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.
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