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arXiv:cond-mat/0004267 · Laser Physics

Formation of Directed Beams from Atom Lasers

Vyacheslav I. Yukalov, Elizaveta P. Yukalova

Published Apr 17, 2000• 0 citations
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Abstract

The problem of creating well-collimated beams of atoms escaping from a trap is studied. This problem is of high importance for the realization of atom lasers. Nonadiabatic dynamics of neutral atoms in nonuniform magnetic fields, typical of quadrupole magnetic traps, has been considered. The main result of this report is that an unusual semiconfining regime of motion is found, when atoms are confined from one side of an axis but are not confined from another side. This regime can be achieved by means of only magnetic fields. The formed atomic beam can be forwarded in arbitrary direction.

DOI
10.48550/arxiv.cond-mat/0004267

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Reply to “Comment on ‘Nonadiabatic dynamics of atoms in nonuniform magnetic fields’ ”

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Nonadiabatic dynamics of neutral atoms in nonuniform magnetic fields, typical of quadrupole magnetic traps, has been considered in my earlier paper [V. I. Yukalov, Phys. Rev. A 56, 5004 (1997)]. The main result of this paper is that an unusual semiconfining regime of motion was found, when atoms are confined from one side of an axis, but are not confined from another side. The author of the Comment [V. E. Shapiro, Phys. Rev. A 60, 719 (1999)] expressed doubts on the validity of this result, advancing several concrete questions. In the present reply I answer all of these questions and show that they do not invalidate the possibility of realizing the semiconfining regime of motion.

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The problem of creating well-collimated beams of atoms escaping from a trap is studied. This problem is of high importance for the realization of atom lasers. Nonadiabatic dynamics of neutral atoms in nonuniform magnetic fields, typical of quadrupole magnetic traps, has been considered. The main result of this report is that an unusual semiconfining regime of motion is found, when atoms are confined from one side of an axis but are not confined from another side. This regime can be achieved by means of only magnetic fields. The formed atomic beam can be forwarded in arbitrary direction.
Dynamics of neutral atoms in nonuniform magnetic fields, typical of quadrupole magnetic traps, is considered by applying an accurate method for solving nonlinear systems of differential equations. This method is more general than the adiabatic approximation and, thus, permits one to check the limits of the latter and also to analyze nonadiabatic regimes of motion. An unusual nonadiabatic regime is found when atoms are confined from one side of the zzz axis but are not confined from another side. The lifetime of atoms in a trap in this semiconfining regime can be sufficiently long for accomplishing experiments with a cloud of such atoms. At low temperature, the cloud is ellipsoidal, being stretched in the axial direction and moving along the zzz axis. The possibility of employing the semiconfining regime for studying the relative motion of one component through another in a binary mixture of gases is discussed.
Nonadiabatic dynamics of neutral atoms in nonuniform magnetic fields, typical of quadrupole magnetic traps, has been considered in my earlier paper [V. I. Yukalov, Phys. Rev. A 56, 5004 (1997)]. The main result of this paper is that an unusual semiconfining regime of motion was found, when atoms are confined from one side of an axis, but are not confined from another side. The author of the Comment [V. E. Shapiro, Phys. Rev. A 60, 719 (1999)] expressed doubts on the validity of this result, advancing several concrete questions. In the present reply I answer all of these questions and show that they do not invalidate the possibility of realizing the semiconfining regime of motion.
We demonstrate an atom laser using all-optical techniques. A Bose-Einstein condensate of rubidium atoms is created by direct evaporative cooling in a quasistatic dipole trap realized with a single, tightly focused CO2-laser beam. An applied magnetic field gradient allows the formation of the condensate in a field-insensitive m(F)=0 spin projection only, which suppresses fluctuations of the chemical potential from stray magnetic fields. A collimated and monoenergetic beam of atoms is extracted from the Bose-Einstein condensate by continuously lowering the dipole trapping potential in a controlled way to form a novel type of atom laser.
Magnetic trapping of laser-cooled neutral atoms has been demonstrated at NBS and is progressing in other laboratories. Confinement of any object requires exchanging kinetic for potential energy, and for neutral atoms, this necessarily derives from shifts of internal energy levels. This is implemented through the force experienced by the atomic magnetic moment in a carefully designed, highly inhomogeneous field. Since magnetic fields of convenient strength can shift atomic energy levels by only a few gigahertz (temperature ≌ 0.1 K), neutral atom traps are very shallow and must be loaded with very cold atoms. We present some motivations for using magnetic traps and then discuss the constraints and optimal configurations of various arrangements. For example, no trap can be isotropic. Next we discuss both the classical and quantum mechanical motions of trapped atoms. These motions are important for at least two reasons: First, several schemes under study for further cooling depend on knowing the position and velocity of the atoms, as well as the vector field at each point along the orbit. Second, magnetic traps depend on the atomic moment remaining aligned with the field as the atom orbits in the trap, and this precludes rapid motion through a low-field region.