[Posted early due to travel]
Long time readers of this column know that I”m an Albert
Einstein groupie and can”t resist anything that deals with another
of his many predictions that has come true. Last week, we
discussed how Dr. William Philips and the workers at NIST (the
National Institute of Science and Technology) have used lasers to
achieve temperatures as low as 40 millionths of a degree Kelvin
above Absolute Zero, or 0.000040 K.
I also mentioned a great University of Colorado (UCOL) Web
site with interactive demos you could play with to illustrate laser
cooling. At the time, I didn”t know why UCOL had such a site. I
should have remembered that it was a collaboration between
UCOL and NIST in 1995 that resulted in a major splash in the
scientific world and even got headlines in the popular media.
This was the attainment of what was termed a “superatom”.
Actually, it goes by the scientific name of a Bose-Einstein
condensate (BEC). A BEC is a most unusual kind of stuff that
Einstein and a fellow named Bose had predicted over 70 years
ago. To make this BEC stuff took another leap in coldness
below the 40 millionths of a degree Kelvin.
Surprisingly, Dr. Phillips did not talk about a BEC or about
something else, a “directional atom laser”, that came out of his
group at NIST and was derived from a BEC. Both the BEC and
the atom laser are truly amazing. In what follows, I have relied
heavily on information contained on the NIST, University of
Colorado and Royal Holloway, University of London Web sites.
First, if you didn”t read last week”s column, I suggest you click on
the archives (bottom of this column) and do so. This will bring
you up to speed on laser cooling and the magnetic bottle. Why
did Dr. Phillips not talk about temperatures lower than 40
millionths of a degree Kelvin in his lecture on laser cooling? In
laser cooling, the sodium atoms were slowed down and trapped
in the magnetic bottle. However, the jolts that follow absorption
and emission of photons from the lasers also keep the atoms on
the go. As long as you shine the laser beams on them, they can”t
go any slower than the speed corresponding to that 0.000040 K.
You need another trick. Let”s turn off the lasers and bring out the
salad bowl and some Ping-Pong balls from last week.
This time let”s pretend we have a collapsible salad bowl, sort of
like those collapsible drinking cups you might take with you on a
trip. Let”s fill the bowl with a bunch of Ping-Pong balls, all
moving around at different speeds. The bowl is big enough to
keep them all inside the bowl. Now collapse the bowl a bit,
lowering the wall. Some of the balls, notably the faster ones,
will escape. The ones left behind will be slower. If our Ping-
Pong balls were atoms, they would keep on moving, banging into
each other. Some would be slowed down, others speeded up. If
we collapse the bowl some more, again the faster balls will
escape and the ones left behind will on average be moving even
more slowly. Slower atoms mean that we”ve lowered the
temperature.
A magnetic bottle is like the salad bowl, only the walls are the
magnetic field, which can be raised or lowered in strength.
Remember last week we said atoms are like tiny magnets? If we
goose up the current in our magnetic bottle, we can make the
magnetic field so strong that those atoms at 0.000040 K can”t get
out. It”s sort of like having a magnetic thermos bottle with the
atoms trapped inside. If we lower the strength of the magnetic
field, it”s like lowering the walls of our salad bowl and the fast
atoms can get out. As the faster atoms leave, the remaining
atoms are the slower ones and the temperature goes down below
0.000040 K. The UCOL Web site has a demo where you can
raise and lower the walls (strength of the field) and see how the
number of slower atoms changes, depending on how fast and
how far you raise or lower the wall.
In Boulder, Colorado in 1995, Eric Cornell of NIST and Carl
Wieman of UCOL used laser cooling followed by magnetic
cooling to cool rubidium atoms. (Rubidium, instead of sodium,
was chosen for its optical and magnetic properties.) They
cooled these suckers all the way down to less than a millionth of
a degree above Absolute Zero and that”s when Bose and Einstein
would have been proud. The rubidium atoms condensed into this
superatom or BEC. In a BEC, every atom is the same. This
may not sound exciting to you but, in physics, the creation of a
BEC is considered one of the greatest achievements of the 20th
century.
