Quantum Light

Physicists at Cambridge MA exhange Light & Matter

For the first time physicisists have stopped and extinguished a light pulse in one part of space and then revived it in a completely separate location. They accomplished this feat by completely converting the light pulse into matter that travels between the two locations and is subsequently changed back to light.

Matter, unlike light, can easily be manipulated, and the experiments provide a powerful means to control optical information. The findings, published this week by Harvard University researchers in the journal Nature, could present an entirely new way for scientists and engineers to manipulate the light pulses used in fiber-optic communications, the technology at the heart of our highly networked society.

"We demonstrate that we can stop a light pulse in a supercooled sodium cloud, store the data contained within it, and totally extinguish it, only to reincarnate the pulse in another cloud two-tenths of a millimeter away," says Lene Hau, Professor of Physics and of Applied Physics in Harvard's Faculty of Arts and Sciences and School of Engineering and Applied Sciences.

Read more on Matter & Light United from Harvard University

Scientists at Harvard University first showed how ultra-cold atoms can be used to freeze and control light to form the "core" – or central processing unit – of an optical computer. Optical computers would transport information ten times faster than traditional electronic devices, smashing the intrinsic speed limit of silicon technology.

This research could be a major breakthrough in the quest to create super-fast computers that use light instead of electrons to process information.

Some substances, like water and diamonds, can slow light to a limited extent. More drastic techniques are needed to dramatically reduce the speed of light. Hau's team accomplished "light magic" by laser-cooling a cigar-shaped cloud of sodium atoms to one-billionth of a degree above absolute zero, the point where scientists believe no further cooling can occur. Using a powerful electromagnet, the researchers suspended the cloud in an ultra-high vacuum chamber, until it formed a frigid, swamp-like goop of atoms.

When they shot a light pulse into the cloud, it bogged down, slowed dramatically, eventually stopped, and turned off. The scientists later revived the light pulse and restored its normal speed by shooting an additional laser beam into the cloud.

Hau's cold-atom research began in the mid-1990s, when she put ultra-cold atoms in such cramped quarters they formed a type of matter called a Bose-Einstein condensate. In this state, atoms behave oddly, and traditional laws of physics do not apply. Instead of bouncing off each other like bumper cars, the atoms join together and function as one entity.

The first slow-light breakthrough for Hau and her colleagues came in March 1998. Later that summer, they successfully slowed a light beam to 38 miles per hour, the speed of suburban traffic. That's 2 million times slower than the speed of light in free space. By tinkering with the system, Hau and her team first made light stop completely in the summer of 2000.
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Light waves that travel very slowly without distortion could eventually help simplify and reduce the cost of high-speed optical communications. (Image courtesy of National Institute Of Standards And Technology)

Light is so fast that it takes less than 2 seconds to travel from the Earth to the moon. This blazing fast speed is what makes the Internet and other complex communications systems possible. But sometimes light needs to be slowed down so that signals can be routed in the right direction and order, converted from one form to another or synchronized properly.

Ultraslow Optical Solitons in a Cold Four-State Medium
Physical Review Letters, Vol. 93. Issue 14

NIST physicists showed it is possible to use a very stable pulsed laser to create a soliton that travels slowly through a cryogenic gas of rubidium atoms for more than 5 centimeters without noticeable distortion. The scientists now plan to translate the theory into practical experiments. Currently, 300 kilometers of fiber are required to delay an optical signal for one thousandth of a second, whereas only a few centimeters of fiber might be needed using the new class of soliton.

Solitons first were discovered in the 1800s when a naval engineer observed a water wave travel more than a mile within a canal without dissipating. Light wave solitons generated within optical fibers are now the subject of intense research worldwide. Their very short, stable pulse shapes might be used to pack more information into fiber-optic communication systems. But when previously known forms of optical solitons are slowed down, attenuations and distortions (and therefore losses of data) occur quickly, before the light has travelled even 1 millimeter.
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