 |
 Physicist Peter Engels.
 Machinist John Rutherford.
Most of the parts they made for Engels were one of a kind. His
machine uses a two-step process to reduce the movement of atoms,
thereby reducing their temperature. It starts with atoms that are a
gas at room temperature. Any one of several elements will work;
Engels uses rubidium, a metallic element related to potassium. He
bombards the atoms with lasers from six directions. Atoms caught at
the point where the beams intersect will slow down—and get
cold.
“We usually think an atom is kind of a massive thing, and a
photon is a kind of light,” says Engels. “If you stick your hand
out in the sunshine, you don’t really feel any recoil down. You
feel the energy, it’s getting warm, but you don’t feel any recoil.
But it’s there.”
At the atomic level, he says, individual atoms do recoil when
they are struck by light of just the right wavelength. With beams
coming at them from six directions, the atoms resemble ping pong
balls confined in an ever-shrinking box; they bounce off one photon
only to hit another and then another, eventually coming to
near-rest in the center of the “magneto-optical trap,” or MOT, as
it is called.
The MOT confines the atoms enough to lower their temperature to
within a few thousandths of a degree above absolute zero. That’s a
few degrees colder than the deep reaches of space, but still far
too warm for the atoms to condense into BEC. At this point, Engels
says, lasers no longer help. Cooling the atoms the last few
fractions of a degree needed to reach BEC requires strong magnets,
an ultrahigh-vacuum environment, and a clever design.
Once the atoms are caged by the laser beams, they are held in
place by a powerful magnet. Each atom carries a minute magnetic
moment, like a tiny bar magnet. That gives Engels a handle on them.
When he slides the magnet along a rail, the atoms follow, all the
way from the MOT, through a glass tube, to a chamber connected to
the ultrahigh vacuum pump.
Engels compares the next step of the operation to cooling a cup
of coffee by blowing over it. Blowing removes the hottest
molecules—the ones that are bouncing around the most—leaving cooler
ones behind. The average temperature of the coffee drops. In
Engels’s machine, electromagnetic radiation “blows” over the atoms,
and the vacuum suctions off the most active ones. Those that remain
cool down even more. If things go well, they get cold enough to
form BEC.
This is a crucial step, possibly the one most difficult for
researchers to achieve. The vacuum system must be absolutely
leak-proof and free of surface contaminants. This may also be the
part of the project where Engels really lucked out: vacuum systems
are one of George Henry’s very favorite things to build.
Page
1
2
3
4
Continued
|
|
Rumor has it that Instrument Shop supervisor George Henry once
worked on parts for the Apollo moon missions.
 An array of mirrors and lenses directs six laser beams into Engels's magneto-optical trap.
| |