When I say the atoms are the same, I mean they all are in the
same quantum mechanical state. It”s not easy to explain, or to
understand, but let”s generalize a bit. In our normal everyday
world, atoms and other elementary particles hate to be in the
same state. I think we”ve mentioned in the past that photons are
one of the rare things that rather enjoy being the same state. In
fact, a BEC is very much like the photons in a laser. In a laser all
the photons are alike – the same color and the same phase. In
other words, they”re all marching in step. As a result, you have
very pure intense color and a laser beam spreads out very little,
as in a laser pointer. In the UCOL-NIST Bose-Einstein
condensate, the rubidium atoms are all the same. You can see a
BEC on the UCOL site. It”s sort of like a cherry pit, the BEC
atoms clustered in the pit while the normal atoms are like a cloud
around the BEC.
You might pose a profound question at this point. “You say that
the atoms in a BEC are like the photons in a laser. Does they
mean that a BEC is a laser?” Strictly, a laser only involves
photons of light. However, in 1997 at MIT, Wolfgang Ketterle”s
laboratory was the site of the first “atom laser”. The MIT
workers used gravity to encourage some atoms to trickle out of
one of these Bose-Einstein condensates. In the MIT atom laser,
the BEC atoms came out in pulses and spread out like ripples in a
pond when stones are thrown into it. Although it was an atom
laser, it didn”t stay in a confined beam like your red laser
pointer”s beam.
In 1999 in the NIST laboratory in Gaithersburg, Maryland,
members of Phillips” group and collaborators managed to cool
sodium atoms down to about 0.00000005 K, 50 billionths of a
degree above Absolute Zero! At that temperature almost all the
atoms were in the BEC. The NIST workers then lined up two
lasers, essentially facing each other, and shined them through the
magnetic bottle. They tuned the lasers to slightly different
frequencies. The BEC atoms absorbed photons from one laser
and emitted photons into the other. This caused a bunch of the
atoms to move in the same direction. In order to overcome the
magnetic field holding the atoms in the ”bottle”, the lasers were
pulsed by increasing the difference in frequencies. This gave an
added ”kick” in the same direction and out popped a clump of
atoms. Doing this pulsing very fast caused the clumps to
actually overlap and it”s almost a continuous beam of atoms,
similar to the continuous beam of photons in a laser pointer.
The NIST ”directional” atom laser beam did not spread out like
the MIT atom laser beam. The NIST beam was about as wide as
a human hair and zipped along at a clip of 6 centimeters, or a
couple of inches a second. Quite a bit less than the speed of light
from a laser! It would take a minute for the atom beam to travel
across my living room. I have my doubts that the beam would be
stable that long but was unable to find anything about how long
or if it can hang together. I think I”ll pause here and try to find
the Science article that first revealed NIST”s accomplishment.
……… I”m back after spending about two hours trying to
understand the paper. Wow! The paper was obviously not
written for the layman or even for this poor chemist, whose
quantum mechanical experience has been limited (to put it
mildly) and dates back to some 40-50 years ago. However, I did
learn that there are typically only about a million sodium atoms
in NIST”s typical BEC. That may sound like a lot of atoms but
it”s less than a millionth of a billionth of a gram of sodium! Not
only that, but their typical experiment lasted less than a
hundredth of a second! So, the little blobs of atoms didn”t get to
travel very far at all. Even so, they did get pictures of the blobs
and they moved as expected from the quantum mechanics.
I also found that they used a rotating magnetic field and had to
synchronize their laser pulses with the field to get those little
suckers out of the bottle. And remember last week spinning top
that levitated in midair over the magnetic ”hotplate”? I
mentioned that atoms can behave like tiny magnets. I may be all
wet in my interpretation of the paper, but it seems to me that to
get those sodium atoms out of the magnetic bottle they had to
change the spin of the atoms, possibly akin to stopping the top
from spinning. In the new spin state, the atoms aren”t trapped by
the magnetic field and go charging out of the ”bottle”. All in all,
that experiment took an awful lot more sophistication than just
cooling down to a few billionths of a degree above Absolute
Zero!
After reading the paper, I can see why Dr. Phillips didn”t talk
about the atom laser in his lecture in Washington. He hadn”t yet
figured out a clever way to demonstrate it to an audience in an
entertaining way such that a non-physicist like myself could be
conned into thinking he really understands it.
You”re probably saying, “Enough already, no more coldness!”
Ok, next week we”ll talk about heat!
Allen F. Bortrum
