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Physical Review A

Rotating magnetic quadrupole field traps for neutral atoms

V. E. Shapiro

Published Aug 1, 1996• 4 citations
Publisher

Abstract

Methods of trapping neutral spin-polarized particles are proposed using highly nonuniform magnetic fields rotating at low frequencies, down to the resonance of the particle's orbiting motion. Such fields beget new trapping mechanisms of vast possibilities. The new resonance methods and the new traps of zero-size equilibrium orbit should give much tighter and more stable confinement than previously known. They also give the possibility of a high, resonance selectivity in magnetic separation and evaporative cooling. A general analysis of trapping with rotating fields is given.

DOI
10.1103/physreva.54.r1018

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Methods of trapping neutral spin-polarized particles are proposed using highly nonuniform magnetic fields rotating at low frequencies, down to the resonance of the particle's orbiting motion. Such fields beget new trapping mechanisms of vast possibilities. The new resonance methods and the new traps of zero-size equilibrium orbit should give much tighter and more stable confinement than previously known. They also give the possibility of a high, resonance selectivity in magnetic separation and evaporative cooling. A general analysis of trapping with rotating fields is given.
The magnetic methods most successful so far for tight, stable confinement of cold neutral atoms in the dark explore three-dimensional-nonuniform magnetic configurations. A different method, using revolving planar-nonuniform fields, is developed in the present work. The paper gives an idea of the method, describes its basic features, and shows its superiority, such as (i) a greater product of curvatures of the trap well in the three spatial directions by a factor of two orders of magnitude given the same magnitude of field gradient, (ii) tight, stable, and more isotropic confinement in both the trap center and an easy-to-change orbit, (iii) vast possibilities to change the form of the trap well, and (iv) the possibility to separately detect the effective spin and translational motion temperatures of the cloud via observations of its density distribution.
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.
We describe a new type of magnetic trap whose time-averaged, orbiting potential (TOP) supplies tight and harmonic confinement of atoms. The TOP trap allows for long storage times even for cold atom samples by suppressing the loss due to nonadiabatic spin flips which limits the storage time in an ordinary magnetic quadrupole trap. In preliminary experiments on evaporative cooling of 87Rb{}^{87}\mathrm{Rb}87Rb atoms in the TOP trap, we obtain a phase-space density enhancement of up to 3 orders of magnitude and temperatures as low as 200 nK.
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.
We discuss a radio-frequency (rf) ion trap, the rotating-radio-frequency (rotating-rf) trap, in which the motion of a charged particle is described by trigonometric functions rather than the usual Mathieu functions of a normal rf trap. In the rotating-rf trap, a rotating quadrupole electric field confines charged particles, whereas in a normal rf trap, an oscillating quadrupole electric field does. Ion motion in a rotating-rf trap is a superposition of two nondegenerate circular secular motions and two corresponding circular micromotions. The cases of applying a uniform dc magnetic field or a quadrupole dc electric field in addition to the rotating-rf field are also discussed. Confinement in a rotating-rf trap can be tighter than in a normal linear rf trap with the same experimental parameter values.
Abstract A novel design for an electromagnetic trap is proposed for confinement of neutral particles having a permanent electric dipole moment. The device uses a combination of a sextupole electric and quadrupole magnetic fields superimposed with a strong constant electric field perfectly aligned along the z-axis. We use the extended dipole model to study the dynamics of particle in this position dependent electromagnetic field. The motion of the centre of mass of the dipole is nonlinearly coupled with its rotation. We find three families of particular solutions for which the orbits are periodic or quasiperiodic. They correspond to the particular cases when the whole energy is accumulated in either translational or rotational degrees of freedom. Stability analysis of chosen particular solutions provides trapping conditions. We present several numerical simulations which illustrate trapping and confinement of an electric dipole in the proposed trap. These simulations were performed for three kinds of polar particles and various fields selections. We hope that the proposed model allows experimental physicists to apply a wide variety of non-destructive methods for manipulation, detection and analysis of trapped particles.