Spring 2013 Magazine

The Brain

Quantum Leap

Prem Kumar’s groundbreaking research in quantum communication gets a boost


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Prem Kumar

Consider light as sand falling through an hourglass: from a distance, the sand looks like a continuous flow. Up close, individual grains are apparent.

For more than 25 years Prem Kumar has harnessed those individual grains of light, called photons, to forge his career in quantum communication and computing. And while the AT&T Professor of Information Technology, professor of electrical engineering and computer science and of physics and astronomy, and director of the Center for Photonic Communication and Computing could now be considered a quantum photonics elder, he doesn’t have time to slow down. As the lead investigator of a major new grant, Kumar hopes to discover new approaches that could help make his dream of quantum communication a reality.

Quantum communication is different from classical communication and computing, which works by processing “bits,” fundamental units of information that can exist in only one of two states, 0 or 1.Quantum communication uses quantum bits (“qubits”), such as photons, ions, or atoms, which operate under the rules of quantum mechanics instead of classical mechanics; in addition to being in 0 or 1, qubits can be in a “superposition”— both 0 and 1 simultaneously. Because the superposition state is able to carry and process significantly more information in less time and with a higher level of security than do classical communication processes, researchers in the field dream of faster computers and a quantum Internet.

“We’re trying to engineer nature in a different way than it naturally likes to be,” Kumar says. “The jury is still out on quantum communication. We are working to achieve the goal of having quantum communication at the same level as classical communication, and we’re nowhere near that yet.”

As the lead investigator on a four-year, $8 million grant from the Defense Advanced Research Project Agency’s (DARPA) Quiness program to a group of university and industrial partners, Kumar plans to combine research from the last two decades to find new approaches to quantum communication. One might come from his groundbreaking work on quantum frequency conversion, by which the frequency of a light beam can be changed while its quantum state is preserved. Other possibilities include using atoms as quantum repeaters (similar to amplifiers) and creating pulses of light (arbitrary optical waveform generation) to better carry information.

“The goal is far-reaching,” Kumar says. “We’re going to use every trick of the trade that we’ve worked on over the years to make it happen.”

Conducting quantum communication on the current telecommunications infrastructure is a challenge. Quantum information is fragile and cannot be amplified using traditional means. While the optical-fiber cables used today in classical communication can carry about 10 terabits of information per second across vast distances, that infrastructure can barely support 1 megabit per second over 100 kilometers using quantum communication techniques.

Kumar couldn’t have anticipated this future when he was growing up in India, but his intuition for the laws that govern the smallest part of the universe showed itself early. In seventh-grade math he argued that the intersection of two lines could not be a point; the lines themselves were a series of small atoms. Two years later, when he picked up a physics textbook and felt for the first time he was reading something “natural,” his future was sealed. In graduate school at the University at Buffalo, he began working in optics, with lasers. “They were beautiful,” he said. “Not only did I get my degree, but I got to play with these toys that looked pretty.” But a postdoctoral stint at MIT led him to quantum optics, a field that was taking off due to research advances and a proliferation of fiber-optic communication.

He joined Northwestern in 1986 after spending five years at MIT as a research scientist, and in the early 1990s he developed the “quantum frequency conversion” technique that a recent Physics Today article highlighted as groundbreaking and fundamental to current research. It might make possible quantum communication across great distances.

Most recently Kumar’s group developed a switching device to enable the ubiquitous fiber-optic infrastructure to be shared among many users of quantum information. Such a system could route a qubit to its final destination, just as email is routed across the Internet today. The switch could also help encode information in photons for deep-space communication.

Much of this success belongs to Kumar’s unique research group, a mix of engineering and physics students and postdocs. The physicists bring the fundamental knowledge of the universe; the engineers bring the ability to apply it. “The sum of the two is much greater than the parts,” Kumar says. “They learn from each other, they teach each other, and it allows us to do better research.”

Acknowledging Kumar’s position among the top in his field, DARPA’s Defense Sciences Office recently appointed him as a program manager; he took a leave and moved to Washington, DC, to help the office oversee and create programs. MIT also invited him to give the Hermann Anton Haus Lecture this spring. It was a return to the place where he began his research and an opportunity to reflect on his career. He thinks about advances still to come during the lifetime of his son, Rajan Kumar, a McCormick junior. The scholars Kumar has trained over a quarter of a century will continue his work, he hopes. “We all had hoped and still hope that there will be elements of quantum communication everywhere, but it’s turning out to be rather hard,” he says. “I want to see the future generations do better than I did.”

By: Emily Ayshford