Wave-guide:  Atom Reflection in a Tapered Magnetic Guide

While linear magnetic guides such as the one shown above trap atoms only in the transverse direction while allowing atoms to move in the longitudinal direction, tapered guides can be used to confine atoms in 3D. The transverse trapping strength in guides is charcterized by the field gradient near the field minima. Most guides are designed with parallel wires and so have a fixed field gradient along the guide for a given current in the wires. Tapered guides on the other hand have a position dependent transverse field gradient. These can be made by tapering the wires in the guide or by using a magnetic material to boost the field gradient at specific locations. The figure below shows a two-wire guide with field tapering accomplished with the aid of 2 tapered magnetic iron. Going from left to right, the atoms see an increasing field gradient as it enters the region without the iron to that with the iron. The magnetic iron boosts the field gradient by up to a factor of 4.



We observe that atoms moving along such a guide can be reflected at the entrance of the taper. The control parameter is determined by the atom's transverse action. The larger the transverse action, the higher the probability that the atom will bounce off the taper. The is shown in the figure below.


The horizontal axis shows the fluorescence of atoms at a location 2.5cm from the start of the taper.  Atoms collected from the MOT at this location falls via gravity down the guide and is reflected back to their starting position at t=400ms and 800ms corresponding to the first and second rebound respectively. The three curves correspond to the case for atoms with average transverse action controlled by displacing the MOT with respect to the guide axis. The bigger the displacement in the x direction, the higher the percentage of atoms that gets reflected.

The reflectivity parameter  R is shown in the figure below.



We have done numerical simulations of the atom dynamics inside such a guide. The color graph shows a position-time map of the atoms released inside the guide. Atoms start at z=-2.5 cm and move down the guide before getting refelcted at around z=-0.5 cm. The reflected atoms remain trapped within the guide and bounces back and forth with a period of 400 ms.

Tapered guides could be developed into a new kind of magnetic atom trap in which the trapping parameter is based on the conservation of its transverse action.  Such a trap is similar to magnetic bottle traps in plasma physics which confine charged particles between regions of converging magnetic field using the adiabatic principle.

Reference:
Atom Reflection in a Tapered Magnetic Guide

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