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发信人: leader (kikizh), 信区: Physics 标题: Stopping and Freezing a Bullet 发信站: BBS 珞珈山水站 (Thu Mar 13 20:31:19 2008) The cooling of atoms to near absolute zero has been possible thanks largely to laser-based techniques, but only about 10 percent of elements in the periodic table have the necessary atomic level structure. A new two-step technique, de scribed in two back-to-back papers in the 7 March Physical Review Letters, pro mises to cool atoms representing 90 percent of the elements. As a demonstratio n, the researchers slowed a beam of atoms with a magnetic braking system, and then--working with a separate sample--showed how to cool the atoms further in the process of switching them from one trap to another. Although the steps hav e yet to be combined, the team believes their method could cool atoms like hyd rogen that have proven difficult to work with using traditional means. Laser cooling has revolutionized atomic physics research and led to the creati on of bizarre quantum systems like the Bose-Einstein condensate. Bouncing lase r photons off of atoms can cool them to the microkelvin regime by slowing down their motion. But the atoms must consistently return to their initial interna l state after each scattering event. Most atom species have multiple final sta tes, which interferes with cycling, so researchers have devised other ways to slow and cool gases using receding walls or electric field pulses, for example . But there's no simple, one-size-fits-all solution that can handle a wide ran ge of atoms. Now Mark Raizen of the University of Texas at Austin and his colleagues think they may have come pretty close to one. Any atom with an unpaired electron is paramagnetic, meaning it has a magnetic dipole--a sort of internal bar magnet that will line up with an external magnetic field. About 90 percent of the ele ments in the periodic table are paramagnetic in their ground states, and many simple molecules are paramagnetic as well. Raizen's team has developed a progr am to slow paramagnetic particles--usually the first step to making cold atoms or molecules--and then trap them at low temperature. In the first of their two papers, Raizen's team describes using magnetic field pulses to slow bunches of excited, paramagnetic neon atoms that were shot lik e bullets through their apparatus. The bunches went through the centers of 64 coils of wire that each produced a microsecond pulse of 5 teslas just as a bun ch passed by. The principle is similar to coilguns being developed by the mili tary to launch projectiles, only "in reverse," Raizen says. To those atoms wit h their dipoles aligned opposite the beam's direction, the pulses are like hil ls they have to climb, except that each hill disappears before the atoms have a chance to slide "downhill." Other groups have been working on atomic coilgun s, but Raizen's group is the first to significantly slow down a beam, from 450 to 56 meters per second, which is a 98 percent reduction in kinetic energy. Eventually the team plans to stop an atom bunch completely, so it can be loade d into a magnetic trap at a temperature of around 100 millikelvin. Their secon d paper describes a proof-of-principle demonstration of the next cooling step using rubidium atoms in a magnetic trap. Above the rubidium atoms, the researc hers placed an optical trap they designed, a 100-micron-wide "box" whose walls were six laser "sheets." Atoms could drift through the walls but would then e ncounter another laser that switched their internal state into one that was re pelled by the walls--a system that can work with most atom types. The researchers slowly lowered their optical trap to "skim off" the slow-movin g atoms at the upper edge of the magnetic trap, a bit like grabbing bouncing b alls at the top of their flight, when they have the least kinetic energy. They extracted 150,000 atoms at a temperature of 7 microkelvin--concentrating thes e coldest atoms by many times, compared with the magnetic trap, according to t he team. Raizen plans to someday combine the gun and optical trap to cool trit ium, a radioactive isotope of hydrogen, because measurements of its decay woul d help researchers determine the neutrino's mass. Claude Cohen-Tannoudji of the École Normale Supérieure in Paris thinks the papers are "a significant step for extending cooling and trapping methods to a lot of atomic and molecular systems." But Kristian Helmerson of the Nati onal Institute of Standards and Technology in Gaithersburg, Maryland, says the work "is not the breakthrough that the field is needing" because it resembles previous "trap-swapping" schemes that have had limited cooling potential. Rai zen admits his group's method has some of those same limitations, but "its imp ortance is that it can be applied to almost any species that can be magnetical ly trapped." --Michael Schirber Michael Schirber is a freelance science writer in Montpellier, France. -- 有限的生命，只读经典的文章 limited life for classic papers ※ 来源:·珞珈山水BBS站 http://bbs.whu.edu.cn·[FROM: 159.226.37.*]
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发信人: leader (kikizh), 信区: Physics
标题: Stopping and Freezing a Bullet
发信站: BBS 珞珈山水站 (Thu Mar 13 20:31:19 2008)

