Atomic physicists tend to tinker away on their own, preferably in dark, hushed labs. When Eric Cornell started as a postdoc with Carl Wieman at JILA, an institute run jointly by the National Institute of Standards and Technology and the University of Colorado in Boulder, in 1990 he did his best to transform their second-floor lab into a basement. "We had these beautiful windows that looked out over the mountains," Cornell says, "and we bought 3-inch-thick Styrofoam and cut it into squares and taped it over them." The quiet and darkness made it easier to fiddle with the homemade lasers they were using to coax atoms into a new state of matter.
Cornell and Wieman were trying to cool a puff of rubidium gas to within a few billionths of a degree of absolute zero—colder than any place in nature, even the 2.73 kelvins of space. They hoped to produce a long-predicted state of matter called a Bose-Einstein condensate (BEC), in which the atoms shed their individual identities and crowd en masse into a single quantum wave. In 1995, they succeeded—a triumph that earned them a share of the 2001 Nobel Prize in Physics. "We finally unplugged that experiment just 2 years ago," Cornell says.
Now, Cornell and other physicists are taking their atomic wisps out of seclusion and into space. Early next year, NASA will launch its $70 million Cold Atom Laboratory (CAL) to the International Space Station (ISS). Once in orbit, the fully automated rig will create BECs and do other cold atom experiments, taking advantage of weightlessness to attain record-low temperatures and break ground for ambitious studies of quantum mechanics and gravity. Miniaturization is the key: Experiments that once required a room full of lasers, optical elements, and vacuum systems can now fit in a device the size of an ice chest, with the atoms trapped on the surface of a microchip. The effort will stretch the culture of atomic physicists, forcing them to share a single remote facility, like users of a space telescope.
"I've certainly been pitching this for 20 years, really from the beginning of BECs, when doing something like this in space seemed crazy," says Robert Thompson, a physicist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, and CAL's project scientist.
There is one main reason to do atomic physics in space. "It's all about getting away from gravity," says Charles Sackett, a physicist at the University of Virginia in Charlottesville and a CAL experimenter. Here's the problem: To make a BEC, physicists use magnets and lasers to trap and chill atoms so that their speeds drop from thousands of meters per second to centimeters per second—slower than a walk. But to probe a BEC, they must release it from its trap and shine laser light on it, creating a shadow that reveals the atoms' distribution.
On Earth, gravity pulls at the atoms the moment they are released, typically giving physicists just 10 to 20 milliseconds to make their measurements before the BEC crashes to the bottom of the vacuum chamber. In the weightlessness of orbit, a BEC should hover for up to 10 seconds before lingering gas in the vacuum chamber warms it up, Sackett says, allowing time for measurements that can't be made on Earth.
Working in orbit should also push atoms to lower temperatures. In making a BEC, the final step begins with the atoms trapped in a magnetic field. Physicists ramp down the magnetic field so that the trap becomes weaker and wider, allowing the gas to expand and cool—just as a can of spray paint gets cold when the gas inside decompresses. In orbit, the trap can get weaker and bigger without losing the atoms, enabling the gases to attain even lower temperatures.
Such weightlessness has been mimicked, fleetingly, on the ground. Since 2007, a multi-institutional team working at the Center of Applied Space Technology and Microgravity in Bremen, Germany, has dropped its reusable cold atom experiments down a 146-meter tower into a bed of polystyrene pellets. The plunge produces nearly 5 seconds of weightless free fall in which to work—and twice as long if researchers catapult the experiment up the tower and allow it to fall back to Earth.
Those few seconds enabled the team to reach temperatures of 50 picokelvins, the coldest ever attained, says Ernst Rasel, a physicist at the Gottfried Wilhelm Leibniz University of Hanover in Germany and leader of the QUANTUS collaboration. (The acronym comes from the German for "quantum gases under weightlessness.") Earlier this year, QUANTUS researchers launched their apparatus on a sounding rocket from Kiruna, Sweden. Rising to an altitude of more than 240 kilometers, the rocket flight offered 6 minutes of free fall. During that time, the automated machinery performed 85 distinct experiments, Rasel says, including producing the first BEC in space. The ISS, however, will give CAL far more time—a year or longer—letting users do even more.
For starters, CAL physicists aim simply to try to reach the lowest temperatures possible, which might allow delicate new quantum effects to emerge. Researchers are confident they can dip down to 100 picokelvins and possibly lower, Sackett says. That may not be quite as low as the QUANTUS team claims in its drop-tower result. But the QUANTUS team can perform just three runs a day, whereas CAL can perform experiments continuously.
In another experiment, Nathan Lundblad, a physicist at Bates College in Lewiston, Maine, and colleagues hope to make hollow shells of BECs, something that gravity squashes on Earth. The bubbles can be fashioned by applying radio waves of the right frequency to a BEC, Lundblad explains, and at first, researchers hope to simply jiggle the bubbles and see how they react.
