Duke Alumnus Basking in the Limelight of Physics

Ronald Walsworth, a Duke alumnus now at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., is literally riding a wave of success since the research group he co-directs, plus another from the same city, announced they could - in a manner of speaking - "stop" light dead in its tracks and start it moving again. The discovery could be of great importance for telecommunications industries, which are increasingly using light as the vehicle for shuttling packets of information through fiberoptic cables at the greatest possible speeds. In Durham to visit his in-laws - his wife, Lisa, is the daughter Donald Burdick, an associate professor at Duke's Institute for Statistics and Decision Sciences - Walsworth sat in the office of his one-time Duke physics mentor, Horst Meyer, talking about the striking and head-spinning findings, which made the front page of The New York Times. He and his research associates found that by shining a "control" laser beam into a heated vapor of the rare-earth metal rubidium they could induce rubidium atoms to capture vital information from a pulse of light of opposite polarization, dubbed the "signal" beam. The rubidium atoms would ordinarily absorb light from the signal beam, thus destroying any information that beam contained. But by first co-mingling the control beam with the atoms in a small glass cell, then also sending through the signal beam, they could make the rubidium adjustably transparent to the signal beam. The effect involved, called electromagnetically induced transparency, or EIT, "was first observed, though not understood, some 40 or 50 years ago," Walsworth said in an interview. It has to do with "destructive interference between two pathways for light to be absorbed," he added. Simply put, the two light waves can put the atoms into a mixture of so-called "spin states" that are so out of phase with each other that the absorption of the signal beam by the two states cancel each other out. The same principal is at work in rooms acoustically designed to contain "dead" spots, where certain sound waves frequencies interfere enough with each other to produce silence, he noted. Likewise, bathed by the control beam within their glass cell, the rubidium atoms may be transparent to the incoming signal beam. But the signal beam is still dramatically slowed and compressed by the gas. In the process, the information the signal beam contains - essential components of that light-- can be transferred to the atoms' spin states, forming a combined light-atom entity called a "polariton." If the control beam is then turned off, the polariton is brought to a halt, and will retain the data that had been on the signal pulse. When the control beam is switched on again, the signal pulse is regenerated with its original information, and can then continue on its way once more at the normal speed of light. Thus, the signal pulse can be said to have been "stopped," with its information stored in the atoms and then reconstituted. In another sense, however, that's an oversimplification. Light can be described both as waves of electromagnetic energy and as particles called photons. And "from one point of view, photons are always streaming through," Walsworth said. "They're not being stopped at all. It's just that the information is being bled out of this type, the signal beam, and written on here, the polariton." Thus, while information might be removed from the signal beam, the photons' energy is not. "It just gets kicked into the control beam," he added. In response, the control beam actually gets a little brighter. His group's findings might help advance the technology of all-optical switching, which is currently hamstrung by the fact that light signals cannot be currently stored in place like electronic signals are. Another possible and exciting application might be in the still largely theoretical realm of quantum information, which seeks to use the often counterintuitive rules of quantum mechanics -- the correct description of atomic-scale physics -- to design faster computers and unbreakable encryption codes. Light is likely the ideal carrier of quantum information. However, computing and storing such information will likely be best accomplished in atoms, he said. Thus, a mechanism to transfer quantum information reversibly between light and atoms may be a key component of a future "quantum Internet." Walsworth, who graduated from Duke in 1984 before going to Harvard for his Ph.D. attended high school in New Canaan, Conn., where he grew up infatuated by the space program as well as by basketball. While he played on his high school's varsity team, "I wasn't going to end up being good enough to play basketball at Duke," he said. "But it is a great school with a nice warm climate." While still in his freshman year, he managed to link up with the world-class cold temperature physics group headed by Meyer, Duke's Fritz London Professor of Physics. He continued working with Meyer and his graduate students over the next 3 years. "It opened up a wider world than one would get just being in with the undergraduates, though that was fun too," he recalled. "I got a good education in the classroom and an even better one in technical details and in life training working side-by-side at the bench top with these people. I would recommend it for anybody." And he still got to play basketball, on an intermural team calling itself The Flaming Mothers of God. "We had to come out with the most outlandish name we possibly could," he explained. "We had a lot of fun in those days, and got a lot of work done too."