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Coldest place in the universe
A mile of light squeezed into thousandths of an inch
The coldest place in the universe is not millions of miles away in a dark corner of outer space but in an exotic laboratory in Cambridge, Mass. It’s a place where Harvard University researchers are slowing and compressing light and probing exotic states of matter, work that could lead to new types of communications systems and computers that your grandchildren will use.
• Video: Light and matter (2007) (1:52)
• Video: Light stopper (2001) (2:52)
The ultracold temperatures are part of a system that makes it possible to suspend a cloud of atoms in a windowed vacuum chamber where they can be studied by scientists standing only a few inches away at room temperatures. Inside the chamber, temperatures drop to within a billionth of a degree of absolute zero (minus 459.7 degrees Fahrenheit), low enough to all but stop the constant jiggling and twisting of atoms.
The cigar-shaped atom cloud, suspended in the chamber by a magnetic field, is only 1/200-inch-long, says Lene Vestergaard Hau, Gordon McKay Professor of Applied Physics and Professor of Physics in the Division of Engineering and Applied Sciences at Harvard. “This easy access allows us to massage the atoms with laser beams and make them do exactly what we want.”
Last year, Hau and her colleagues used the atom cloud to slow, then stop, a beam of light zipping along at 186,282 miles per second. After parking the light for a few thousandths of a second, they brought it back to full speed and intensity. Until a few years ago, such a thing was thought to be impossible, even by someone as imaginative as Albert Einstein.
However, Einstein, along with physicist Satyendra Nath Bose, did imagine clouds of ultracold atoms that form an entirely new state of matter. If cooled down enough in a deep vacuum, atoms can lose their individuality and lock together to form a single superatom that takes on a totally different character from gases, liquids, and solids. For example, this condensate, as it’s called, exhibits some of the properties of a superfluid, a fluid that flows without any resistance.
Einstein and Bose proposed such a state of matter 75 years ago, but a Bose-Einstein condensate was not actually made in a laboratory until 1995. In 1998, Hau used it to slow the speed of light from 186,282 miles per second to about 37 mph. Then in 2000, her team stopped a beam of light cold. The atom cloud held the light until Hau released it with a laser beam. Once freed, the light continued on, retaining its original characteristics and brightness. One practical use for such a system would be new types of optical systems for information storage.
Recently, Hau reversed her experiments, employing light to probe the Bose-Einstein condensate rather than using the condensate to slow light. Light is compressed as it slows down, and she broke down the superfluidity of the frigid cloud with the help of an ultraslow light pulse. This was the first time a thing like that was ever done.
The point was to try to understand Bose-Einstein condensates better, just as you might take a carburetor apart to see how it works. Hau and her colleagues, Zachary Dutton, Christopher Slowe, and Michael Budde describe their results in the July 27 issue of the journal Science.
Slowing and stopping light offers the possibility of much more efficient computers, machines based on changes in the excited states of atoms that would be controlled by light pulses. This research also points the way toward purely optical communications systems, which would eliminate the devices now necessary to turn information carried by optical fibers into electronic signals and back again. Such conversions are now necessary to create computer images, telephone messages, and other electronic communications.
The superfluidity experiments do not offer such commercial possibilities not yet, anyway. “Our imaginations have not figured out yet how to exploit our results for practical uses,” Hau admits.
Lighting up computers
So far, Hau’s group has done its experiments at the Rowland Institute for Science, a private research facility a few miles from Harvard’s main campus in Cambridge. That was the first site of the coldest place in the universe. Now, Hau is setting up two more ultracold vacuum chambers at the Cruft Laboratory located near historic Harvard Yard and part of the University’s Division of Engineering and Applied Sciences.
The experimental setup at the Roland Institute occupies a small room; the facility at Harvard sprawls across several rooms. Hau’s ultimate object, however, is to put ultracold clouds of atoms and hypervacuums on chips the size of a fingernail. Equipment to make atom chambers and laser systems with dimensions measured in nanometers (billionths of a meter) will soon be in place in Cruft Laboratory.
Such chips might run data storage devices capable of containing vastly more information in much tighter spaces than now possible. And they might run so-called quantum computers capable of working on several problems simultaneously, a capability beyond today’s most powerful machines. Light processing chips, whether as elaborate as atom-cloud devices or not, could make it practical to communicate entirely by light.
Information can be transmitted faster by light than by electrons zipping through wires. But when light pulses shining through glass fibers reach their destination, they must be converted to electrons to process the information coded in photons, or units of light. If you reply or otherwise interact with the incoming signal, it must be converted back from electrons to photons.
“At present, light is extremely difficult to process,” Hau notes. “This difficulty can be overcome by feeding the light into a Bose-Einstein condensate and processing the signal by means of a magnetic field.”
Such systems are one, two, or more decades away, but Hau and her colleagues have begun to make them possible by using their custom-made coldest places in the universe.
In a Bose-Einstein condensate the flowing particles are neutral, that is, they lack an electric charge. If charged particles flow through very cold wires, it’s possible to have electricity flowing without resistance, a phenomenon known as superconductivity. Superconductivity has become more and more practical in the past 15 years since its discovery, due to finding ways to achieve it without colossally cold (and expensive) conditions. Fifteen years from now, we might be talking about new ways to profitably exploit superfluidity.
