Contact:
Steve Bradt
617.496.8070
Physicists Observe Information Exchange Between Quantum Dots
Find boosts fledgling field of quantum computing by showing how non-adjacent 'bits' cooperate
Cambridge, Mass. - March 25, 2004 - Scientists have found that the fundamental elements of a quantum computer can exchange information and work in tandem even when they're separated by a considerable distance. The result represents the first success at controlling the transfer of information between quantum particles located some distance apart, rewriting the belief that "quantum dots" must be neighbors to operate in unison and advancing scientists' progress toward developing massively parallel spin-based quantum computing.
The work, by researchers at Harvard University and the University of California, Santa Barbara, is reported March 25 on the web site of the journal Science.
"In quantum computing, the goal is not to have information shared only between neighbors, but rather to develop broader networks of data exchange," says Charles M. Marcus, professor of physics in Harvard's Faculty of Arts and Sciences. "This observation of communication between two remote quantum dots shows that it is possible to control the exchange of information between non-adjacent particles."
Still a theoretical technology, quantum computing would take advantage of a particular property of quantum systems where two particles are said to be "entangled": the status of one is wholly dependent on the status of the other. Because such a linkage depends on the exchange of electrons between particles, some researchers have believed that entanglement can occur only between neighboring quantum dots, also known as qubits.
"Entanglement is a concept unique to quantum systems and is much stronger than the kinds of correlations we see in everyday life," Marcus says. "To give an example, if we have one set of car keys and I go to work and discover the keys are in my pocket, it means that my wife can't use the car. This is a correlation: If the keys are with me, then they are definitely not with her. Other correlated events will follow from this fact as well -- but the act of reaching into my pocket did not suddenly take the keys away from home.
"In quantum mechanics, the act of measuring determines the state not only of the object being measured, but of other objects, perhaps very far away, that are correlated with the object," Marcus adds. "It's surprising, but it seems that this is how the world works, at least in a quantum mechanical system. In fact, it is this parallelism -- that an object holds the potential to be in any state until measured -- that confers quantum computing's strength."
Marcus is part of a multi-investigator effort to harness the power of quantum parallelism by using the spin, or intrinsic angular momentum, of electrons trapped in artificial atoms known as quantum dots, which are nanofabricated into a semiconductor. Quantum computing's practical value would be severely limited if only neighboring dots were able to correlate their spins. The discovery by the Marcus group that entanglement can be controlled across an open conducting region could be as crucial to quantum information processing as bus lines and interconnects are to today's classical electronic computers.
"Because quantum computing is still in its infancy, the focus is now mostly on achieving and controlling entanglement between neighboring quantum bits," Marcus says. "But as we have seen in classical computers, the wiring problem -- namely, how elements of a multi-element system will communicate -- is also crucial."
Marcus and his colleagues characterize remote quantum dot communication of spin information in terms of the RKKY interaction, named for theoreticians Ruderman, Kittel, Kasuya and Yosida, who first described long-range spin interaction in the 1950s.
The lead experimentalist on this project is Harvard undergraduate Nathaniel J. Craig. The team also includes Harvard graduate student Jacob M. Taylor, Middlebury College undergraduate Edward A. Lester, and Micah P. Hanson and Arthur C. Gossard of the Department of Materials at the University of California, Santa Barbara. The Harvard work was supported by the Defense Advanced Research Projects Agency, the Army Research Office, the National Science Foundation and Harvard University.
###