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MAGNETIC FIELD
ELECTRIC FIELD
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MAGNETIC AND ELECTRIC FIELDS can
exert forces on atoms even though
atoms are only slightly magnetic and
electrically neutral. An atom in a mag-
netic field will be drawn toward the re-
gion of strongest field if the south pole
of the atom points toward the north
pole of the field. An atom in an electric
fleM will be attracted toward the re-
gion of highest field as well. The elec-
trie field pulls on the negative charges
in the atom, white repelling the positive
charges. As a result of the new distribu-
tkm of charge, the atom is attracted to-
ward the positively charged rod.
The magnitude of this scattering
force is quite low. If an atom absorbs a
single photon, its change in velocity is
tiny compared with the average veloci-
ty of atoms in a gas at room tempera-
tare. (The change is on the order of one
centimeter per second, the crawling
speed of an ant, whereas an atom at
room temperature moves at the speed
of a supersonic jet.)
This scattering force was first detect-
ed in 1933, when Otto R Frisch used it
to deflect a beam of sodium atoms. He
prepared the atoms by vaporizing sodi-
um in a container. To form the beam,
he allowed the atoms to pass through a
hole in the container and a series of
slits. Once established, the beam was
bombarded with light from a sodium
lamp. Although, on average, each sodi-
um atom absorbed only a single pho-
ton, Frisch was able to detect a slight
deflection of the beam.
The scattering force that Frisch gen-
erated was far too weak to capture
\ atoms. Decades later workers realized
that the photon-scattering rate could
be increased to more than 10 million
photons per second, corresponding to
a force 100,000 tunes greater than the
pull of gravity by the earth. The first
dramatic demonstration of the scattering
force on atoms was made by two
separate groups led by Phillips and John
L. Hall at the National Bureau of Standards.
In 1985 they stopped a beam of
atoms and reduced the temperature of
the atoms from roughly 300 kelvins
(room temperature) to 0.1 kelvin.
The power of the scattering force
attainable with lasers gave researchers
hope that they could not only stop
atoms but trap them as well. But attempts
to configure several laser beams
so that they could collect and concentrate
atoms in some region of space
seemed doomed to failure. According to
a principle known as the Optical Earnshaw
Theorem, it is impossible to fashion
a light trap out of any configuration
of light beams if the scattering force is
proportional to the light intensity. The
problem is that the beams cannot be
arranged to generate only inward directed
forces. Any light that enters a trapping
region must eventually escape and
must therefore carry outward directed
forces as well. Even if Luke Skywalker
were a physicist, the (scattering) force
would not always be with him.
F
ortunately, an atomic trap can be
based on another kind of force
that light can exert on atoms. To
understand this force, it is instructive
to consider how small particles can be
attracted to a positively charged object,
such as a glass rod rubbed with cat's
fur. The rod produces an electric field
that polarizes the particle. Consequently,
the average position of positive
charges in the particle will be slightly
farther away from the rod than the average
position of the negative charges.
This asymmetric distribution of charge
is said to have a dipole moment. The attractive
dipole force exerted by the electric
field on the negative charges of the
particle is stronger than the repulsive
force on the positive charges. As a result,
the particle is pulled toward the
regions where the electric field is
strongest. Notice that this force is analogous
to the magnetic dipole force first
used to trap neutrons and atoms. If the
charge on the rod were negative, the
electric field would induce a dipole moment
of reversed polarity, and the particle
would still be attracted to regions
of high electric field.
Because of the dipole force, atoms can
be trapped by an electric field that has a
local maximum of some point in space.
Could such fields be produced by some
clever arrangement of electric charges?
For any system of fixed charges, the
answer is no. Yet an electric fieldwith a
local maximum can be achieved in a
dynamic system. In particular, because I
light is made up of rapidly oscillating
electric and magnetic fields, a focused
laser beam can produce an alternating
electric field with a local maximum. \
When the field interacts with an atom,, '
it alters the distribution of electrons
around the atom, thereby inducing an
electric dipole moment. The atom will
thus be attracted to the local maximum
in the field, just as the charged particle
was drawn toward the rod.
The fact that the electric field changes
rapidly does not present a problem. As
the field changes polarity, the dipole
moment of the atom also switches
around. As long as the field changes at
a rate slower than the natural oscillation
frequencies of the atom, the dipole
moment remains aligned with the field.
The atom therefore continues to move
toward the local maximum. As a result,
this dipole force can be used to confine
atoms. In 1968 Vladilen S. Letokhov
first proposed that atoms could be
trapped in a light beam using the dipole
force, and 10 years later Arthur
Ashkin of AT&T Bell Laboratories suggested
a more practical trap based on
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focused laser beams.
Although the dipole-force trap is ele-
gant in conception, it had practical
problems. To minimize the scattering
force, the light must be tuned well be-
low the frequency at which the atoms
readily absorb photons. At those large
detunings, the trapping forces are so
feeble that atoms as cold as OJOl kelvin
cannot be held in the trap. Even when
colder atoms were placed in the trap,
they would boil out of the trap in a mat-
ter of a few thousandths of a second as
a result of the ever present photon scat-
tering. In addition, the task of injecting
atoms into the trap seemed daunting
because the volume of the trap would
only be 0.001 cubic millimeter.
For these reasons, the challenges to
optical trapping seemed formidable.
Then, in 1985, a scheme for a workable
optical trap became apparent after
atoms were laser cooled in all dimen-
sions and to much lower temperatures
than the stopped atomic beams. The
laser-cooling idea was first proposed in
1975 by Theodor Hansch and Arthur
Schawlow of Stanford University. In the
same year, a similar scheme for cooling
trapped ions with lasers was proposed
by David J. Wineland and Hans G. Deh-
melt of the University of Washington
The researchers predicted that an
atom could be cooled if it is irradiated
from two sides by laser light at a fre-
quency slightly lower than the frequen-
cy needed for maximum absorption. If
the atom moves in a direction oppo-
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