But the shells might also enable them to probe the wave nature of the BEC in a new way. Mathematical consistency demands that the undulating quantum wave in the BEC wrap around the sphere and merge smoothly with itself. As it does so, it might generate tiny whirlpools called vortices. Physicists have already produced vortices by spinning a BEC. In Lundblad's experiment, however, vortices would emerge in a new way—through the interplay of the quantum wave and the geometry of the bubble.
Others on the CAL team plan to probe an odd bit of quantum mechanics known as the Efimov effect that enables certain atoms to form weakly bound three-atom molecules, even though no two atoms will stick together. The molecules are the atomic equivalent of the Borromean rings, a topological curiosity in which three rings intertwine so that removing any one ring causes the other two to fall apart. To create the molecules, JILA's Cornell and Peter Engels and Maren Mossman of Washington State University in Pullman will apply a magnetic field to ultracold atoms of potassium-39. The gas won't be dense enough to form a BEC, but at certain magnetic field strengths, the isolated atoms should be coaxed into forming three-atom molecules.
The effect has been seen on Earth, but theory predicts that the molecules will form, break up, and reform at successively stronger magnetic fields. The size of molecules should grow each time by a factor of 22.7. Cornell and colleagues aim to spot the second Efimov state, in which the molecules become giants, about the size of bacteria. To do that will require letting the gas expand until it is 1/1000 as dense as in earlier experiments—something that would be difficult on Earth. "It's going to be doubly hard because we don't get to hover around [the experiment] with an army of graduate students," Cornell says.
Perhaps CAL's biggest goal is a type of experiment called atom interferometry. Laser light can split the quantum wave of a BEC into two halves that move apart and recombine. Thanks to the weirdness of quantum theory, that splitting means that each atom in the BEC literally takes both paths at once. If the split paths are separated vertically, one path will be infinitesimally farther from Earth, giving it slightly more gravitational energy than the other path and causing the quantum wave to undulate slightly faster along that path. As a result, when the waves merge, they will interfere with each other to create a rippled density distribution in the BEC. The pattern should reveal exactly how much the atoms accelerate under gravity as they orbit Earth.
If precise enough, an orbiting atom interferometer could have many scientific applications. Atom interferometers might be used in spacecraft as inertial navigation systems that would be more accurate than current devices, which rely on laser gyroscopes. And by testing the effect of gravity on BECs of two different types of atoms, an atom interferometer could test the principle that all objects, no matter their weight or composition, accelerate at the exact same rate under gravity's pull—as Galileo Galilei supposedly demonstrated by dropping balls of different materials off the Leaning Tower of Pisa in Italy. That "equivalence principle" now serves as the cornerstone of Albert Einstein's theory of gravity, general relativity, and physicists are keen to test it in as many ways as they can.
Because of equipment problems, however, CAL won't be able to do atom interferometry right away. To produce BECs, CAL's developers at JPL are using a system made by ColdQuanta, Inc., in Boulder. Its heart is a vacuum chamber about the size of a stick of butter. At one end of the chamber, a microchip helps trap and cool the atoms. In both the original device and its backup, the chip leaked, Thompson says. To keep the project on schedule, researchers switched to a simpler design, also made by ColdQuanta, without the tiny mirrors needed for atom interferometry. In a year or so, they plan to send up an upgrade package capable of atom interferometry. "It's very clear to me that this problem is absolutely solvable," says Dana Anderson, ColdQuanta's CEO and co-founder.
CAL is only the beginning of cold atoms in space. The CAL and QUANTUS teams plan to join forces on a second space station mission, called BECAL, which would launch in 2020 or 2021. It would focus on atom interferometry, and achieving the sensitivity to surpass the best tests of the equivalence principle. "I'm really happy about the collaboration," Rasel says, because even the sounding rocket flights are much too short to reach his group's goals. "For me, it's an opportunity to realize dreams."
The merger highlights the groups' different technological approaches. To whip CAL together in just 5 years, NASA relied on commercial, off-the-shelf technology. "A lot the things they bought they had to rejigger for themselves," says Dan Stamper-Kurn, a physicist at the University of California, Berkeley. "It wasn't headache-free." In contrast, German physicists built their own equipment, which even CAL researchers say works better. So BECAL will put the guts of the German system inside JPL's space station package.
To fulfill the true promise of cold atoms in space, physicists eventually hope to launch a dedicated satellite mission. Although the space station offers weightlessness, it's also relatively noisy, shaken by the rattle of pumps and other machinery. A quieter satellite might allow cold atom experiments to reach higher precisions and sensitivities. That might open the door to using an atom interferometer to measure tiny variations in Earth's gravity with far greater precision than current satellites, providing a new tool to map the flows of mass around the globe due to processes such as the draining of subsurface aquifers and the melting of ice sheets.
But first, scientists have to learn how to do atomic physics in space. CAL aims to teach them just that. Nobody is sure what they'll find along the way. "I'm convinced that we're going to come up with a couple of cool thing that nobody thought about," Sackett says, "once we get it up and running."