Pitt Physicist Dutt, Harvard-Led Team Report in 'Nature' a Major Step Toward Quantum Networks

So peculiar is quantum physics that when an experiment by Pitt physics and astronomy professor Gurudev Dutt and colleagues from Harvard University turned weird, it got them published in the Aug. 5, 2010, edition of Nature.

The team reported the first linkage of a diamond-encased electron with a packet of light waves to achieve quantum entanglement, or “quantum weirdness”—the indivisible connection between atoms that is fundamental to developing practical quantum networks and quantum computers. Dutt served as one of the main designers of the experiment and coauthored the Nature paper with principal investigator and Harvard physics professor Mikhail Lukin, lead author Emre Togan, a graduate student in physics at Harvard, and researchers from various institutions, including the California Institute of Technology, Texas A&M, and the University of Copenhagen.

The electron and the photon cluster represent, respectively, a solid-state qubit and a flying qubit, with a qubit being the basic unit of quantum information, Dutt said. Where it gets “weird” is that by coupling these two qubits, the researchers took a major step toward creating networks capable of transfering vast amounts of quantum information over long distances via a continuous stream of photons without the solid-state components ever having physical contact. Quantum computers are of particular interest to the U.S. government for the theoretical ability of the highly efficient devices to crack any code, particularly those protecting electronic communication.

“Einstein called this capability ‘spooky,’ and fundamentally it is very, very weird that two objects never connect but can still talk to each other through quantum entanglement,” Dutt said. “But we’ve shown that quantum networks can happen and that photons can be entangled with a solid-state qubit. Taking these results to the next step will require a lot of technical improvement, but we’ve addressed the fundamental challenges.”

The team’s success with the electron is itself a notable accomplishment, Dutt said. Past demonstrations of quantum entanglement connected photons to single atoms that were trapped to create qubits. Atom-based qubits not only require a significant amount of equipment and energy to create, but they also die off rather quickly. The Harvard team used a single electron stored in the nanoscale defects of a diamond—a hardy qubit ready for repeated entanglement.

“The beauty of this experiment,” Dutt said, “is that we now have two qubits in a robust environment that are open to all types of quantum experiments again and again and again.”

The work was supported by the Defense Advanced Research Projects Agency, the Harvard-MIT Center for Ultracold Atoms, the National Science Foundation, the National Defense Science and Engineering Graduate Fellowship, and the Packard Foundation.

— By Morgan Kelly