Record-shattering cold at MIT

By Anne Barnard, Globe Staff, 9/12/2003

A team of physicists at Massachusetts Institute of Technology has created the coolest thing in the world.

Literally.

Using a labyrinth of lasers, lenses, and magnetic fields, the scientists chilled sodium gas to the lowest temperature ever recorded, half a billionth of a degree above absolute zero. Absolute zero is the point at which atoms, the basic building blocks of the universe, have been drained of all energy and cease to move.

Physicists say the feat -- and the device invented to achieve it -- may someday help them test the behavior of matter under conditions that until now have been conceptualized only in theory. And the ultra-cold atoms might someday be used to measure time and motion more precisely than ever.

But for now, the main victory is in the bragging rights, which are no small thing.

"Sometimes your strategy is just to go for the record," said Wolfgang Ketterle, one of the team's leaders, a long-distance runner who compared breaking the record to when he ran a marathon in less than three hours. "Just for the heck of it -- going for the record brings out the best in you."

Ketterle shared a Nobel Prize in 2001 for cooling gas to a temperature so cold that its atoms entered a never-before-seen state of matter: They moved in unison instead of jumping around randomly at varying speeds as they usually do. Such a substance, called a Bose-Einstein condensate, had been envisioned by Albert Einstein and Indian physicist Satyendra Nath Bose seven decades earlier, but never observed in nature.

That breakthrough by Ketterle, Eric Cornell, and Carl Wieman spawned an entire breed of physicists who focus on cooling substances as close as possible to absolute zero. Absolute zero is -460 degres Fahrenheit, -267 degrees Celsius, and zero degrees on the Kelvin scale used by scientists.

The previous record was three- billionths of one degree Kelvin. That is one-billionth of the temperature of interstellar space, where the lingering energy of the Big Bang keeps the temperature at about 3 degrees Kelvin. That's still pretty cold: At that temperature, every substance but helium is frozen solid.

Now, working in a small room off a linoleum-floored corridor on the Cambridge campus, the MIT team has cooled sodium gas even further, to one-sixth the temperature of the previous low mark. It reports the findings in today's issue of the journal Science.

"It shows the field is moving forward," Ketterle said.

Scientists want to chill atoms to such extreme temperatures because cooling means atoms slow down -- from the speed, at room temperature, of a moving jet airplane to one inch every 30 seconds at the record low. That allows scientists to study tiny atomic movements that are drowned out by the frenetic motion of higher temperatures, explained Ketterle's co-author David Pritchard, a principal investigator at the MIT-Harvard Center for Ultracold Atoms, who helped train the Nobel Prize-winning team.

"We just want to get these suckers to stay still and stay in our apparatus," he said.

Slowing atoms also improves the precision of devices that use atomic properties to measure time, velocity, rotation, and acceleration. Atomic clocks, for instance, measure time according to predictable oscillations of atoms -- a second is defined as a certain number of oscillations of the cesium atom. The colder those atoms are, the less the measurements are distorted by what Ketterle calls the "blurring effect" of atoms' random motion and collisions with one another.

One possible use for super-cold atoms, which has attracted the attention of military planners, is to replace global positioning systems, which map their location by triangulating with satellites, with devices that could measure their own location, direction, and speed without communicating with the outside world.

Current GPS systems rely on atomic clocks that must be accurate to a billionth of a second to map location to within a foot, so colder temperatures could improve them by making the clocks more accurate, Pritchard said. But even more intriguingly, he imagines a device that would use the atoms' position as a reference point to map its own motion. With such a device a submarine could map its location without any radio contact.

Ultra-cold atomic clocks already work in satellites, and a prototype has flown in a military airplane, but the MIT team's record-breaking machine -- about the size of a small car -- can't yet be used on the move.

"We have to bulletproof the apparatus," Pritchard said. Right now, "you're not going to load it onto a truck and expect it to work."

In the MIT lab, the scientists use a complex process to produce a tiny amount of super-cold gas, explained Aaron Leanhardt, a 26-year-old doctoral candidate who is the paper's lead author.

First, sodium, solid at room temperature, is heated to 300 degrees Celsius to make it a gas. Next, techniques called laser cooling and evaporative cooling get the gas to the starting point for the latest experiment -- the temperature that won Ketterle the Nobel Prize.

"It's hot in our world," Leanhardt said.

The researchers force the gas through a meter-long tube, where it slams into light particles from a laser being fired in the opposite direction. In a fraction of a second, the collisions slow the atoms from 1,000 meters per second to 30 meters per second. They are cooled further in a spherical container where lasers pummel them from all sides. They are then placed in a magnetic bowl where the hottest atoms float away like steam evaporating.

But wait: High school physics says gases become liquid and then solid as they cool. In the MIT experiments the researchers keep the atoms in a vacuum container -- the opposite of a pressure cooker. It keeps them at such a low density, much thinner than air, that they can't join up to form solids.

The new MIT technique solved a perennial problem: As atoms cool, gravity pulls them downward until they stick to their container, making them useless for experiments. Leanhardt and his team essentially created an invisible magnetic tabletop that levitates the atoms. They built a dime-sized wire coil coursing with electrical current that creates a magnetic field. The atoms are captured between the upward pressure of that field and the downward force of gravity. To measure the temperature, the scientists photograph the shadow of the gas -- usually no more than several human hairs across. The atoms take up less space as they cool.

The researchers named their invention, unpoetically, a gravito-magnetic trap.

"Unfortunately, we're not English majors," Leanhardt said.

Anne Barnard can be reached at abarnard@globe.com.