT. In both cases the optimal mechanism is found to operate by a``vibrationally selective coherent population trapping''. The trapping condition is that the instantaneous phase of the laser is locked to the phase of the transition dipole moment of v 0 with the excited population. The molecules that reach v 0 by spontaneous emission are then trapped, while the others are continually repumped. For vibrational cooling to v 2 and rotational cooling, a dierent mechanism operates. The ®eld completely changes the transient eigenstates of the Hamiltonian creating a superposition composed of many states. Finally this superposition is transformed by the ®eld to the target energy eigenstate. Ó
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We adapt an earlier semiclassical theory of laser cooling of an arbitrary multistate atom [J. Javanainen, Phys. Rev. A 44, 5857 (1991)]to the limit of low light intensity. The formal theory is implemented analytically on a computer using MATHEMATIcA. Expressions of light-pressure force and diffusion are provided for jl=2 j2=-, ' and jl=l~j2=2 atoms in a one-dimensional optical confinement area in which the pair of counterpropagating waves has orthogonal polarizations. In our models the cooling temperature decreases as the level degeneracy increases.
Laser cooling of internal degrees of freedom of molecules by dynamically trapped states
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In the last several years we have discovered a variety of remarkable pulse strategies for manipulating molecular motion by employing a design strategy we call "" local optimization.ÏÏ Here we review the concept of local optimization and contrast it with optimal control theory. By way of background, we give highlights from two recent examples of the method : (1) a strategy for eliminating population transfer to one or many excited electronic states during strong Ðeld excitation, an e †ect we call " optical paralysis Ï ;
Broadband laser cooling of trapped atoms with ultrafast pulses
Journal of the Optical Society of America B, 2006
We demonstrate broadband laser cooling of atomic ions in an rf trap using ultrafast pulses from a modelocked laser. The temperature of a single ion is measured by observing the size of a timeaveraged image of the ion in the known harmonic trap potential. While the lowest observed temperature was only about 1 K, this method efficiently cools very hot atoms and can sufficiently localize trapped atoms to produce near diffraction-limited atomic images.
Physical Review A
We propose a scheme that combines velocity-selective coherent population trapping (CPT) and Raman sideband cooling (RSC) for subrecoil cooling of optically trapped atoms outside the Lamb-Dicke regime. This scheme is based on an inverted Y configuration in an alkali-metal atom. It consists of a Λ formed by two Raman transitions between the ground hyperfine levels and the D transition, allowing RSC along two paths and formation of a CPT dark state. Using statedependent difference in vibration frequency of the atom in a circularly polarized trap, we can tune the Λ to make only the motional ground state a CPT dark state. We call this scheme motionselective coherent population trapping (MSCPT). We write the master equations for RSC and MSCPT and solve them numerically for a 87 Rb atom in a one-dimensional optical lattice when the Lamb-Dicke parameter is 1. Although MSCPT reaches the steady state slowly compared with RSC, the former consistently produces colder atoms than the latter. The numerical results also show that subrecoil cooling by MSCPT outside the Lamb-Dicke regime is possible under a favorable, yet experimentally feasible, condition. We explain this performance quantitatively by calculating the relative darkness of each motional state. Finally, we discuss on application of the MSCPT scheme to an optically trapped diatomic polar molecule whose Stark shift and vibration frequency exhibit large variations depending on the rotational quantum number.
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We report on Bose-Einstein condensation (BEC) in a gas of strontium atoms, using laser cooling as the only cooling mechanism. The condensate is formed within a sample that is continuously Doppler cooled to below 1 µK on a narrow-linewidth transition. The critical phase-space density for BEC is reached in a central region of the sample, in which atoms are rendered transparent for laser cooling photons. The density in this region is enhanced by an additional dipole trap potential. Thermal equilibrium between the gas in this central region and the surrounding laser cooled part of the cloud is established by elastic collisions. Condensates of up to 10 5 atoms can be repeatedly formed on a timescale of 100 ms, with prospects for the generation of a continuous atom laser.
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We demonstrate the possibility of energy-selective removal of cold atoms from a tight optical trap by means of parametric excitation of the trap vibrational modes. Taking advantage of the anharmonicity of the trap potential, we selectively remove the most energetic trapped atoms or excite those at the bottom of the trap by tuning the parametric modulation frequency. This process, which had been previously identified as a possible source of heating, also appears to be a robust way for forcing evaporative cooling in anharmonic traps.
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