The cooling of atoms to near absolute zero has been possible thanks largely to
laser-based techniques, but only about 10 percent of elements in the periodic
table have the necessary atomic level structure. A new two-step technique, de
scribed in two back-to-back papers in the 7 March Physical Review Letters, pro
mises to cool atoms representing 90 percent of the elements. As a demonstratio
n, the researchers slowed a beam of atoms with a magnetic braking system, and
then--working with a separate sample--showed how to cool the atoms further in
the process of switching them from one trap to another. Although the steps hav
e yet to be combined, the team believes their method could cool atoms like hyd
rogen that have proven difficult to work with using traditional means.

Laser cooling has revolutionized atomic physics research and led to the creati
on of bizarre quantum systems like the Bose-Einstein condensate. Bouncing lase
r photons off of atoms can cool them to the microkelvin regime by slowing down
their motion. But the atoms must consistently return to their initial interna
l state after each scattering event. Most atom species have multiple final sta
tes, which interferes with cycling, so researchers have devised other ways to
slow and cool gases using receding walls or electric field pulses, for example
. But there's no simple, one-size-fits-all solution that can handle a wide ran
ge of atoms.

Now Mark Raizen of the University of Texas at Austin and his colleagues think
they may have come pretty close to one. Any atom with an unpaired electron is
paramagnetic, meaning it has a magnetic dipole--a sort of internal bar magnet
that will line up with an external magnetic field. About 90 percent of the ele
ments in the periodic table are paramagnetic in their ground states, and many
simple molecules are paramagnetic as well. Raizen's team has developed a progr
am to slow paramagnetic particles--usually the first step to making cold atoms
or molecules--and then trap them at low temperature.

In the first of their two papers, Raizen's team describes using magnetic field
pulses to slow bunches of excited, paramagnetic neon atoms that were shot lik
e bullets through their apparatus. The bunches went through the centers of 64
coils of wire that each produced a microsecond pulse of 5 teslas just as a bun
ch passed by. The principle is similar to coilguns being developed by the mili
tary to launch projectiles, only "in reverse," Raizen says. To those atoms wit
h their dipoles aligned opposite the beam's direction, the pulses are like hil
ls they have to climb, except that each hill disappears before the atoms have
a chance to slide "downhill." Other groups have been working on atomic coilgun
s, but Raizen's group is the first to significantly slow down a beam, from 450
to 56 meters per second, which is a 98 percent reduction in kinetic energy.

Eventually the team plans to stop an atom bunch completely, so it can be loade
d into a magnetic trap at a temperature of around 100 millikelvin. Their secon
d paper describes a proof-of-principle demonstration of the next cooling step
using rubidium atoms in a magnetic trap. Above the rubidium atoms, the researc
hers placed an optical trap they designed, a 100-micron-wide "box" whose walls
were six laser "sheets." Atoms could drift through the walls but would then e
ncounter another laser that switched their internal state into one that was re
pelled by the walls--a system that can work with most atom types.

The researchers slowly lowered their optical trap to "skim off" the slow-movin
g atoms at the upper edge of the magnetic trap, a bit like grabbing bouncing b
alls at the top of their flight, when they have the least kinetic energy. They
extracted 150,000 atoms at a temperature of 7 microkelvin--concentrating thes
e coldest atoms by many times, compared with the magnetic trap, according to t
he team. Raizen plans to someday combine the gun and optical trap to cool trit
ium, a radioactive isotope of hydrogen, because measurements of its decay woul
d help researchers determine the neutrino's mass.

Claude Cohen-Tannoudji of the École Normale Supérieure in Paris thinks
the papers are "a significant step for extending cooling and trapping methods
to a lot of atomic and molecular systems." But Kristian Helmerson of the Nati
onal Institute of Standards and Technology in Gaithersburg, Maryland, says the
work "is not the breakthrough that the field is needing" because it resembles
previous "trap-swapping" schemes that have had limited cooling potential. Rai
zen admits his group's method has some of those same limitations, but "its imp
ortance is that it can be applied to almost any species that can be magnetical
ly trapped."

--Michael Schirber
Michael Schirber is a freelance science writer in Montpellier, France.
--
有限的生命，只读经典的文章

limited life for classic papers

※ 来源:·珞珈山水BBS站

http://bbs.whu.edu.cn·[FROM: 159.226.37.*]

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