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European Quantum Electronics Conference (EQEC)

Transient laser cooling of two-level quantum systems with narrow natural line widths

Jozsef S. Bakos, Gagik P. Djotyan, Zs. Sörlei

Published Aug 24, 2005• 0 citations

Abstract

Splitting of the atomic wave packet in velocity space due to the action of ultrashort laser pulses in the coherent regime of interaction was obtained in @2,9,10#. The transient regime of cooling by a sequence of short laser pulses of the narrowband traveling-wave field under the condition that the atoms decay completely between pulses was investigated in @11# .I t was shown in @12# that more deceleration of neutral atoms may be achieved with the train of ultrashort p pulses separated by a few radiative lifetimes than would be achived with the same average intensity used for cw generation. Laser cooling on transitions with a linewidth narrower than the recoil shift was studied by the Monte Carlo method using broadband excitation in @13#. The force of the stimulated light pressure on an atom arising from the action of two near-resonant standing waves with different frequencies modeling two counterpropagating waves with modulated in time amplitudes was studied in @14#.

DOI: 10.1109/eqec.1994.698368

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Splitting of the atomic wave packet in velocity space due to the action of ultrashort laser pulses in the coherent regime of interaction was obtained in @2,9,10#. The transient regime of cooling by a sequence of short laser pulses of the narrowband traveling-wave field under the condition that the atoms decay completely between pulses was investigated in @11# .I t was shown in @12# that more deceleration of neutral atoms may be achieved with the train of ultrashort p pulses separated by a few radiative lifetimes than would be achived with the same average intensity used for cw generation. Laser cooling on transitions with a linewidth narrower than the recoil shift was studied by the Monte Carlo method using broadband excitation in @13#. The force of the stimulated light pressure on an atom arising from the action of two near-resonant standing waves with different frequencies modeling two counterpropagating waves with modulated in time amplitudes was studied in @14#.

A new method is proposed for laser cooling atoms with Doppler shifts an order of magnitude larger than the homogeneous linewidth of the atomic transition. Two traveling waves cross at a small angle and form standing waves whose wavelength is much longer than that of either traveling wave. This allows one to extend the region in which the laser cooling force is proportional to velocity. The resulting broadband stimulated cooling forces are predicted to stop 0.1% of the flux of a 500-K atomic sodium beam in a distance of 300 μm and a time of a few microseconds. This is more than 100 times faster than current methods. Numerical integration and the method of Gordon and Ashkin have been used to calculate the cooling forces which result when two traveling waves cross at an angle 2a. The stimulated cooling forces are reduced in intensity by sin2a but are proportional to velocity over an interval which is larger by 1/sin a. This allows one to stop a substantial fraction of an atomic beam without chirping the laser or Zeeman tuning the atoms.

The laser cooling of atoms with a narrow-line optical transition, i.e., in regimes of quantum nature of laser-light interactions resulting in a significant recoil effect, is studied. It is demonstrated that a minimum laser-cooling temperature for two-level atoms in a standing wave reached for red detuning close to three recoil frequencies is vastly different from the theory used for a semiclassical description of Doppler cooling. A set of dimensionless parameters uniquely characterizing the time evolution and the steady state of different atoms with narrow-line optical transitions in the laser field is introduced. The results can be used for analysis of optimal conditions for laser cooling of atoms with narrow lines such as Ca, Sr, and Mg, which are of great interest for atomic clocks.

We propose to extend coherent laser cooling from narrowband to broadband transitions by using trains of ultrashort broadband pulses. We analytically study two possible methods to reduce the momentum spread of a distribution by several units of photon momentum in a single spontaneous emission lifetime. We report on numerical simulations of one-dimensional laser cooling of a two-level system with realistic parameters. With this scheme, coherent laser cooling can in particular be implemented in the case of fast species with a short lifetime such as positronium.

We study the laser cooling of atoms with a narrow-line optical transition in monochromatic laser light, i.e. in regimes of quantum nature of laser-light interactions resulting in a significant recoil effect. It is demonstrated that a minimum laser cooling temperature for two-level atom in standing wave reached for red detuning close to 3 recoil frequency, greatly different from the theory used for a semiclassical description of Doppler cooling. A set of dimensionless parameters uniquely characterizing the time evolution and the steady state of different atoms with narrow-line optical transitions in the laser field is introduced. The results can be used for analysis of optimal conditions for laser cooling of atoms with narrow lines such as Ca, Sr, and Mg, which are of great interest for atomic clocks.

We propose and investigate theoretically a novel scheme for transient slowing and cooling of two-level quantum systems with narrow transition linewidths by a sequence of counterpropagating, short, linearly polarized laser pulses with special frequency chirping. Both internal degrees of freedom and the motion of the center of mass of quantum systems are considered quantum mechanically. Interaction with a large number of laser pulses during the decay time permits a drastic decrease in the cooling time of such systems.